Chapter IV (1)

THE MICROSCOPIC AND OPTICAL CHARACTERS OF MINERALS.

OPAL.

ISOTROPIC. AMORPHOUS.

COMPOSITION: SiO_{2}.nH_{2}O, generally soluble in caustic alkalies.

▄Usual Appearance in Sections▄: Colorless patches, incrustations or
veins, also at times with sphærulitic structure (hyalite), showing
interference cross, of negative character, between crossed nicols.
Often shows anomalous double refraction due to strains. The refractive
index is very low (1.46) so that the surface of the opal appears
rough.

REMARKS: Found as a secondary mineral in many acid volcanic rocks,
rhyolite, trachyte, andesite, etc., and also in basic basalts. H., 5.5
to 6.5. Sp. gr., 2.2.

LIMONITE.

AMORPHOUS.

COMPOSITION: Fe_{2}(OH)_{6}, Fe_{2}O_{3}, frequently quite impure.

▄Usual Appearance in Sections▄: Brownish and opaque, in very thin
sections may be translucent.

REMARKS: Limonite is essentially a decomposition product, often
forming pseudomorphs after ferruginous silicates or halos about the
iron ores.

PYRITE, Pyrites.

ISOMETRIC.
COMPOSITION: FeS_{2}.

▄Usual Appearance in Sections▄: Cubes, pyritohedrons, combinations of these forms; or in irregular grains. Outline of cross-sections generally square.

_Opaque_, and by reflected light, bright yellow, with strong metallic lustre.

Alters very easily to the oxides of iron (rust).

REMARKS: May be present in all kinds of rocks, igneous, metamorphic
and sedimentary. Not noticeably acted on by hydrochloric acid. H., 6
to 6.5. Sp. gr., 4.9 to 5.2.

PYRRHOTITE, Magnetic Pyrites.

COMPOSITION: FeS. Distinguished from pyrite by being practically
always in irregular masses and not in crystals, and by bronze yellow
color with reflected light. Found in basic eruptive rocks, more rarely
in schists.

MAGNETITE, Magnetic Iron Ore.

ISOMETRIC.

COMPOSITION: Fe_{3}O_{4}, often contains Ti.

▄Usual Appearance in Sections▄: Grains and crystals (generally octahedra), Fig. 33 B. Skeleton crystals frequent in highly ferruginous eruptive rocks.

_Twinning._—Common, according to _Spinel_ law.

FIG. 33.—_A_, Zircon crystals (isolated from granite) in balsam,
showing high relief. _B_, Magnetite crystals. _C_, Ilmenite, showing
partial decomposition to _leucoxene_ along crystallographic
directions.
]

_Opaque_, and by reflected light, bluish-black, with strong metallic lustre.

▄Distinguished from▄: HEMATITE, CHROMITE, ILMENITE and GRAPHITE, by being easily separated from powdered rock by weak magnet.

REMARKS: Very widely distributed in eruptive rocks and crystalline
schists. In the eruptive rocks magnetite belongs to the oldest
secretions from the magma, immediately followed by chrysolite,
biotite, hornblende, augite, etc.; hence often appears as inclusions
in these and other minerals. Magnetite grains may form with other
substances pseudomorphs after hornblende, biotite, hypersthene, etc.
Such pseudomorphs appear to be caused by “resorption.” Magnetite is
strongly magnetic and soluble in hydrochloric acid. H., 5.5 to 6.5.
Sp. gr., 4.9 to 5.2.

CHROMITE.

ISOMETRIC.

COMPOSITION: FeCr_{2}O_{4}.

▄Usual Appearance in Sections▄: Octahedral crystals, grains and in the
olivine rocks sometimes in dense aggregates. May be surrounded by
green, pleochroic halo of chrome ochre.

_Opaque_, and by reflected light, brownish-black to black, with
general absence of metallic lustre. Usually translucent and brownish
on the edges (by transmitted light), with a very rough surface due to
high index of refraction (_n_ = 2.1).

▄Distinguished from▄:

(_a_) MAGNETITE by brownish-black to black color and general absence
of metallic lustre (by reflected light) and by grains being usually
translucent and brownish on the edges (by transmitted light).

(_b_) SPINEL (Picotite), see under Spinel.

REMARKS: Common in crystalline rocks, rich in magnesia, and in
serpentine. Chromite is not acted on by acids, is non-magnetic and
gives chromium bead test. H., 5.5. Sp. gr., 4.3 to 5.6.

SPINEL.

ISOTROPIC. ISOMETRIC.

COMPOSITION: Mg(AlO_{2})_{2}. Pleonaste (Fe, Mg spinel), Picotite (Cr
spinel).

▄Usual Appearance in Sections▄: Octahedral crystals and twins (after
spinel law), less often in grains. Fracture cracks. Always optically
normal and never decomposed in rocks. Usually colorless or dark green
(pleonaste) to brown (picotite). The refractive index is high (_n_ =
1.72, spinel proper, to 2.00, chrome spinel), hence the _relief_ is
marked and the surface rough.

▄Distinguished from▄:

(_a_) GARNET when colorless by octahedral shape of crystals (garnet
forms being 110 and 211), when brown (picotite) from melanite garnet
by common zonal coloration of the latter, but may require chemical
tests. Furthermore spinel may have green color and is never
decomposed.

(_b_) PEROVSKITE by the lower index of refraction and the absence of
reaction for Ti.

(_c_) CHROMITE chemically or by density or hardness.

REMARKS: Found in gneiss, granulite, lherzolite and in regions of
contact metamorphism and secondary bedding formations (picotite),
olivine-basalt and serpentine. Spinels are insoluble in hydrochloric
acid. H., 8. Sp. gr., 3.5 to 4.1.

GARNET.

ISOTROPIC. ISOMETRIC.

COMPOSITION: R″_{3}R‴_{2}(SiO_{4})_{3}. R″ is Ca, Mg, Fe or Mn; R‴ is Al, Fe‴, or Cr, rarely Ti.

▄Usual Appearance in Sections▄: Irregular grains, Fig. 13, or simple crystals, showing forms (110) and (211), alone or in combination, Fig. 14 _b_. Zonal structure not infrequent, especially in the titanium varieties, Fig. 34.

_Color._—Colorless, or nearly so, to yellowish, reddish or brownish.

_Index of Refraction._—_n_ = 1.750–1.856, hence _relief_ high and surface very rough.

_Fracture._—Irregular cracks occur, but no cleavage noticed.

▄Crossed Nicols▄: As garnets are isotropic, sections remain dark during complete rotation. Optical anomalies may however occur, but are generally confined to, titanium free, lime garnets and manganese garnets. The effect being to divide the crystal symmetrically into different areas, “dodecahedral structure.”

FIG. 34.—Garnet, with zonal structure, in gneiss. (From Cohen.)
]

▄Alteration▄: Garnets are usually fresh, but may be found altered to chlorite or hornblende.

▄Distinguished from▄: SPINEL and PEROVSKITE.—See under the latter.

REMARKS: Found principally in granulites, metamorphic rocks, contact
rocks, crystalline schists, etc. Certain varieties may be found in
eruptive rocks or olivine rocks. May form pegmatitic borders with
pyroxene, spinel, etc. Garnets are practically insoluble in
hydrochloric acid. H., 6.5 to 7.5. Sp. gr., 3.4 to 4.3. The
insolubility in acids and the high sp. gr. help in separating garnet
from a powdered rock.

LEUCITE.

ISOTROPIC. ISOMETRIC.[80]

COMPOSITION: KAl(SiO_{3})_{2}.

▄Usual Appearance in Sections▄: Crystals or grains, which vary greatly in size. Cross-sections often nearly round. When very small and free from inclusions may be easily overlooked. Sometimes the grains are surrounded by tangentially arranged needles of different minerals.

FIG. 35.—Leucite, with radial and tangential inclusions, Vesuvius
Lava. (From Cohen.)
]

_Color._—Colorless.

_Index of Refraction._—_n_ = 1.509, hence no _relief_ and generally smooth surface.

_Fracture._—May be noticed, but no cleavage observed.

_Inclusions._—Common, radially or zonally arranged, consisting of minerals or glass, Fig. 35.

▄Crossed Nicols▄: The smaller crystals appear isotropic; the larger crystals show characteristic intersecting systems of twin lamellæ, Fig. 36.

