Rocks and rock minerals; a manual of the elements of petrology without the use of the microscope, for the geologist, engineer, miner, architect, etc., and for instruction in colleges and schools online

Montana and India, in peridotites in North and South
Carolina and Alabama, and in other igneous rocks in the
Urals and in California. An unrecorded occurrence is in
syenite in Orange County, New York State. Many more
such will doubtless be discovered. The variety sapphire
has been found in basaltic rocks in Montana, the Rhine
district and elsewhere. Corundum also occurs in the
contact zone of igneous rocks. In these cases it is usually



88 ROCKS AND ROCK MINERALS

in thin tabular crystals. It also occurs in metamorphic
rocks, sometimes in thick beds of the variety called
emery. Probably in many of these occurrences it ante-
dates the period of metamorphism and is of igneous
origin.

Determination. The crystal form, when present, and
color indicate the presence of this mineral which is readily
confirmed by a test of its hardness since it cannot be
scratched by another of the rock minerals. These tests
may be confirmed by the other described properties.

THE IRON ORES.

The term ore is commonly applied to the oxides, sul-
phides and carbonates of the heavy metals as the sources
from which they are obtained in commercial quantities.
Of these minerals the only ones, which by reason of their
wide distribution and common occurrence as components
of rocks, may be considered of general importance from
the petrological standpoint are the oxides and sulphides
of iron. Even these play only a subordinate role in
rock-making and are considered as accessory minerals,
except in certain cases where they have been concen-
trated by geologic processes into considerable masses.
They are considered accessory because, in one form or
another, they are found scattered in small quantities
through most rocks and in each of the three great classes
of rocks and do not therefore have the same importance
and value in classification that those minerals, such as
feldspars and pyroxenes have, which occur in large and
varying amounts. They are mentioned here because they
are the most common of accessory rock-minerals and are
of importance in other ways as well. They include
magnetite, ilmenite, hematite, limonite and pi/rite. There
are other oxides and sulphides of iron but they are rela-
tively of small petrographic importance.



KOCK-MAKING MINERALS



89



MAGNETITE.



Form. Magnetite crystallizes in the isometric system,
most commonly in octohedrons, Fig. 44, sometimes in
dodecahedrons, Fig. 45, sometimes in a combination of






Fig. 44



Fig. 45



Fig. 46



both, Fig. 46. It is sometimes seen in distinct crystals
in rocks but usually is in small grains whose form cannot
be made out and is sometimes in larger irregular masses.

General Properties. No distinct cleavage but some-
times a parting parallel to the octahedral faces resembling
cleavage. Fracture, uneven. Brittle. Color, dark gray
to iron-black; opaque; luster, metallic, fine to dull.
Resembles often bits of iron or steel in the rocks. Streak,
black. Magnetic. Hardness, 5.5-6.5. Specific gravity,
5.2. The chemical composition is Fe 3 O 4 = FeO . Fe 2 O 3 ,
or FeO = 31.0 per cent, Fe 2 3 = 69.0. Difficultly fusi-
ble before the blowpipe and in the oxydizing flame be-
comes non-magnetic. Slowly soluble in hydrochloric acid.

Occurrence. Magnetite is one of the most widely
distributed of all minerals. It is found in f all kinds of
igneous rocks, usually in small grains, but sometimes
segregated into considerable masses. It occurs also in
rocks produced by contact metamorphism and in the
crystalline schists, sometimes in large bodies. It is
uncommon in the unmetamorphosed sedimentary rocks.
It is one of the most important ores of iron.

Determination. The appearance of magnetite in small
dark metallic-looking particles is usually sufficient to dis-
tinguish it in the rocks, and this may be confirmed by a



90 ROCKS AND ROCK MINERALS

test of its hardness, streak and magnetism, together with
the other properties described above. It is not liable to
be confused with any other mineral except ilmenite.

SPINELS.

Magnetite may be regarded as the type of a group of minerals
known as the spinels. They have the general chemical composition
RO . R 2 O 3 and crystallize in isometric octahedrons as illustrated in
magnetite. In them the RO is either MgO, FeO, MnO or ZnO or
mixtures of them; R 2 O 3 is Fe 2 O 3 , A1 2 O 3 or Cr 2 O 3 or mixtures of
them. True spinel is MgAl 2 O 4 (MgO . A1 2 O 3 ) and when trans-
parent and of good color is sometimes cut as a gem. Hercynite is
iron spinel, FeAl 2 O 4 , and chromite is FeCr 2 O 4 , more or less mixed
with other spinel molecules. Depending on their composition the
spinels have various colors, black, green, red and gray. They are
extremely hard, 7-8, without good cleavage and are of high luster
to pitchy. Some of the spinels are constituents of igneous rocks,
especially of those low in silica and rich in iron and magnesia like
peridotite and dunite; others are found in metamorphic rocks,
especially those produced by contact metamorphism. In all cases
they form only accessory and not important components of the
rocks and, except in some contact rocks, are rarely found in crystals
sufficiently large to make them of megascopic importance.

