Louis V. (Louis Valentine) Pirsson.

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

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Online LibraryLouis V. (Louis Valentine) PirssonRocks 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 text (page 9 of 35)
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times fibrous, the fibers fine, flexible and easily separable,
like asbestus. Massive varieties have a conchoidal or
splintery fracture. Has a smooth, greasy feel. The
color of massive varieties is green, bright yellowish green,
olive green, to blackish green, or nearly black; the fibrous
varieties are apt to be brownish, yellowish brown, pale
brown or nearly white. Luster of the massive varieties
greasy, wax-like, glimmering and usually feeble to dull; of
fibrous varieties pearly to opalescent. Translucent to
opaque. Hardness, 2.5-3.0; apparent greater hardness
is caused by presence of remains of the original mineral
or by infiltrated and deposited silica. Specific gravity
somewhat variable, fibrous 2.2-2.4, massive 2.5-2.7.
Composition, H 4 Mg 3 Si 2 O 9 = 2 H 2 O . 3 MgO . 2 SiO 2 . A
small part of the MgO is usually replaced by ferrous oxide,
FeO. Before the blowpipe difficultly fusible, fine fibers


fuse more readily. In the closed glass tube yields water
on ignition. The finely powdered or divided material
decomposes in boiling hydrochloric acid with separation
of silica but does not gelatinize. The solution may be
tested for iron and magnesia. Easily told from epidote
and other common green silicates which may resemble it
by its greasy feel and softness.

Occurrence. Serpentine is a secondary mineral result-
ing from the alteration of previously existing silicates
containing magnesium. Thus pyroxene, amphibole and
especially olivine may be altered to this substance. In
the case of olivine the process can be illustrated by the
following equation :

Olivine + Water + Carb. diox. = Serpentine + Magnesite
2 Mg 2 Si0 4 + 2 H 2 O + C0 2 = H 4 Mg 3 Si 2 9 + MgCO 3

This would explain the frequent association of the mineral
magnesite, MgCOs, with serpentine; or it might be taken
into solution by the carbonated water and removed.

A still simpler method would be by the action of heated
waters containing some soluble silica.

3 Mg 2 Si0 4 + 4 H 2 O + SiO 2 = 2 I^MggSiaOg.

Therefore as a product of alteration of such minerals,
especially by the action of heated waters, serpentine is a
common and widely diffused mineral and is found both
in igneous and metamorphic rocks. It may occur dis-
seminated in small scattered masses in the rocks or form
large independent bodies of itself, as described further
under the chapters dealing with the rocks. Besides the
common massive form, many sub-varieties of serpentine
are known; the most important of these is the finely
fibrous one, often taken for asbestus, which is known as
chrysotile. It usually occurs in seams in the massive
variety. Bright green massive material is known as
precious serpentine and is cut for ornamental purposes.



General Properties. The exact crystal form of talc is
doubtful, but this is not a matter of importance since it so
rarely occurs in distinct crystals. It is usually seen in
compact or strongly foliated masses, sometimes in scaly
or platy aggregates which may be grouped into globular
or rosette-like forms. Like mica it has a perfect cleavage
in one direction, but the laminae though flexible are not
elastic; it is sectile. It has a soft greasy feel. The
cleavage face has a mother of pearl luster. The color is
white, often inclining to green; apple-green; sometimes
gray to dark gray. Usually translucent. Hardness
= 1-1.5, easily scratched with the finger nail. Specific
gravity, 2.7-2.8. Streak, light, usually easily seen on
dark cloth. Composition, H 2 Mg 3 (SiO 3 )4, acid metasili-
cate of magnesium. Before the blowpipe it whitens,
exfoliates and fuses with difficulty on the edges. Only
yields water in the closed glass tube on intense ignition.
Scarcely acted on by hydrochloric acid. It is easily
recognized by the properties mentioned above.

.Occurrence. Talc is a secondary mineral produced by
the action of circulating fluids on magnesium silicates,
especially those free from alumina, such as olivine, hypers-
thene and some pyroxenes and amphiboles. The process
could be illustrated by the following equation.

Enstatite + Water + Garb. diox. = Talc +Magnesite

4 MgSiO 3 + H 2 O + CO 2 = HsMg., (Si0 3 ) 4 + MgCO 3

Thus talc occurs at times in the igneous rocks as an
alteration product of such silicates, especially in the
peridotite and pyroxenite groups. The place, however,
where it plays an important function is in the meta-
morphic rocks, where alone it may form independent
masses, as in steatite or soapstone, or be an important
component of several varieties of schistose rocks as in
talcose schists.