FIG. 36.—Leucite, showing complicated, interpenetration twinning
between crossed nicols.
]

_Double Refraction._—Very weak (γ − α = 0.001). In thin sections it may be necessary to use a sensitive color plate to prove double refraction.

_Interference Colors._—Very low 1st order, dark gray, etc.

▄Alteration▄: Quite frequent to fibrous or granular zeolites.

▄Distinguished from▄: ANALCITE—see under analcite.

REMARKS: Almost entirely confined to younger eruptive rocks,
phonolite, tephrite and other leucite rocks and their tuffs. Often
found with plagioclase, nephelite, augite, etc. It is more or less
attacked by hot hydrochloric acid. H., 5.5 to 6. Sp. gr., 2.4 to 2.5.
The isolation of leucite from rock powder can be better accomplished
by specific gravity than by chemical methods.

ANALCITE.

ISOTROPIC. ISOMETRIC.

COMPOSITION: NaAlSi_{2}O_{6} + H_{2}O.

▄Usual Appearance in Sections▄: Secondary colorless grains, with no
very characteristic microstructure or properties. Cleavage parallel to
cube (100) usually seen. Index of refraction low (n = 1.488), hence
rather rough surface. Between _crossed nicols_ may show optical
anomalies, but not so marked as in garnet.

_Distinguished from_: LEUCITE, SODALITE and NEPHELITE. These minerals
are most easily confused with analcite and recourse must be had to
chemical tests, detection of optical anomalies, gelatinization test or
turbidity by heating.

REMARKS: Occurs as a secondary product (commonly from nephelite or
leucite) in alkali-rich eruptive rocks. Considered also as a primary
mineral in igneous rocks.[81] Gelatinizes with hydrochloric acid, and
becomes turbid by heating. H., 5.5. Sp. gr., 2.25.

SODALITE GROUP.
Sodalite, Haüynite (Haüyne) and Noselite (Nosean).

ISOTROPIC. ISOMETRIC.

COMPOSITION:

Sodalite, 3NaAlSiO_{4} + NaCl.
Haüynite, 2(Na_{2}Ca)Al_{2}(SiO_{4})_{2} + (Na_{2}Ca)SO_{4}.
Noselite, 2Na_{2}Al_{2}Si_{2}O_{8} + Na_{2}SO_{4}.

▄Usual Appearance in Sections▄: Dodecahedral rounded crystals or (S)
irregular grains. Colorless, yellowish, brownish, greenish to deep
blue. Refractive index low [_n_ = 1.483(S) to 1.503(H)], hence the
surface appears rather rough in sodalite and slightly rough in
haüynite. Inclusions, abundant and characteristic, often rod-like and
arranged regularly, making section translucent and especially dark at
border, Fig. 37. Dodecahedral cleavage sometimes seen. Optical
anomalies may occur.

▄Alteration▄: Takes place easily to aggregates of natrolite, other
zeolites, mica, etc.

▄Distinguished from▄:

(_a_) ONE ANOTHER only by chemical tests. Gelatinization test with
hydrochloric acid will show in addition to jelly, salt crystals for
(S), abundant gypsum crystals (CaSO_{2} + 2H_{2}O) for (H) and few if
any gypsum crystals (absence of Ca) for (N). (H) and (N) turn blue
when heated, but test will not work if minerals are decomposed. When
treated with hydrochloric acid and nitrate of silver the black
sulphide of silver will show on (H) and (N), but the white chloride on
(S).

(_b_) NEPHELITE (Elæolite) by being isotropic.

(_c_) ANALCITE by no turbidity when heated.

REMARKS: These minerals are found in the basic, soda-rich rocks. (S)
in elæolite-syenite also in trachyte and phonolite, (H) and (N) common
in phonolite and leucite-porphyry. H., 5.5 to 6. Sp. gr., 2.3.

FIG. 37.—Haüynite, showing dark centre and border, in nepelinite.
(From Cohen.)
]

PEROVSKITE, Perofskite.

ISOTROPIC. ISOMETRIC.
COMPOSITION: CaTiO_{3}.

▄Usual Appearance in Sections▄: Microscopic, octahedral crystals or
larger grains, pale brownish in color and not very transparent, darker
colored in the larger grains. In reflected light grains appear
yellowish with adamantine lustre. Refractive index very high (_n_ =
2.38), hence _relief_ very strong.

Between _crossed nicols_, the little crystals generally appear
optically normal and remain dark; but the larger crystals may show a
complicated penetration twinning.

▄Distinguished from▄: GARNET (melanite) and SPINEL (picotite) by much
higher refractive index and reaction for titanium and by zonal
coloration of melanite. When opaque it might be mistaken for the iron
ores, but has no metallic lustre.

REMARKS: Found in the younger basic eruptive rocks, especially
melilite-basalt. Commonly associated with the iron ores nephelite,
augite and chrysolite. Insoluble in hydrochloric acid. H., 5.5. Sp.
gr., 4.1.

RUTILE.

ANISOTROPIC. UNIAXIAL. TETRAGONAL.
COMPOSITION: TiO_{2}. _ć_ = c ELONGATION ∥ _ć_.

▄Usual Appearance in Sections▄: Sharp, elongated, prismatic crystals
when microscopic, but granular when the individuals are large. Grains
may be almost opaque, with adamantine lustre by reflected light. Knee-
or heart-shaped twins, Figs. 38 and 39, common in the smaller
crystals, the larger individuals may also show geniculated twinning.
Small crystals sometimes form net-shaped groups (sagenite), by
crossing one another at angles of 60°. Pleochroic halos may surround
crystals. Color, yellowish to reddish-brown. Index of refraction very
high (_n′_ = 2.712, α = 2.616, γ = 2.903), hence _relief_ marked and
surface very rough. Prismatic cleavage present in larger individuals,
not observed in microscopic crystals. Pleochroism and strong
absorption may be noticed, especially in the larger grains, but may
fail entirely.

▄Crossed Nicols▄: Double refraction _very_ strong (γ − α = 0.287).
Interference colors[82] very high order, only seen in the microlitic
crystals which do not appear dark due to total reflection; in other
cases may not show at all. _Extinction_ parallel to prisms. In
_convergent light_ optical character (+).

▄Alteration▄: May take place to a white or yellowish, fibrous or
granular substance, strongly refracting, and similar to the alteration
product of ilmenite. May be surrounded by grains of titanite.

▄Distinguished from▄: The OPAQUE ORES by adamantine lustre with
reflected light; ZIRCON and CASSITERITE in concentrates by chemical
tests. May not be possible to distinguish from cassiterite in
sections.

REMARKS: Found in the metamorphic schists, amphibolites, slates,
contact and fragmentary rocks, etc.; also as inclusions in quartz and
mica. Especially common as a secondary product of titaniferous
hornblende and biotite. The “sagenite” webs of the decomposed micas
are probably secondary. Rutile is insoluble in hydrochloric acid. H.,
6 to 6.5. Sp. gr., 4.2. It is easily separated from rock powder by its
insolubility in acid and its high sp. gr.

Rutile twins.

FIG. 38.—Twin plane (101). FIG. 39.—Twin plane (301).
]

ZIRCON.

ANISOTROPIC. UNIAXIAL. TETRAGONAL.
COMPOSITION: ZrSiO_{4}. _c_ = ć. ELONGATION ∥ c′.

▄Usual Appearance in Sections▄: Small, short prismatic crystals, Fig. 33A, and grains. Shell-like (zonal) structure may be noticed. When enclosed in black-mica, hornblende, cordierite, etc., often surrounded by characteristic pleochroic halos.[83]

_Color._—Colorless, rarely pale brownish.

_Index of Refraction._—_n′_ = 1.95, (α = 1.931, γ = 1.993) hence _relief_ very high and surface rough.

▄Polarized Light▄:

_Pleochroism._—Not usually noticeable.

▄_Crossed Nicols_▄:

_Double Refraction._—Very strong (γ − α = 0.062).

_Interference Colors._—Very high (4th) order, minute crystals show brilliant colors.

_Extinction._—As zircon is uniaxial, basal sections remain dark during rotation of stage. In all other sections extinction is parallel to _ć_.

▄_Convergent Light_▄: Basal sections, which are large enough to give interference figures, show several rings in addition to dark cross. Optical character (+).

▄Alteration▄: Very rarely takes place.

▄Distinguished from▄:

(_a_) APATITE.—By much higher relief and stronger double refraction.