ILMENITE.

General Properties. Ilmenite crystallizes in the hex-
agonal system like hematite, but it is so rarely seen in
good megascopic crystals in rocks that its crystal form
is not a matter of importance. It usually occurs in
embedded grains and masses, sometimes in plates of
irregular to hexagonal outline. No cleavage; fracture,
conchoidal; brittle. Color, iron-black, sometimes with
faint reddish to brownish tinge; luster, submetallic; streak,
black to brownish-red. Opaque. Hardness, 5-6. Spe-
cific gravity, 4.5-5. Composition, FeTiO 3 = FeO . TiO 2 .
FeO = 47.3, Ti0 2 = 52.7. Is not generally pure, but
more or less mixed with hematite, Fe 2 3 , with which it
is isomorphous. Before the blowpipe very difficultly
fusible; in the reducing flame becomes magnetic. After
fusion with carbonate of soda can be dissolved in hydro-



ROCK-MAKING MINERALS 91

chloric acid and the solution boiled with tin becomes
violet showing titanium. Fresh mineral difficultly soluble
in acids; decomposed by fusion with bisulphate of potash.
The solutions give reaction for iron with potassium
ferricyanide. The test for titanium is the safest method
to determine the mineral.

Occurrence. Ilmenite or titanic iron ore, as it is often
called, is a widely spread mineral occurring as a common
accessory mineral in igneous rocks in the same manner
as magnetite which it often accompanies. In the same
way it is found in gneisses and schists. Unless the
embedded grains are of such size that they can be safely
tested it cannot usually be discriminated from that
mineral by simple inspection. The most important
megascopic occurrences are in the coarser grained gabbros
and anorthosites where the mineral is very common and is
indeed not infrequently segregated in places into such
large beds and masses that it would be a useful ore of iron
if some method of profitably smelting it could be dis-
covered.

HEMATITE.

Form. Hematite crystallizes in the rhombohedral
division of the hexagonal system but is so rarely in distinct
well-formed crystals of observable size as a rock con-
stituent that this is not a matter of practical importance.

It occurs as a rock-mineral in three different forms: as
specular iron ore, micaceous hematite, and as common red
hematite.

In the first case it forms masses and plates, the latter
sometimes hexagonal in outline. Its color is black to
steel-gray with sometimes a faint reddish tone. It is
opaque, has a metallic luster which is sometimes very fine
or splendent so that it resembles polished steel or iron, at
other times it is rather dull but metallic-looking. Fracture,
subconchoidal; no cleavage.

As micaceous hematite it is in thin flakes which some-
what resemble mica; often they are so thin as to be trans-



92 ROCKS AND ROCK MINERALS

lucent and then have a deep red color. The luster is
submetallic to metallic, sometimes splendent like the
specular form. The thin leaves are usually of ragged
outlines but sometimes hexagonal.

Common red hematite does not appear crystallized.
The mineral is massive, sometimes columnar or granular,
often in stalactitic or mamillary forms and sometimes
earthy. It is dull, without metallic luster, opaque and
of a dark red color.

General Properties. The streak of hematite is of a red
color, from bright Indian red to brownish red and fur-
nishes the most convenient method of distinguishing it
from magnetite and limonite. Before the blowpipe it is
very difficultly fusible except in very fine splinters. After
heating in the reducing flame magnetic. Dissolves slowly
in hydrochloric acid and the solution gives reactions for
iron.

The composition is Fe2Os, ferric oxide. The hardness
varies from 5.5-6.5; specific gravity of the specular
variety is 5.2.

Occurrence. Hematite is one of the most widely diffused
of minerals. The specular variety is a common accessory
component of feldspathic igneous rocks, such as granite.
It is also found in the crystalline schists, often in thick
beds and masses.

Micaceous hematite occurs in the crystalline schists
in megascopic form, as in itabirite, and also in minute
microscopic scales it is the red coloring matter found in
igneous and metamorphic rocks. The red color of many
potash feldspars is due to it and so is that of many
slates.