The zeolites are a group of hydrous silicates, composed
like the feldspars of aluminum with alkali and alkali-
earth metals. They are indeed for the most part second-
ary minerals which have been formed at the expense of
feldspars and feldspathoids by the action of heated cir-
culating waters and steam and are thus chiefly found in
igneous and especially volcanic rocks. They do not
form a group so closely related in crystallization and
other properties as the feldspars, but still, in many ways,
they have certain common properties by which they may
be distinguished. These will be first described, and then,
out of the many species, the individual characters of a
few of the most important will be treated.

Group Properties. The zeolites are nearly always well crystallized,
the crystals presenting the forms characteristic of the different
species. They have a vitreous luster, are usually colorless or white,
sometimes tinted yellow or red, like feldspar. They are usually of
inferior hardness and can be scratched by the knife. Their specific
gravity is low, 2.1-2.4. They fuse very readily before the blowpipe,
most of them with intumescence (whence the name, fe<V, Greek,
to boil), but some quietly, to white glasses or enamels. They dis-
solve in hydrochloric acid, sometimes gelatinizing and sometimes
with separation of slimy silica. Some of the more common varieties
are, analcite, natrolite, stilbite and heulandite.

Analcite. This zeolite crystallizes in isometric trapezohedrons
like garnet, which easily enables one to recognize it. Generally
colorless to white. Before the blowpipe first becomes opaque, then
fuses quietly to a clear glass, coloring the flame yellow. Dissolves
in hydrochloric acid with separation of silica but does not gelatinize.
Its composition is NaAl(SiO 3 ) 2 + H 2 O.

Natrolite. Crystallizes in orthorhombic prisms which are
generally long, slender and even needle-like and arranged in diver-
gent bunches or compacted into fibrous, often radiating masses.
Before the blowpipe fuses easily and quietly to a clear glass; fuses in
a candle flame. Dissolves in acid with gelatinization. Composition,
Na 2 Al(AlO) (SiO 3 ) 3 + 2 H 2 O.

Stilbite. Crystallizes in complex monoclinic crystals, which are
usually so compounded together that the aggregate has the form of
a sheaf. There is a perfect cleavage in one direction and this appears


on the side of the sheaf with pearly luster. Sometimes in divergent,
sometimes in globular groups. White or red in color. Before the
blowpipe swells, intumesces and fuses to a white enamel. Dissolves
in acid without gelatinization. Composition,

H 4 (CaNa 2 )Al 2 (SiO 3 ) 6 + 4 H 2 O.

Heulandite. Crystallizes in flattened monoclinic crystals which
aggregate into compound individuals, the crystals being grown
side by side with the flattened surfaces together. There is a perfect
cleavage parallel to this flattened side which has a pearly luster.
The cleavage plates are often curved and have a lozenge-shaped
outline. Blowpipe and chemical characters like stilbite. Com-
position, H 4 CaAl 2 (SiO 3 ) 6 +. 3 H 2 O.

Occurrence. As stated above, the zeolites are second-
ary minerals chiefly found in igneous rocks. They are
found in these especially when they have been subjected
to the action of circulating waters and steam which have
attacked the feldspars and feldspathoids. Thus, for
example, a mixture of albite and nephelite with water
would yield analcite, as follows :

Albite + Nephelite + Water = Analcite
NaAlSi 3 O 8 + NaAlSiO 4 + 2 H 2 O = 2 NaAl(SiO 3 ) 2 . H 2 O

Thus where feldspathic rocks have been somewhat altered
they are very apt to contain zeolites in small amounts
scattered through them; in some rare cases it has been
found that a considerable part of the rock mass is com-
posed of them, especially of analcite. Ordinarily the
presence of these minerals is not to be detected megascopi-
cally, though it may be discovered by heating some of the
powdered rock in a closed glass tube, when the easy evolu-
tion of abundant water would indicate their presence.

Their especial home, from the megascopic point of view,
is in the lavas, particularly basaltic ones. Here they are
found coating and lining cavities and the sides of jointing
planes, and composing the materials of the amygdaloids
in lavas, as described under amygdaloidal structure and
under basalt. They may be associated with crystals of


quartz and of calcite in such occurrences, and, in addition
to the common kinds mentioned above, many others may
occur whose description must be sought for in the larger
manuals on minerals.


The carbonates are salts of carbonic acid, H 2 C03, and
are secondary minerals, in that their metals have been
derived from previously existent minerals acted upon by
water and carbon dioxide either from the supply already
in the atmosphere or coming from interior sources deep
within the earth. The carbonates thus formed have been
either deposited directly where we now find them, or, being
soluble in water containing carbon dioxide, they have been
carried in solution into lakes and into the sea and rede-
posited by the agencies of organic life in the form of
chalk, limestone, etc. The shifting about of carbonates
on the earth's surface, owing to their solubility in water
containing carbon dioxide, is a geologic process of great
importance and gives rise to a variety of products which
are described in their appropriate places as rock forma-
tions. Here they are treated simply as minerals and out
of the considerable number of kinds only two are of such
importance that they will be considered in detail calcite
and dolomite.