(_b_) TITANITE.—By uniaxial character.

(_c_) RUTILE.—See under the latter.

Easily confused with xenotime, which, however, has higher interference colors and more distinct pleochroism; but chemical tests may be necessary.

REMARKS: Found widely distributed but not in quantity in eruptive and
metamorphic rocks. Occurs in granite, syenite, diorite, gabbro,
gneiss, etc. It is one of the oldest constituents of the rocks in
which it occurs, and may often be found as inclusions in the
ferro-magnesium minerals. Zircon is insoluble in hydrochloric acid.
H., 7.5. Sp. gr., 4.5 to 4.7. It can easily be separated from rock
powder on account of its high sp. gr., insolubility in acid and
non-magnetic properties. The crystals can then be examined separately,
or chemical tests made to prove the presence of Zr.

SCAPOLITE GROUP, Wernerite, etc.

ANISOTROPIC. UNIAXIAL. TETRAGONAL.
COMPOSITION: Silicates of Ca, Al _ć_ = a. ELONGATION ∥ a′.
and Na.

▄Usual Appearance in Sections▄: Colorless grains, lath-like
individuals or prisms (dipyre, in contact metamorphic limestone).
Index of refraction (_n′_ = 1.551 to 1.584;, α = 1.542 to 1.558, γ =
1.555 to 1.597) about the same as quartz, hence usually no _relief_
and surface smooth. Cleavage distinct parallel to square prism.
Inclusions (carbonaceous) may be abundant in contact rocks.

▄Crossed Nicols▄: Double refraction usually strong, but varies (γ − α
= 0.013 to 0.039), increases with the Ca percentage. Interference
colors upper 1st or 2d order, more brilliant than those of most of the
colorless minerals. Basal sections (showing cleavages intersecting at
90°) isotropic. _Extinction_ parallel in longitudinal sections. In
_convergent light_ basal sections show distinct uniaxial interference
figure; optical character (−).

▄Alteration▄: Takes place easily to a fibrous substance or to kaolin,
muscovite, etc.

▄Distinguished from▄:

(_a_) FELDSPARS (not showing twinning) and IOLITE (Cordierite) by
uniaxial character, cleavage and higher order interference colors.

(_b_) QUARTZ by cleavage, higher order interference colors and optical
character; quartz is (+)

(_c_) APATITE (in grains) by lower index of refraction, cleavage and
higher order interference colors.

REMARKS: Found especially in metamorphosed diabases and gabbros
(Norwegian); also in gneisses, crystalline schists, metamorphosed
limestones, etc. Dipyre occurs in contact zones of limestones and
schists, where it might be confused with andalusite, but
cross-sections show uniaxial character. The minerals of this group are
more or less soluble in hydrochloric acid. When the scapolite contains
Cl, the following test can be made on fresh material. Treat with a
solution of silver nitrate in hydrofluoric acid and the jelly will be
impregnated with chloride of silver which will turn brown. H., 5.5.
Sp. gr., 2.68.

VESUVIANITE, Idocrase.

ANISOTROPIC. UNIAXIAL. TETRAGONAL.
COMPOSITION: _ć_ = a. ELONGATION ∥ a′.
Ca_{6}Al_{3}(OH.F)(SiO_{4})_{6}.

▄Usual Appearance in Sections▄: Grains or prismatic crystals. Almost
colorless to reddish (when containing Mn). Index of refraction high
(_n′_ = 1.715, α = 1.701 to 1.726, γ = 1.705 to 1.732), hence _relief_
marked and surface rough. Cleavage imperfect, parallel to prism.
Pleochroism generally very faint.

▄Crossed Nicols▄: Double refraction very weak (γ − α = 0.001 to
0.006), may vary in different portions of the same crystal (optical
anomalies due to the mineral being at times a mixture of isomorphous
individuals). Interference colors very low 1st order, dark gray, etc.,
may often appear zonal. Basal sections isotropic when normal, but may
show division into biaxial portions. _Extinction_ parallel in sections
elongated ∥ _ć_ axis. In convergent light basal sections show a faint
cross when normal; optical character generally (−).

▄Alteration▄: Not known in rock-making vesuvianite.

▄Distinguished from▄: EPIDOTE (Pistacite) by the very low order
interference colors. CORUNDUM by weaker double refraction. GARNET
(Grossularite), ZOISITE, and APATITE may be easily confused with this
mineral and hard to distinguish from it.

REMARKS: Found in limestones, that have undergone alteration by
contact with igneous rocks, and in metamorphic schists. Also may occur
in dense (nephrite-like) aggregates in serpentine. It is insoluble in
hydrochloric acid. H., 6.5. Sp. gr., 3.3 to 3.8.

MELILITE.

ANISOTROPIC. UNIAXIAL. TETRAGONAL.
COMPOSITION: Na_{2}(Ca,Mg)_{11}(Al,Fe)_{4}Si_{9}O_{36}?.
_ć_ = a. ELONGATION ∥ c′.

▄Usual Appearance in Sections▄: Almost colorless, tabular (∥
base) crystals or irregular grains or shreds. Sections very
commonly lath-shaped, and often characterized by the peculiar
“peg-structure,”[84] the lines or markings being ∥ _ć_ (⟂
elongation of the section). Index of refraction (_n′_ = 1.630, α
= 1.629, γ = 1.631) higher than that of the other associated
colorless materials, hence _relief_ rather marked. Cleavage,
parallel to base, very imperfect.

▄Crossed Nicols▄: Double refraction very weak (γ − α = 0.003), and
diminishes with a decrease of Al. Interference colors the lower 1st
order, grays, etc.; anomalous interference colors may show.
_Extinction_ parallel to cleavage or the peculiar markings or lines.
Optical character usually (−), but when poor in Al (+).

▄Alteration▄: Takes place frequently to a fibrous aggregate.

▄Distinguished from▄: NEPHELITE and FELDSPAR by higher relief, shape,
“peg-structure” and usual dull appearance with reflected light.

REMARKS: Abundant in the leucite and nephelite rocks (associated with
these minerals and with augite, perovskite and chrysolite), and takes
the place of a feldspar in the melilite-basalt. It gelatinizes easily
with hydrochloric acid. H., 5. Sp. gr., 2.9.

GRAPHITE.

HEXAGONAL.

COMPOSITION: C.

▄Usual Appearance in Sections▄: Minute particles, or flakes and grains
of irregular shape, seldom crystallized.

_Opaque_, and by reflected light, black with metallic lustre.

▄Distinguished from▄: The similarly appearing ores by its insolubility
in acids and the possibility of making it disappear by heating.

REMARKS: Graphite is widely distributed in the oldest rock formations,
especially in the schists. It is often associated with rutile and the
iron oxides. Graphite is not acted on by acids. H., 1 to 2. Sp. gr.,
2.09 to 2.25. It is burnt with great difficulty in thin sections on
platinum foil; but this test may vary, in many cases the graphite
(when in bladed flakes) not being consumed even after long heating.
When heated it may expand into worm-like forms.

_Carbonaceous Matter._—Occurs in opaque, grayish-black particles
having no lustre; and is found finely disseminated, sometimes in
larger aggregations, in clay slates, limestones, etc.

HEMATITE.

HEXAGONAL.

COMPOSITION: Fe_{2}O_{3}.

▄Usual Appearance in Sections▄: Irregular scales, minute grains or
earthy. Distinct crystalline forms not often observed in rocks.

_Opaque_, and by reflected light, black with metallic lustre, or red
without lustre. May also be transparent in red tints. No marked
pleochroism observed.

REMARKS: Found widely distributed in acid eruptive rocks, crystalline
schists, etc. Also as inclusions in minerals, and as a red pigment in
many rocks. It is insoluble in hydrochloric acid, and non-magnetic,
unless attached to grains of magnetite. H., 5.5 to 6.5. Sp. gr., 4.9
to 5.3.

ILMENITE, Menaccanite.

HEXAGONAL.

COMPOSITION: (FeTi)_{2}O_{3}.

▄Usual Appearance in Sections▄: Irregular masses, without crystallographic outline, rhombohedral crystals, or skeleton-like growths. Also in brownish, translucent mica-like forms.

_Opaque_, and by reflected light, iron-black with metallic lustre.

When _translucent_: pleochroism brown to yellow; double refraction not very strong; optically (−).