Common red hematite is found in sedimentary and
metamorphic rocks in beds and masses, often of great
size and forming one of the most valuable ores of iron.
It is the interstitial cement of many stratified rocks, such
as red sandstones, and as a red pigment in the form of
powder it is everywhere distributed in all classes of rocks



ROCK-MAKING MINERALS 93

and in soils, though possibly in some cases it may be
replaced by turgite (hydrohematite), 2Fe 2 03.H 2 O, which
often closely resembles it. Earthy red hematite, usually
more or less mixed with clay, is called red ocher.

LIMONITE.

Form. Limonite does not crystallize, but occurs in
earthy formless masses in the rocks, and when found in
considerable deposits very frequently exhibits compact
stalactitic or mammillary shapes which have a fibrous or
radiating structure and are sometimes concretionary;
sometimes in earthy beds or deposits.

General Properties. No cleavage. Luster of compact
varieties often silky to sub-metallic, but generally dull
and earthy. Color, various shades of brown from very
dark to brownish yellow. The surface of the compact
stalactitic or mammillary forms often has a varnish-like
skin. Opaque. Streak, yellow-brown. The hardness of
the compact mineral varies from 5-5.5 and the specific
gravity from 3.6-4.0. The composition is 2 Fe 2 3 .3 H 2 O
or Fe 2 (OH) 6 .Fe 2 3 , partly dehydrated ferric hydroxide.
Fe = 59.8; = 25.7; H 2 O = 14.5 = 100. Difficultly
fusible before the blowpipe; becomes magnetic in the
reducing flame. Heated in closed glass tube gives off
water. Slowly soluble in hydrochloric acid, the solution
giving reactions for iron. The yellow streak is the most
convenient means of distinction from hematite.

Occurrence. Limonite occurs in several different ways.
In all cases it is strictly a secondary substance formed at
the expense of previously existing minerals, by the various
agencies of weathering and alteration. In igneous and
metamorphic rocks it is frequently seen as small, earthy,
yellowish to brownish masses which represent the decay
of some previous iron-bearing mineral, such as pyrite,
hornblende, etc. Accumulated in beds, as explained under
sedimentary rocks, it frequently has the compact form
with stalactitic and mamillary or concretionary structure.



ROCKS AND ROCK MINERALS



As bog iron ore it is loose, porous and earthy. Mixed
with more or less clay it forms yellow ocher and is the
yellow pigment of many soils and sedimentary rocks.

PYRITB.

Form. Pyrite almost invariably occurs in crystals in
the rocks, very seldom in grains and masses. It crystal-
lizes in the isometric system. It is frequently seen in
cubes or in the twelve-sided form seen in Fig. 47 and called
the pyritohedron because this mineral so commonly shows
it. Combinations of the two are also very common as





Fig. 47



Fig. 48



Fig. 49



Fig. 50



shown in Fig. 48. Very often the cubic faces are striated
by fine lines as seen in Fig. 50 produced by oscillating or
repeating combinations of the pyritohedron on the cube
faces. The octahedron is less frequent and is apt to be
modified by the pyritohedron combining with it as in
Fig. 49. Other more complex forms also occur.

General Properties. No good cleavage; fracture, con-
choidal to uneven. Color, brass yellow; luster, metallic,
splendent, duller when tarnished. Opaque. Streak,
greenish to brownish black. Hardness, 6-6.5; specific
gravity, 5.0. Composition, FeS2j iron = 46.6, sulphur =
53.4 = 100. Easily fusible before the blowpipe, burning
and giving off sulphur dioxide gas, and leaving a magnetic
globule. In the closed glass tube on heating gives a sub-
limate of sulphur and leaves a magnetic residue. Insol-
uble in hydrochloric but decomposes in boiling nitric
acid with separation of sulphur.

The color and crystallization are usually sufficient to



ROCK-MAKING MINERALS 95

at once identify pyrite and distinguish from other rock
minerals. From pyrrhotite, FeuSi 2 , and chalcopyrite,
FeCuS 2 , other sulphides of iron which occasionally may
be seen in rocks, the test of hardness discriminates it
from chalcopyrite (3.5) which can be readily scratched
with the knife and gives reactions for copper; pyrrhotite
has a bronze color, is also scratched by the knife and gives
little or no sulphur in the closed tube.

Occurrence. Pyrite is a mineral which has many
different modes of origin and in consequence is found in
all kinds of rocks as a scattered accessory component,
usually in small distinct crystals, less commonly aggre-
gated. The largest masses are found in ore deposits,
chiefly formed in contact zones of igneous rocks through
the action of mineralizing solutions. In igneous rocks it
appears as a primary product of crystallization from the
molten magma. In sedimentary rocks it is frequently
found replacing fossils, and its occurrence must be due to
reactions between the sulphur of albuminous materials
of organic life and the iron in the rocks. It is common in

coal seams.