Form. Calcite crystallizes in the rhombohedral system.
The crystals are often very fine, perfect, and sometimes
of very large size. It displays a great variety of crystal
forms many of them being often very complex. Some
simple ones are shown in the annexed figures. Fig. 51 is
a simple flat rhombohedron, three faces above and three
below. Fig. 52 is the unit rhombohedron, so called
because the faces are parallel to the cleavage. Fig. 53 is
a very acute rhombohedron. Fig. 54 is a very short
prism with six prism faces ra and the flat rhombohedron
e above and below; Fig. 55 is similar but the prism faces



w are elongated. Fig. 56 is an acutely pointed form, the
scalenohedron. All of these are common crystal forms

Fig. 5i


Fig. 53

Fig. 54

shown by calcite; they occur when it is found lining
cavities in rocks, in druses, in amygdaloids, in geodes and
on the surfaces of joint planes and fissures and in caves;
in short in all those places where it could be deposited by
infiltrating water containing it in solution. As a rock-
making mineral it is massive; of-
ten crystalline-granular and coarse
to fine in structure as in marble, or
compact as in ordinary limestone,
or loose and powdery in texture as
in chalk. It is sometimes spongy
in structure as in tufa, or rounded,
stalactic, mamillary, etc., as in
cave deposits and in concretions,
and not uncommonly fibrous.
Cleavage. Calcite has a very perfect rhombohedral
cleavage; in three directions parallel to the faces or of the
crystal shown in Fig. 52. While this
is, of course, best produced in iso-
lated crystals it can be well observed
on the fractured surfaces of coarsely
crystalline massive forms, as in many
marbles and related rocks and in the
*** 57- massive calcite of veins. The angles

of the face of the rhomb produced by cleavage are just
about 78 and 102 degrees, as shown in Fig. 57, and small

Fig- 55


Fig. 56


cleavage pieces can be readily tested by applying them
to the edges of the figure on the paper.

General Properties. The hardness is 3; readily scratched
or cut by the knife; chalky varieties are of course softer.
The specific gravity is 2.71 if pure. The natural color
of calcite is colorless or white but it frequently displays a
great variety of exotic colors owing to the presence of
impurities; thus it may be reddish or yellowish from iron
oxides or gray to black from organic matter, or green,
purple, blue, etc., from ether substances. Streak, white
to gray. The luster of the crystallized calcite is vitreous;
of massive forms glimmering to dull. In the same way
it varies from transparent to translucent to opaque.
Chemical composition, CaC0 3 ; or CaO . C0 2 . CaO = 56
per cent, C02, 44 per cent = 100.

Before the blowpipe it is infusible, colors the flame
reddish yellow and after ignition if placed on moistened
yellow turmeric paper colors it brown. Fragments
effervesce freely in cold (difference from dolomite) very
dilute acids.

Occurrence. Calcite is one of the most common and
widely diffused of all minerals. It is found in the igneous
rocks as the result of alteration of the lime-bearing sili-
cates by waters containing carbon dioxide in solution,
other substances being formed as by products at the same
time. Thus, for example,

Pyroxene- + Water 4- Garb. diox. = Calcite + Serpentine + Quartz
3 CaMg(SiO 3 ) 2 + 2 H 2 O + 3 CO 2 = 3 CaCO 3 + H 4 Mg 3 Si 2 O9 + 4 SiO 2

The calcite thus formed may remain for a time in the
rock but eventually, as the latter breaks down into soil,
it is to a greater or lesser extent removed in solution and
carried away. The mineral has also been found to occur
very commonly in minute cavities in unaltered igneous
rocks, especially intrusive ones, and its origin is probably
due to infiltration and deposition of the material from
neighboring rock masses. In these cases just mentioned


the mineral is ordinarily not observable megascopically,
but its presence is easily ascertained by immersing a
fragment of the rock in cold dilute acid and seeing if it
effervesces and gives off carbonic acid gas. Calcite also
occurs in amygdaloidal cavities in lavas, especially in
basalts, and often in good crystals.

In the sedimentary and metamorphic rocks calcite
plays a much more important part. It is very commonly
found distributed through them in fine particles or acting
as a cement to the other mineral granules. From this
role, if we examine a whole series of these rocks, we find
it increasing more and more in abundance and importance
as a constituent, until finally there are enormous, widely
extended rock-masses, such as the chalks, limestones
and marbles, composed practically and in some instances
wholly of this substance. Such rocks are found described
in their appropriate places in this work ; it is sufficient here
to mention that in the sedimentary rocks calcite plays an
important part in chalk, limestone, calcareous marls,
calcareous sandstones, etc.; in chemical deposits in
calcareous tufas, sinters, stalagmitic deposits, veins, etc.,
and in the metamorphic rocks in marbles and in rocks
which are mixtures of calcite and various silicates.