▄Alteration▄: Often takes place to a whitish, strongly refracting, substance only slightly transparent, called _leucoxene_. This alteration product frequently develops along definite rhombohedral directions, Fig. 33 C. Also a change to titanite or rutile may occur, or the ilmenite may be surrounded by these minerals.

▄Distinguished from▄: MAGNETITE and HEMATITE.—By whitish, strongly refracting decomposition product. At times the distinction may be very difficult.

REMARKS: Ilmenite occurs principally in the soda-rich and basic
eruptive rocks. The mica-like form is limited to the porphyritic
eruptives. The brown pigment in the plagioclase of certain gabbros may
be ilmenite. It is attacked slowly by hot hydrochloric acid, and the
solution when heated with tin becomes violet. Pure ilmenite is
indifferent towards the magnet, hence strong magnetic properties would
indicate a mixture with magnetite. H., 5 to 6. Sp. gr., 4.5 to 5.

CORUNDUM.

ANISOTROPIC. UNIAXIAL. HEXAGONAL.
COMPOSITION: Al_{2}O_{3}. _c_ = a.

▄Usual Appearance in Sections▄: Pyramidal or prismatic crystals,
grains or basal plates. Zonal structure or twinning may be noticed.
Colorless or with patches of blue. Index of refraction high (_n′_ =
1.766, α = 1.760, γ = 1.769), hence _relief_ well marked and surface
very rough. Rhombohedral cleavage may show in larger individuals.
Pleochroism only marked when color is deep.

▄Crossed Nicols▄: Double refraction weak (γ − α = 0.009), like quartz.
Interference colors middle 1st order, white to yellow. _Extinction_
parallel in elongated sections. Optical anomalies very rarely noticed
in microscopic individuals. In _convergent light_ basal sections show
a rather indistinct cross; optical character (−).

▄Distinguished from▄:

(_a_) APATITE and VESUVIANITE by brighter interference colors.

(_b_) TOURMALINE (light colored) by not having such strong absorption.

(_c_) CYANITE by uniaxial character.

Corundum may need to be isolated from the rock in order to be
determined with certainty.

REMARKS: found in contact metamorphic rocks, eruptive rocks, granular limestones, etc. It is insoluble in hydrochloric acid. When rock sections are ground with emery, care must be taken not to confuse grains of emery with corundum in the rock. H., 9. Sp. gr., 3.9 to 4.

QUARTZ.

ANISOTROPIC. UNIAXIAL. HEXAGONAL.
COMPOSITION: SiO_{2}. _ć_ = c.

▄Usual Appearance in Sections:▄ Allotriomorphic in the granitoid rocks, when apparently the last mineral to form, Fig. 5. More or less chemically corroded pyramidal crystals (with cross-sections six-sided or rhombic with an angle of about 100°) in the porphyritic rocks. Rounded or angular grains in the “clastic” rocks; granular mosaic in crystalline schists and contact rocks, very rarely in distinct crystals in any rocks. May at times be mutually interpenetrated with an acid feldspar (the areas of quartz and feldspar extinguishing as entire crystals), producing “micro-pegmatitic” structure, Fig. 67. Finally may appear as pseudomorphs after other minerals, but may then consist of some of the other forms of silica.

_Color._—Colorless, although by reflected light it may appear colored or cloudy if it contain many inclusions.

_Index of Refraction._—_n′_ = 1.547 (α = 1.544, γ = 1.553) hence no _relief_ and surface smooth.

_Cleavage._.—Rarely noticed, an important fact in determining quartz. Quartz breaks irregularly.

_Inclusions._—Minute fluid, gas and mineral inclusions, often in irregular trains, are very characteristic of quartz in granite rocks and crystalline schists. The inclusions are not so abundant in porphyritic rocks, but a few glass inclusions may occur, filling up “negative” crystals in the quartz. Rutile, amphibole, etc., may occur as needle-like inclusions in quartz.

_Polarized Light:_

_Pleochroism._—None.

_▄Crossed Nicols:▄_

_Double Refraction_.—Weak (γ − α = 0.009).

_Interference Colors._—The middle 1st order, white, yellow, etc.

_Extinction._—As quartz is uniaxial, basal sections remain dark during a complete rotation of stage. In the other sections extinction is not characteristic, due to the absence of cleavage and crystallographic outlines. Thin sections do not show circular polarization.

_▄Convergent Light:▄_ Basal sections show a dark cross, without any rings. Optical character (+).

▄Alteration:▄ Does not take place, so quartz always appears fresh and unweathered in sections.

▄Distinguished from:▄

(_a_) SANIDINE (in fresh grains).—By use of convergent light. Feldspar is biaxial, or sections which appear uniaxial are (−).

(_b_) NEPHELITE.—By almost entire absence of hexagonal outline, stronger double refraction, fresh, unweathered appearance and (+) optical character.

(_c_) IOLITE (Cordierite), SCAPOLITE and TOPAZ.—See under the latter minerals.

Quartz may be distinguished from all silicates by being dissolved without residue in hydrofluoric acid.

REMARKS: Quartz occurs widely distributed, as in the great sandstone
formations. It is also a characteristic mineral of all acidic rocks,
being common in granite, aplite, rhyolite, quartz-porphyry,
quartz-diorite, dacite, etc. Quartz is very brittle and hence is a
good indicator of the dynamic forces which have affected the rocks. It
may show traces of mechanical deformation by peripheral shattering of
the larger grains or by “wavy extinction”;[85] and also evidences of
chemical corrosion by curved and looped contours. In some diabases the
quartz may be surrounded by a rim of hornblende or augite needles
(“quartz augen”). “Cataclastic” quartz may be biaxial. The “secondary
enlargement” of quartz in clastic rocks may be noticed by the
deposition of silica in crystallographic orientation around the
clastic grains,[86] the new portion extinguishing at the same time as
the core. Quartz is not attacked by ordinary acids. H., 7. Sp. gr.,
2.6 to 2.7.

_Chalcedony._—This variety of SiO_{2} has a radially fibrous structure and shelly parting. It may form sphærulites, central sections through which show a dark cross between crossed nicols, or line cavities in rocks.

The index of refraction is a little lower than for ordinary quartz. The optical character is (−), which must be determined by a mica or gypsum plate, _ć_ = a. Elongation ∥ a′.

Chalcedony occurs in the ground mass of very silicious porphyritic
rocks, which have microfelsitic development; and is found as a
secondary mineral in all kinds of silicate rocks.

TRIDYMITE.

▄Usual Appearance in Sections▄: This form of SiO_{2}, which is soluble in boiling caustic soda, appears in “tile-like” aggregates of minute colorless plates (pseudo-hexagonal) and is always secondary. The refractive index is extremely low (_n′_ = 1.477), hence the surface appears rough.

Between _crossed nicols_ the interference colors are very low in order (γ-α = 0.002), and the tablets may show a division into different areas (optical anomalies). In _convergent light_ an indistinct biaxial figure is generally seen.

REMARKS: Chiefly a volcanic mineral, found in rhyolite, trachyte and andesite. Commonly associated with opal and chalcedony.

CALCITE.

ANISOTROPIC. UNIAXIAL. HEXAGONAL.

COMPOSITION: CaCO_{3}. Ca may be replaced by small quantities of Mg, Fe, Mn, etc. _ć_ = a.

▄Usual Appearance in Sections:▄ Grains and aggregates. May be fibrous or oölitic. Only in crystals in certain rocks.[87]

FIG. 40.—Calcite, crossed twin lamellæ, in granular limestone. (From
Cohen.)
]

_Twinning._—Polysynthetic, parallel to one or more faces of −½ R. (10̄12). Very common in crystalline limestones, and may have been produced by pressure or by the grinding of the section. Shows itself between crossed nicols as a series of light and dark bands, parallel or intersecting, Fig. 40, about parallel to longer diagonal of cleavage rhombs. When the composition face of the twins is oblique to the face of the section, interference colors can be seen without the analyzer.

_Color._—Colorless when pure, but may appear colored by transmitted light, due to organic pigments.

_Index of Refraction._—_n′_ = 1.601 (α = 1.487, γ = 1.659) hence with ordinary light _relief_ not marked. Due to the great variation in refractive indices of the two rays, with polarized light, the surface will appear either quite smooth or rather rough, depending upon which vibration direction lies over the plane of the polarizer. This marked variation in appearance (sometimes called “twinkling”) serves as a good test for calcite.