APATITE.

Apatite crystallizes in hexagonal prisms either rounded at the
ends or capped by a six-sided pyramid. It is scratched by the
knife, has a vitreous luster and is white to green or brown in color.
No good cleavage. Brittle. Transparent in small crystals to
opaque in large. Very difficultly fusible. Dissolves in nitric acid
and ammonium molybdate solution added to a few drops of the
nitric acid solution gives a bright yellow precipitate showing the
presence of phosphorus. Composition (CaF)Ca 4 (PO 4 ) 3 ; phosphate
of lime with fluorine ; the fluorine is often replaced wholly or in part
by chlorine. Apatite is found in large, sometimes huge, crystals in
pegmatite dikes and in metamorphosed limestones in the crystalline
schists: these may be said to be the chief megascopic modes of
occurrence. In these, however, it cannot be said that its function
as a rock-mineral is of any wide or general importance. In addition
to this it occurs in minute microscopic crystals, which can seldom
be detected with the eye or lens, in all kinds of igneous rocks and in
many metamorphic ones. Microscopical study of the thin sections
of such rocks has shown that in this form the mineral has a nearly



96 ROCKS AND ROCK MINERALS

universal distribution as a constant accessory component. Although
the relative proportion of the mineral is small, rarely rising above
two or three per cent of the rock, its presence is a matter of great
importance, since by it the phosphorus, so necessary to vegetable
and animal life (in bones, etc.), is furnished to the soil which is
formed when the rocks decay and break down under the action of
the various agents of weathering.

SEC. 2. Hydrous Silicates.

The minerals of this group are of purely secondary
origin; they are formed from previously existent ones
by the agencies of weathering/ water containing carbon
dioxide or vegetable acids and by heated water or its
vapors circulating in already solid, existent rocks. Thus
they do not play any important part in fresh unchanged
igneous rocks; only as these alter do they become of
importance in them; their true home is in the meta-
morphic and sedimentary ones, which at times are made
up wholly of these minerals.

The important ones to be considered in this section are
kaolin, chlorite, serpentine, talc and zeolites. Some micas
would also naturally be considered here and among
secondary minerals also limonite, but, for reasons pre-
viously stated, these have been treated in the foregoing
section.

KAOLIN CLAY.

Under the heading of clay are included certain hydrous
silicates of alumina having well-known physical properties
by which they are distinguished. By far the most com-
mon and important of these is kaolin which may be taken
as a type of the group, and the only one which need be
considered here in detail.

General Properties. Kaolin crystallizes in the mono-
clinic system forming thin plates or scales often with
hexagonal outlines which are flexible and recall mica but
are inelastic; these are generally so minute and aggre-
gated together that the crystal form is not a matter of



ROCK-MAKING MINERALS 97

importance in megascopic determination of the substance.
Usually in masses, either compact, friable or mealy.
Color white, often tinted yellow, brown or gray. Neither
the hardness (2-2.5) nor the specific gravity (2.6) can be
used for practical tests. On rubbing between the fingers
kaolin has a smooth, unctuous, greasy feel, which helps
to distinguish it from fine aggregates of some other
minerals occurring in nature: thus its presence in soils
can usually be told by rubbing out the fine, gritty
particles of quartz, feldspar, etc., and observing if there
is a smooth, unctuous residue of clay.

It is infusible before the blowpipe, but moistened with
cobalt nitrate and ignited turns blue showing presence of
alumina. Heated in the closed glass tube it yields water.
Insoluble in hydrochloric acid. In the phosphorus bead
before the blowpipe, undissolved silica is left; this test
helps to distinguish it from bauxite, a hydrated oxide of
aluminum (A1 2 O(OH) 4 ) which very much resembles it
and sometimes occurs in considerable deposits. Bauxite
dissolves in the phosphorus bead completely. The
chemical composition of kaolin is H 4 Al 2 Si 2 9 ' a combi-
nation of A1 2 3 . 2 Si0 2 . 2 H 2 0.

Occurrence. Kaolin is always a secondary mineral
formed by the alteration or weathering of previously
existent aluminous silicates and chiefly feldspar. The
reaction by which it is formed from alkalic feldspar is one
of the most important in nature, for by it soil is chiefly
made and the alkali necessary for plant food liberated
and converted into soluble form. It is expressed as fol-
lows:

Orthoclase + Water + Garb. diox. = Kaolin + Potas. Garb. + Quartz.
2 KAlSi 3 O 8 + 2 H 2 O + CO 2 = H 4 Al 2 Si 2 O9 + KzCOs + 4 SiO 2 .