FIG. 41.—Calcite, section parallel to face of rhombohedron, showing
rhombohedral cleavage. (From Cohen.)
]

_Cleavage._—Parallel to unit rhombohedron (10̄11), appearing in thin sections as many sharp cracks, whose angles of intersection depend on the position of the section, Fig. 41. Newton’s colors may be seen along cleavage cracks.

▄Polarized Light:▄

_Pleochroism._—None.

_▄Crossed Nicols:▄_

_Double Refraction._—Very strong (γ − α = 0.172).

_Interference Colors._—Pale, iridescent colors of very high order.

_Extinction._—As calcite is uniaxial, basal sections remain dark during rotation. Extinction angles with respect to the cleavage cracks vary with the position of the section.

_▄Convergent Light:▄_ Basal sections, even when very thin, show distinct interference figure, with cross and rings. Optical character (−).

▄Distinguished from▄:

(_a_) Other CARBONATES.—By ease with which it is attacked by cold dilute acids, test can be made on slide after removing cover.

(_b_) MAGNESIUM-BEARING CALCITE.—By micro-chemical tests.

(_c_) TITANITE (Sphene).—See under the latter.

REMARKS: Calcite is very widely distributed, in addition to the
extensive sedimentary limestone deposits. Common limestone consists of
dense aggregates of crystalline grains. Calcite is often a secondary
product of the lime-bearing silicates in the more basic eruptive
rocks. Pseudomorphs of calcite after olivine are noteworthy. Coarse
aggregates of calcite occur in the crystalline schists and contact
rocks. Calcite is exceedingly plastic to pressure and mechanical
deformation may be recognized by curving of the cleavage cracks,
crumpling of the twin lamellæ and “wavy” extinction. Calcite is easily
attacked and completely dissolved with effervescence by cold dilute
acids. H., 3. Sp. gr., 2.72.

DOLOMITE.

ANISOTROPIC. UNIAXIAL. HEXAGONAL.

COMPOSITION: CaMg(CO_{3})_{2}, when pure CaO = 30.4, MgO = 21.7, CO_{2} = 47.8. Proportions of Mg and Ca vary, and Fe and Mn also occur. _ć_ = α.

▄Usual Appearance in Sections:▄ In rocks chiefly as crystals, even dense homogeneous aggregates showing tendency towards crystalline boundaries (saccharoidal structure). Crystals almost always unit rhombohedron (10̄11) with tendency to curved surfaces.

_Index of Refraction._—_n′_ = 1.622 (α = 1.503, γ = 1.682, γ − α = 0.179), a little higher than that of calcite. For variation in appearance of surface with polarized light, see under calcite.

The microscopic characters are similar to those of calcite, from which it may be _distinguished_ by not being so easily attacked by cold dilute acid (test can be made on slide with cover off), by tendency towards crystalline boundaries, by absence of twin lamellæ (or when present parallel to _−2R._ (20̄21), hence about parallel to shorter diagonals of cleavage rhombs), and by micro-chemical tests. The distinction at times may be very difficult.

REMARKS: Occurs in sedimentary formations and as crystals in limestone
and other rocks. In certain rocks the dolomite crystals may not have a
very good “bond,” and a “drusy” structure may also be characteristic
of the cavities between dolomite crystals in rocks. Only slightly
attacked by cold dilute acids, but if acid is heated it dissolves
easily with effervescence.

APATITE.

ANISOTROPIC. UNIAXIAL. HEXAGONAL.
COMPOSITION: _ć_ = a. ELONGATION ∥ a′.
Ca_{5}(Cl.F)(PO_{4})_{3}.

▄Usual Appearance in Sections▄: Minute, slender hexagonal prisms, cross-sections having regular hexagonal boundaries, needles, and grains. Figs. 14 _a_ and 42.

FIG. 42.—Apatite, showing cross fracture, in nepheline-basalt. (From
Cohen.)
]

_Color._—Generally colorless, seldom bluish or brownish (only in eruptive rocks).

_Index of Refraction._—_n′_ = 1.635 (α = 1.634, γ = 1.637), hence _relief_ more marked than that of the colorless associated minerals.

_Cleavage._—Seldom observed microscopically.

_Parting._—Long columnar crystals generally show a transverse jointing, so that the pieces may be more or less separated.

_Inclusions._—Gas and fluid may be present.

▄Polarized Light▄:

_Pleochroism._—None shown by the colorless crystals, the colored crystals show stronger absorption parallel to _ć_.

_▄Crossed Nicols▄_:

_Double Refraction._—Weak (γ − α = 0.003).

_Interference Colors._—The lower first order, generally grayish-blue or white.

_Extinction._—As apatite is uniaxial, basal sections remain dark during rotation of stage. In all other sections extinction is parallel to _ć_ axis.

▄_Convergent light_▄: Basal sections show a cross, without rings. Optical character (−).

▄Alteration▄: Does not usually take place, apatite being found perfectly fresh in decomposed rocks, which is quite remarkable considering its easy solubility in acids.

▄Distinguished from▄:

(_a_) SILLIMANITE and TREMOLITE.—By weak double refraction and elongation ∥ a′.

(_b_) NEPHELITE.—By being relatively much smaller and longer than the nephelite crystals, which are often decomposed. Also by higher relief and negative results with gelatinization test.

(_c_) ZIRCON.—By lower relief and much weaker double refraction.

(_d_) FELDSPARS (when granular and undecomposed).—By higher relief and uniaxial interference figure.

(_e_) VESUVIANITE and ZOISITE.—May be only possible by chemical tests.

(_f_) CORUNDUM.—See under the latter.

REMARKS: Found in most igneous rocks and crystalline schists. In the
eruptive rocks it appears as one of the oldest secretions from the
magma, and hence is often found as inclusions in other minerals,
especially biotite, hornblende, etc. Apatite is easily soluble in
hydrochloric and nitric acids. H., 4.5 to 5. Sp. gr., 3.19. On account
of its high sp. gr., apatite, in rock powder, comes down in heavy
solutions with the metallic minerals, and can be separated from them
by the use of a magnet. This residue can also be tested for phosphorus
in the wet way with ammonium molybdate.

NEPHELITE,[88] Nepheline, Elæolite.

ANISOTROPIC. UNIAXIAL. HEXAGONAL.
COMPOSITION: 7NaAlSiO_{4} + _ć_ = α.
NaAl(SiO_{3})_{2}, with partial
replacement of Na by K.

▄Usual Appearance in Sections▄: Nephelite in short hexagonal prisms and grains in the younger volcanic rocks, hence sections rectangular or hexagonal, Fig. 43; elæolite allotriomorphic in the older plutonic rocks.

_Color._—Colorless.

_Index of Refraction._—_n′_ = 1.539 (α = 1.538, γ = 1.542), hence no relief and surface smooth.

_Cleavage._—Imperfect, parallel to prism (10̄10) and base (0001). More marked in elæolite than in nephelite, especially when decomposition has commenced.

FIG. 43.—Nephelite sections, showing zonal inclusions. (From
Reinisch.)
]

_Inclusions._—Microscopic needles of augite, etc., also fluid and gas. Mostly in zones. Elæolite may be much clouded by inclusions and alteration products.

▄Polarized Light▄:

_Pleochroism._—None.

_▄Crossed Nicols▄_:

_Double Refraction._—Very weak (γ − α = 0.004), may only be detected by using a test-plate.

_Interference Colors._—The lower first order, grayish-white, etc., a little lower than the feldspar colors.

_Extinction._—As the mineral is uniaxial, basal sections remain dark during rotation of stage. In all other sections extinction takes place and is parallel to cleavage lines when these appear.

_▄Convergent Light▄_: Basal sections show a broad cross, without rings. Optical character (−).

▄Alteration▄: Takes place easily to fibrous zeolites (natrolite), or in certain rocks to mica.

▄Distinguished from▄: OTHER MINERALS by gelatinization test and staining with fuchsine. When present in small interstitial individuals (as is often the case in basalts) it is very difficult to distinguish without this test; but it must be remembered that other minerals, zeolites, etc., will also gelatinize. QUARTZ has stronger double refraction, rarely shows hexagonal outline, is always fresh and optically (+). FELDSPAR is biaxial and often shows twinning. ANALCITE.—See under the latter.

REMARKS: Nephelite bears the same relation to elæolite as sanidine
does to orthoclase. It occurs only in the younger volcanic rocks; with
sanidine in phonolite, with plagioclase in tephrite, without feldspar
in nepheline-basalt, and with leucite in leucite-basalt. It is not
found with primary quartz. Elæolite occurs with orthoclase in
elæolite-syenite, etc. nephelite and elæolite frequently occur with
the sodalite group. Nephelite gelatinizes with acids. H., 5.5 to 6.
Sp. gr., 2.5 to 2.6.

TOURMALINE, Schorl.

ANISOTROPIC. UNIAXIAL. HEXAGONAL.
COMPOSITION: Uncertain, _ć_ = a. ELONGATION ∥ a′.
R_{18}B_{2}(SiO_{5})_{4}. R
chiefly Al, K, Mn, Ca, Mg, Li.

▄Usual Appearance in Sections▄: Staff-like individuals, bunched or in radiating aggregates, Fig. 44 B, or prismatic crystals, Fig. 44 A. Basal sections may be nine-sided.

_Color._—Varies greatly, grayish-blue, brown and green most common. Li-tourmaline (rare in rocks) is colorless. Zonal structure may be indicated by differences in color.

FIG. 44.—_A_, Tourmaline, showing strong absorption at right angles to
direction of elongation (_P_ = plane of vibration of polarizer).
_Quartzite_, Black Hills, D. _B_, Tourmaline in radiate aggregate.
_Granite_, Cornwall.
]

_Index of Refraction._—_n′_ = 1.633 (precious) to 1.674 (α = 1.620 to 1.651, γ = 1.640 to 1.685), hence _relief_ is marked and surface rough.

_Cleavage._—Not seen in thin sections, but irregular, transverse and longitudinal cracks may appear.

▄Polarized Light▄:

_Pleochroism._—Distinct, even in light colored varieties, increasing with the depth of color. The greatest absorption takes place at right angles to the direction of elongation of the crystal, Fig. 44 A. The other minerals having this very strong absorption are hornblende, dark colored mica (distinguished by cleavage and lamellar form) and allanite. Pleochroic halos may be noticed surrounding inclusions.

▄_Crossed Nicols_▄:

_Double Refraction._—Quite strong (γ − α = 0.017 (precious) to 0.034).

_Interference Colors._—Bright upper first or second order, but may not be noticeable, due to absorption of parts of the light.

_Extinction._—As tourmaline is uniaxial, basal sections remain dark during rotation of stage. In all other sections extinction is parallel to _ć_ axis.

▄_Convergent Light_▄: Cross-sections show a sharp cross. Optical character (−).

▄Alteration▄: Does not take place.

▄Distinguished from▄:

(_a_) HORNBLENDE.—By absence of cleavage, and by the fact that the greatest absorption takes place at right angles to the longitudinal axis, while in hornblende it takes place approximately parallel to the longitudinal axis, or to the cleavage lines which are parallel to this axis.

(_b_) APATITE (when colored).—By strong absorption at right angles to longitudinal axis.

(_c_) CORUNDUM.—See under the latter.

In some cases where recognition is difficult, chemical tests, to prove presence of boracic acid, must be made.

REMARKS: The black schorl is the only primary tourmaline and is found
in granitoid rocks. Tourmaline in other rocks results from “fumarole”
action; hence occurs in pegmatite, tin and copper veins, clay
deposits, also (light colored) in contact rocks and crystalline
schists. The hemimorphic terminations may sometimes be noticed.
Tourmaline is not acted on by acids. H., 7 to 7.5. Sp. gr., 3 to 3.2.
It can be separated from powdered rock by sp. gr. solutions combined
with magnetic methods.

ANDALUSITE

ANISOTROPIC. BIAXIAL. ORTHORHOMBIC.
COMPOSITION: Al_{2}SiO_{5}. _ć_ = a. ELONGATION ∥ a′.

▄Usual Appearance in Sections▄: In short, rounded, prismatic crystals,
with almost square cross-section. Colorless or at times pale reddish
and spotted. Index of refraction medium (_n′_ = 1.637, α = 1.632, γ =
1.643), hence _relief_ well marked and surface rough. Cleavage,
parallel to almost square prism, may show. Pleochroism only marked in
colored varieties, being reddish ∥ _ć_ (the direction of elongation or
cleavage). Carbonaceous inclusions are characteristic, arranged as in
macroscopic specimens (_Chiastolite_), Fig. 45. Pleochroic halos may
surround inclusions.

▄Crossed Nicols▄: Double refraction weak (γ − α = 0.001). Interference
colors middle 1st order, white to yellow. _Extinction_ in general
parallel to _ć_ axis in longitudinal sections, symmetrical in
cross-sections. In _convergent light_ Ax. pl. ∥ (010), Bx_{_a0_}. ∥
_ć_; axial angle very large (2_E_ > 180°); optical character (−).

▄Alteration▄: Often takes place to dense aggregate of mica, when the
pseudomorph may be hard to recognize.

▄Distinguished from▄:

(_a_) SILLIMANITE by much weaker double refraction, less elongated
crystals and by elongation ∥ a′ (Sillimanite elong. ∥ c′).

(_b_) DIOPSIDE by weaker double refraction, rhombic cross-section and
parallel extinction in longitudinal sections.

REMARKS: Very characteristic of metamorphic schists and of contact
zones of clay slates with granite, etc., but not found in rocks which
have been formed at great pressure. The andalusite grains may often be
arranged in divergent or finger-like manner. May also form parallel
growths with sillimanite. It is insoluble in hydrochloric acid. H., 7
to 7.5. Sp. gr., 3.18.

FIG. 45.—Chiastolite, showing characteristic carbonaceous inclusions.
(From Cohen.)
]

FIG. 46.—Sillimanite aggregate, showing cross fracture, in mica
schist. (From Cohen.)
]

SILLIMANITE, Fibrolite.

ANISOTROPIC. BIAXIAL. ORTHORHOMBIC.
COMPOSITION: Al_{2}SiO_{5}. _c_ = c′. ELONGATION ∥ c′.

▄Usual Appearance in Sections▄: Long, slender, colorless prisms or
needles; often in felt-like aggregates. Crystals often bent. Index of
refraction rather high (_n′_ = 1.664, α = 1.656, γ = 1.677), hence
_relief_ marked. Transverse fractures common, Fig. 46.

▄Crossed Nicols▄: Double refraction rather strong (γ − α = 0.021).
Interference colors upper first or lower second order, red, purple,
blue, etc. _Extinction_ parallel to prisms. Ax. pl. ∥ (100), Bx_{_a_}.
∥ _ć_, 2_E_ = 35° to 55°. Optical character (+).

▄Distinguished from▄:

(_a_) APATITE by higher order interference colors and by elongation ∥
c′ (apatite has elongation ∥ a′).

(_b_) TREMOLITE by always parallel extinction and small size of axial
angle.

(_c_) ANDALUSITE, see under the latter mineral.

REMARKS: Found especially in clay-rich contact rocks, gneisses and
schists, often occurring with iolite (cordierite). Crystals may appear
in bands. It is insoluble in hydrochloric acid. H., 6 to 7. Sp. gr.,
3.24.

TOPAZ.

ANISOTROPIC. BIAXIAL. ORTHORHOMBIC.
COMPOSITION: _ć_ = c.
Al(Al(O.F_{2}))SiO_{4}.

▄Usual Appearance in Sections▄: Colorless crystals of short prismatic
habit, grains or rod-like radiating aggregates. Index of refraction
about the same as that of calcite (_n′_= 1.608 to 1.632, α = 1.607 to
1.629, γ = 1.618 to 1.637), hence _relief_ medium. Cleavage perfect,
parallel to base, but does not show as many cracks. Fluid inclusions
abundant.

▄Crossed Nicols▄: Double refraction weak (γ − α = 0.008 to 0.011),
about the same as that of quartz. Interference colors middle first
order, white, yellow, etc. _Extinction_ parallel to cleavage. In
_convergent light_, Ax. pl. ∥ (010), Bx_{_a_}. ∥ _ć_, axial angle
large (2_E_ = 86° to 126°); interference figure obtained from basal
sections (_i. e._, from sections showing no cleavage); optical
character (+).

▄Alteration▄: May take place to kaolin or muscovite, by loss of F and
taking up of H_{2}O and alkalies.