This process and reaction have been already described
under feldspars. The feldspathoids also yield kaolin and
the process could be expressed as follows :

Nephelite + Water + Garb. diox. = Kaolin + Sodium Garb.
2 NaAlSiOi + 2 HsjO + CO 2 = I^Al^Oo +



98 ROCKS AND ROCK MINERALS

They are more apt however to first change into musco-
vite or zeolites and these ultimately to clay.

From what has been said it is clear that feldspathic
rocks furnish kaolin, and every stage of the change may be
observed in nature as described more completely in the
chapter dealing with the origin of sedimentary rocks and
soils. Thus kaolin occurs intimately mixed with the
feldspar substance of such rocks as are undergoing this
change; it is found occasionally in quite extensive deposits
where such rocks have been completely altered in place,
and the products of decay yet remain where they have
been formed, and lastly it occurs in extensive beds in the
sedimentary formations. Since the particles of kaolin
are very minute, light and flat they remain much longer
in suspension than the other products of land waste, and
thus in erosive and sedimentary processes there is a con-
stant tendency to separate them from the other particles.
We find beds of clay with every degree of admixture with
sand, etc., that pass into sandstones and other rocks but
not infrequently they are of a high degree of purity.

CHLORITES.

The chlorites are an ill-defined group of hydrous silicates
so named on account of their green color (Greek ^Xwpo's,
green), which are always secondary and formed at the
expense of previously existing silicate minerals which
contain aluminum, iron and magnesia. Outwardly they
resemble the micas, but unlike them their folia are soft
and inelastic. They are hydrous silicates of aluminium
with ferrous iron and magnesium. They have certain
common properties by which they may generally be easily
recognized as a group, but the identification of the different
members is usually a difficult matter and for ordinary
purposes of megascopic petrography of little importance.
In the description which follows then it is these general
group properties which are given, though these are based



ROCK-MAKING MINERALS 99

largely on the species clinochlore which is perhaps the
most common and best known of the group.

Form. The chlorites are really monoclinic in crystal-
lization, but, like the micas, when crystal form can be
observed they are generally in six-sided plates and tablets.
More commonly they occur in irregular leaves and scales
which are aggregated together into fine granular or coarse
leafy massive forms or arranged into fan-like or rosette-
like groups. The scales are sometimes flat; often bent or
curled.

General Properties. Chlorite, like mica, has a highly
perfect cleavage in one direction parallel to the flat base
of the plates. The cleavage leaves are flexible and tough
but unlike mica they are inelastic. Luster of cleavage
face rather pearly. Color green, variable, usually a
rather dark green. Usually translucent. Hardness, 2-2.5
soft, just scratched by the finger nail. Specific Gravity
about 2.7. Streak pale green to white. The chemical
composition of the chlorites is not definitely understood
and seems to be complex: it may be illustrated by the
formula assigned to clinochlore, H 8 (MgFe) 5 Al 2 Si30i8,
which may be written 4 H 2 O . 5 (MgFe)O . A1 2 O 3 . 3 Si0 2 :
Ferrous iron and magnesia are isomorphous. In kam-
mererite, a rare violet-red variety, part of the alumina is
replaced by chromic oxide, Cr 2 O 3 . Before the blowpipe
chlorites are infusible or very difficultly so; with the
borax bead they react for iron. Heated in the closed
glass tube they yield water. They are insoluble or
difficultly so in hydrochloric acid but are decomposed in
sulphuric acid. These reactions are those of the common
kinds.

Occurrence. The chlorites are a widely spread group of
minerals, and occur wherever previously existent rocks
containing silicates composed of alumina, iron and mag-
nesia, such as dark micas, amphibole, pyroxene and
garnet, are being altered by geologic processes. To
chlorite many igneous rocks owe their green color, the



100 ROCKS AND ROCK MINERALS

original ferro-magnesian silicates having been broken
down by decay and changed more or less completely into
this substance. They are apt to lose their original bright,
clean appearance and hard clear-cut fracture and become
dull green and more or less soft and even earthy. This
change can also be often observed in the case of single
embedded crystals of the above-mentioned minerals,
which become soft, dull green masses.

Chlorite is also of common occurrence in the schistose
rocks; in chlorite-schist it is the prominent component
accompanied by other minerals; other schists often owe
their green color to its presence, as in green slates for
example. Thus in finely disseminated particles it is a
common coloring matter.

SERPENTINE.

General Properties. Serpentine does not crystallize,
and therefore has no crystal form of its own, but it is
sometimes found in the crystal form of other minerals
which have been altered to this substance. It is usually
massive, sometimes finely granular or even slaty; some-