▄Distinguished from▄:

(_a_) QUARTZ by higher relief, cleavage and biaxial character.

(_b_) SILLIMANITE (when topaz is in radiating aggregates) by lower
refraction and double refraction.

REMARKS: Common in greisen and all granite rocks containing tin ore.
When formed by “fumarole” action (tin veins) the mineral shows
rod-like radiating forms. It is insoluble in hydrochloric acid. H., 8.
Sp. gr., 3.5.

STAUROLITE.

ANISOTROPIC. BIAXIAL. ORTHORHOMBIC.
COMPOSITION: ELONGATION ∥ ć.
Fe(AlO)_{4}(AlOH)(SiO_{4})_{2},
but varying, may contain Mg or
Mn. _ć_ = c.

FIG. 47.—Staurolite, showing twinning at 90° _b_ and 60° _c_, also
granular quartz inclusions. (From Reinisch.)
]

▄Usual Appearance in Sections▄: Short, flat prisms, which may be
twinned at 90° or 60°, Fig. 47, or grains. Color yellowish to
reddish-brown. Index of refraction rather high (_n′_ = 1.741, α =
1.736, γ = 1.746), hence _relief_ marked and surface rough. Cleavage,
both prismatic and pinacoidal, variable. Inclusions of minute quartz
grains and carbonaceous matter found in larger crystals, but not in
microscopic crystals. Pleochroism distinct but not strong, showing red
∥ _c_ (direction of elongation). Pleochroic halos may surround
inclusions.

▄Crossed Nicols▄: Double refraction weak (γ − α = 0.010). Interference
colors middle first order, white to yellow, etc. (about like quartz).
_Extinction_ in general parallel or symmetrical (in cross-sections) to
cleavages or crystal outline. In _convergent light_, Ax. pl. ∥ (100),
Fig. 48. Bx_{_a_}. ∥_ć_, axial angle large (2_E_ > 180°); optical
character (+).

FIG. 48.—Staurolite, cross-section.
]

▄Alteration▄: Rarely takes place.

▄Distinguished from▄: TITANITE, see under the latter mineral.

REMARKS: Found in metamorphic schists, associated with cyanite
(disthene), iolite (cordierite), andalusite, etc. It is one of the
minerals produced by thermal metamorphism, hence found in rocks of
granite contact zones. It does not occur in the eruptive rocks or in
schists rich in amphibole. Staurolite is not acted on by hydrochloric
acid. H., 7 to 7.5. Sp. gr., 3.4 to 3.8.

THE ORTHORHOMBIC PYROXENES.
Enstatite and Hypersthene.

ANISOTROPIC. BIAXIAL. ORTHORHOMBIC.
COMPOSITION: (Mg.Fe)SiO_{3}. _ć_ = c. ELONGATION ∥ c′.

Enstatite contains little, if any, Fe. Hypersthene contains more Fe, its optical characters beginning to show with about 10 per cent.

FIG. 49.—Enstatite, showing columnar or fibrous structure ∥ _ć_ axis.
_Norite_, Harzburg.
]

▄Usual Appearance in Sections▄: Irregularly bounded individuals (E) or rounded prismatic-pyramidal crystals (H). Columnar or fibrous structure ∥ _ć_ often shows in (E), Fig. 49. Prism angle about 92°. Outline of crystal sections very similar to that of monoclinic pyroxenes.

_Twinning._—Not so common as in monoclinic pyroxenes. Parallel growths with monoclinic pyroxene (diallage) occur.[89]

_Color._—Varies with Fe per cent., (E) colorless, (H) brownish.

_Index of refraction._—_n′_ = 1.665 (E) to 1.723 (H) (α = 1.660 to 1.716, γ = 1.670 to 1.729) (about the same as in monoclinic pyroxene), hence _relief_ marked and surface rough.

_Cleavage._—Variable, parallel to prism (angle 92°) common to all pyroxenes. Also cleavage or parting parallel to brachy pinacoid (010) (prominent) and macro pinacoid (100).[89]

_Inclusions._—Parallel oriented, brownish plates and rods, producing “schiller” structure on the principal cleavage faces, Fig. 15. Glass inclusions abundant in (H).

▄Polarized Light▄:

_Pleochroism._—Almost absent in (E), but distinct in (H), increasing with Fe per cent. The change in color may be very marked, from brownish-red to greenish ∥ _ć_.

_▄Crossed Nicols▄_:

_Double Refraction._—Weak, much weaker than in the monoclinic pyroxenes, increasing with Fe per cent. (γ − α = 0.010 (E) to 0.013 (H).)

_Interference Colors._—Higher first order, about the same or a little higher than quartz.

_Extinction._—Parallel to cleavages in longitudinal sections, which are parallel to _a_ or _b_, and bisecting angles of intersecting prismatic cleavages in basal sections.

_▄Convergent Light▄_: Axial plane parallel to brachy pinacoid (010),[89] _i. e._, parallel to best pinacoidal cleavage. Bx_{_a_}. ∥ _ć_ (E), ∥ _a_(H). Axial angles large (2_E_ = 95° to > 180°). Optical character for (_E_)(+), for (_H_)(−). On account of weak double refraction the interference figures are not very marked.

▄Alteration▄: Takes place to bastite, serpentine, etc.

▄Distinguished from▄: The MONOCLINIC PYROXENES and AMPHIBOLES.—See under these species.

REMARKS: Found in the granular rocks of the gabbro-peridotite series,
also in the olivine basalts (E); and in crystals in porphyritic
andesite (H). These minerals are in general not attacked by acids. H.,
5 to 6. Sp. gr., 3.1 to 3.5.

_Bronzite_ is the name give to the variety containing about 5% Fe and
having the characteristic bronzy lustre due to inclusions.

_Bastite_ (an alteration product of the orthorhombic pyroxenes poor in Fe).—Composed of fibers, often traversed by irregular cracks. Color light yellowish or greenish and index of refraction about the same as Canada balsam. Pleochroism faint (only seen in thick sections), the greatest absorption taking place parallel to the fibers. Double refraction weak and extinction parallel to the fibers. Axial angle large and axial plane at right angles to principal cleavage face (010). The position of the axial plane is the surest distinction between bastite and the orthorhombic pyroxenes.

CHRYSOLITE, Olivine.

ANISOTROPIC. BIAXIAL. ORTHORHOMBIC.
COMPOSITION: (Mg.Fe)_{2}SiO_{4}. ELONGATION ∥ a′ or
c′.

▄Usual Appearance in Sections▄: Prismatic crystals or in large angular fragments or grains. Longitudinal sections more or less lath-shaped, with pointed ends, Figs. 50 and 51, cross-sections six or eight-sided. Outlines of crystals often rounded or corroded. Skeleton forms may occur, and sometimes twinning may be observed.

Chrysolite.

FIG. 50. FIG. 51.
Basal section. Macro pinacoid section.
]

_Color._—Nearly colorless, may be reddish (with high Fe per cent.).

_Index of Refraction._—_n′_ = 1.675 (α = 1.661, γ = 1.697), hence _relief_ marked and surface rough.

_Cleavage._—Parallel to brachy pinacoid (010), less distinct parallel to macro pinacoid (100), Fig. 50. Often only made visible by decomposition. An irregular fracturing occurs, which increases with alteration into serpentine.

_Inclusions._—Chromite, opaque earths, apatite and the brown plates so common in hypersthene; also glass and slag (in basaltic rocks) and fluid (in peridotites and olivinfels).

▄Polarized Light▄:

_Pleochroism._—In general none, but noticed in the reddish varieties, when the absorption is a little stronger parallel to _ć_.

▄_Crossed Nicols_▄:

_Double Refraction._—Very strong (γ − α = 0.036).

_Interference Colors._—Rather high in order (second or third), higher than the colors of augite.

_Extinction._—In general parallel to cleavage lines.

_▄Convergent Light▄_: Axial plane parallel to base (001) and always at right angles to cleavage cracks, Fig. 51. Bx_{_a_}. ∥ _a_. Axial angle very large (2_E_ > 180°). Optical character (+).

▄Alteration▄: Into serpentine very common,[90] producing “mesh-” or “lattice-” structure (see under serpentine, p. 114); also into amphibole, etc. In certain basaltic rocks the rims of grains may be changed into gœthite?, and in certain gabbros the crystals may be surrounded by a radial rim of amphibole.

▄Distinguished from▄:

Light colored MONOCLINIC PYROXENES.— By the absence of extinction angles, cleavage (the intersecting prismatic cleavages of augite being of equal distinctness), stronger double refraction and by axial plane being parallel to base, hence always at right angles to best cleavage (in augite axial plane lies in clino pinacoid, bisecting angles of intersecting prismatic cleavages). Also by gelatinization with acids.

REMARKS: Found only in basic rocks, as peridotite, diabase, gabbro,
norite, basalts, etc. Chrysolite (olivine) is a very brittle mineral
and shows under mountain making pressure “cataclastic” structure.
Chromite is a characteristic associated mineral. When not too poor in
Fe, chrysolite becomes permanently red and pleochroic when strongly
heated. Chrysolite is decomposed by hydrochloric and sulphuric acids,
with separation of gelatinous silica. H., 6.5 to 7. Sp. gr., 3.3 to
3.4.

_Hyalosiderite_ (a more ferruginous chrysolite) and _Fayalite_ (Fe_{2}SiO_{4}) may be reddish in sections, and common in the basic porphyritic eruptive rocks.

IOLITE, Cordierite, Dichroite.

ANISOTROPIC. BIAXIAL. ORTHORHOMBIC.
COMPOSITION: _ć_ = a. ELONGATION ∥ a′.
Mg_{3}(Al.Fe)_{6}Si_{8}O_{28}.

▄Usual Appearance in Sections▄: Grains, more rarely crystals of short
prismatic habit, which often form pseudo-hexagonal interpenetration
twins. Crystals may have edges rounded or corroded. Colorless, but may
be bluish. Index of refraction a little lower than quartz (_n′_ =
1.539, α = 1.535, γ = 1.544), hence _relief_ low and surface smooth.
Cleavage very variable, parallel to brachy pinacoid (010), especially
noticeable when decomposition has taken place. Inclusions of
sillimanite, zircon, rutile, etc., may be seen. Pleochroism usually
not observed, but noticed in blue sections (yellowish white ∥ _ć_ to
blue). Pleochroic halos (yellow) surrounding inclusions common, see p.
59.

▄Crossed Nicols▄: Double refraction weak (γ − α =0.009), like quartz.
Interference colors middle first order, white to yellow. _Extinction_
in general parallel to cleavage cracks. In _convergent light_, Ax. pl.
∥ (100), Bx_{_a_}. ∥ _ć_; axial angle large (hyperbolas only seen
without ellipses) (2_E_ = 64° to 150°); optical character (−).

▄Alteration▄: Takes place readily, forming greenish mica-like
decomposition products, the decomposition commencing along the
crevices or about the inclusions.

▄Distinguished from▄: QUARTZ by observation in convergent light
(quartz is uniaxial), decomposition and pleochroism or pleochroic
halos. The section can also be treated with hydrofluosilicic acid,
when the evaporated solution yields characteristic prismatic crystals
of magnesium fluosilicate.

REMARKS: Found in gneiss, hornstone, granite, granulite, etc., and in
some volcanic rocks. It is often associated with garnet, biotite,
sillimanite, etc. In a thick section heating to redness makes the
pleochroism more distinct. Iolite is only slightly acted on by acids.
H., 7 to 7.5. Sp. gr., 2.6. It is hard to make a mechanical separation
from quartz, on account of similarity in sp. gr.

NATROLITE.

ANISOTROPIC. BIAXIAL. ORTHORHOMBIC.
COMPOSITION: _ć_ = c. ELONGATION ∥ c′.
Na_{2}Al_{2}Si_{2}O_{10} +
2H_{2}O.

▄Usual Appearance in Sections▄: Aggregates of colorless, fibrous
crystals, which may have sphærulitic structure, showing a dark cross
between crossed nicols. Index of refraction lower than balsam (_n′_ =
1.483, α = 1.478, γ = 1.490), hence (in large crystals) the surface
would appear rather rough.

▄Crossed Nicols▄: Double refraction weak (γ − α = 0.012). Interference
colors the middle first order (yellow, etc.), a little higher than
those of quartz. _Extinction_ parallel to fibres. Optical character
(+).

REMARKS: Never a primary mineral in rocks, but found in igneous rocks
filling amygdaloidal cavities, and also as a very common alteration
product of sodalite, noselite, nephelite and acid plagioclases. It
gelatinizes easily with hydrochloric acid. H., 5 to 5.5. Sp. gr., 2.2.

OTHER ZEOLITES.

COMPOSITION: Hydrous silicates; Al, Ca and Na being the chief bases.

▄Usual Appearance in Sections▄: The form depends on the individual
mineral species, but the majority appear in elongated crystals or
fibers. They are all colorless and most of them have a small index of
refraction, hence no _relief_ (prehnite has distinct relief).

▄Crossed Nicols▄: The double refraction is generally very weak
(between that of nephelite and quartz), giving very low order
interference colors (prehnite and thomsonite have strong double
refraction).

REMARKS: The zeolites are always secondary minerals in rocks. They
gelatinize with hydrochloric acid.

GYPSUM.

ANISOTROPIC. BIAXIAL. MONOCLINIC.
COMPOSITION: CaSO_{4} + 2H_{2}O.

▄Usual Appearance in Sections▄: Colorless grains or fibers. May be
colored, however, by inclusions of carbonaceous matter, iron oxides,
etc. Index of refraction about the same as orthoclase (_n′_ = 1.525, α
= 1.521, γ = 1.531), hence no _relief_ and surface smooth. Twinning
lamellæ abundant. Cleavage parallel to (010) gives abundant cracks,
other cleavages may also be noticed.

▄Crossed Nicols▄: Double refraction weak (γ − α = 0.010), the same as
quartz. Interference colors middle first order, white to yellow.
_Extinction_ parallel to most perfect cleavage cracks in sections
parallel to _b_ axis; large extinction angles noticed with reference
to less perfect cleavages. In _convergent light_, Ax. pl. ∥ (010),
_i.e._, ∥ to most distinct cleavages; Bx_{_a_}. (c) Λ = 54° front;
2_E_ = 104°; optical character (+). As the characters of gypsum are
not always very marked it may be necessary to employ micro-chemical
tests.

REMARKS: Forms a rock by itself, often associated with rock salt. It
also occurs as an alteration product of anhydrite. Gypsum is soluble
in hydrochloric acid. H., 1.5 to 2. Sp. gr., 2.2 to 2.4.

MONOCLINIC PYROXENES, Augite, etc.

Including the monoclinic minerals of the Pyroxene Group, which show
distinctly the characteristic cleavage parallel to an almost
right-angled prism.

ANISOTROPIC. BIAXIAL. MONOCLINIC.
ELONGATION ∥
c′.[91]

COMPOSITION: RSiO_{3}, R = Ca, Mg, Mn, Fe, Al chiefly, with the Ca predominating over the Mg.

▄Usual Appearance in Sections▄: Both in crystals and more or less irregular grains, Figs. 4 and 12, the habit varying with the chemical composition as follows:

_Diopside_ (Ca, Mg varieties), long columnar crystals and grains.

_Augite_ (ditto, but containing also Al and Fe), short prismatic crystals and grains.

_Diallage_, granular or lamellar (|| (100)), may show fibrous structure ∥ _ć_.

Prism angle = 87° 06′ (important in cross-sections). Sections of crystals nearly at right angles to the vertical axis _ć_ are octagonal or square with truncated corners, Figs. 4 and 53, while those parallel to the _ć_ axis are lath-shaped. Pyroxene also occurs in skeleton crystals and acicular microlites in eruptive rocks.

FIG. 52.—Augite, section parallel to _ć_ axis showing prismatic
cleavage, in leucite-basal. (From Cohen.)
]

Zonal structure (especially in augites) may be marked by differences in color or extinction, and in some basalts the crystals have the “hour-glass” structure.

_Twinning._—Common, usually the twinning plane being the ortho pinacoid (100). Twin lamellæ may be noticed. Intergrowths occur with orthorhombic pyroxene and amphibole.

_Color._—From almost colorless through green (diopsides, Na pyroxenes, etc.) to brown (augites); the red to brownish-red color of certain augites has been considered due to manganese. Yellow color very rare.

_Index of Refraction._—_n′_ = 1.68 to 1.72 (α = 1.671 to 1.706, γ = 1.700 to 1.728), hence _relief_ high and surface rough.