Determination. Calcite when sufficiently coarsely crys-
tallized is easily recognized by its inferior hardness and
rhombohedral cleavage. This is confirmed chemically
by its ready solubility in cold dilute acids with efferves-
cence of 062 gas and if necessary a test for the presence
of lime. For the distinction from dolomite, reference
should be made to the description of that mineral.


Form. Dolomite crystallizes in the rhombohedral
system and, like calcite, it is found in simple rhombo-
hedral crystals whose faces are. parallel to the cleavage;
Fig. 52 of calcite. Unlike that mineral it rarely occurs
in complicated crystals and the simple rhombohedron, in


which it is generally seen when showing outward crystal
form, usually has its faces curved as represented in Fig.
58 instead of flat. Moreover the curved crys-
tals are apt to be compound, made up of a
number of sub-individuals. This is the way it
occurs when lining druses and cavities, but as
a rock-making mineral it is nearly always
massive, often crystalline-granular and coarse
to fine in texture as in some marbles. It is also often
compact- massive as in some limestones; more rarely
columnar or fibrous.

Cleavage. The cleavage, like that of calcite, is perfect
in three directions parallel to the faces of the simple
rhombohedron. The angles of the cleavage rhombs
differ only a degree or so (74 and 106 degrees, nearly)
from those of calcite and therefore by form alone they
cannot be distinguished by the eye.

General Properties. The natural color is white and
while this is often seen the mineral is very apt to be
tinted some exotic color by other substances; thus it may
be reddish, brown, greenish, gray or even black. Luster,
vitreous, sometimes pearly to dull or glimmering in com-
pact varieties. Translucent to opaque. The hardness
is 3.5-4.0, harder than calcite but easily scratched with a
knife. Specific gravity, 2.8-2.9. Chemical composition,
CaMg(C0 3 ) 2 ; CaO 30.4, MgO 21.7. C0 2 47.9 = 100.
Before the blowpipe infusible, but placed on moist tur-
meric paper after ignition colors it brown. Does not
dissolve or is very little acted upon in cold dilute acid but
on boiling effervesces and goes into solution (difference
from calcite). The solution may be tested for lime and
magnesia as directed in the chapter on mineral tests.

Occurrence. Dolomite occurs as a scattered accessory
component of certain crystalline schists and in beds of
gypsum, etc., but its chief importance as a rock-making
mineral lies in the fact that alone it forms immense
extended beds both in the sedimentary series and in the


metamorphic rocks. It thus exactly parallels calcite,
and in limestones and in marbles we have every degree of
transition between these two substances, thus marbles
composed of calcite alone, others with increasing amounts
of dolomite until pure dolomite marble is reached. This
is more fully described under the carbonate rocks.

Determination. The rhombohedral cleavage and infe-
rior hardness separate dolomite, like calcite, from other
common rock minerals. The frequent curved surfaces
help to distinguish it from calcite but the test with hot
and cold acid mentioned above, together with the finding
of magnesia in the solution, is the only safe way.

Siderite, Magnesite and Breunerite. As an appendix to calcite
and dolomite these carbonates, which are sometimes of local impor-
tance, may be mentioned. Siderite or spathic iron ore is FeCOg,
ferrous carbonate, while magnesite is magnesium carbonate, MgCO a ,
and breunerite is an isomorphous mixture of the two, (MgFe)CO 3 .
In crystallization, cleavage, hardness, etc., they closely resemble
the other carbonates described. Siderite is usually light to dark
brown in color; magnesite white; breunerite brownish. Siderite
chiefly occurs more or less massive and impure in certain sedimentary
deposits, in the so-called " clay iron stone " and is a valuable ore of
iron. Magnesite occurs chiefly in certain metamorphic rocks and
is apt to be associated with serpentine, talc, etc. It may be acccom-
panied or replaced by breunerite.


The sulphates, like the carbonates, are in general
minerals of a secondary nature; the metals they contain
have been taken from previously existent minerals, the
sulphuric acid has been furnished for the most part by
the oxidation of metallic sulphides or by exhalations in
regions of igneous activity. With a few exceptions they
are readily soluble and the great bulk of them, which has
been formed during geologic time, has therefore been
transferred to the sea, which, with the salt lakes in the
interior of continents, is now the great reservoir of these
substances as well as of many other soluble salts such as



the chlorides. As rock-making minerals only two of the
large number of sulphates known are of importance,
gypsum and anhydrite. Barite, BaSO-i, which is one of
the few insoluble sulphates, is a very common material in
veins and is also found in concretions, but it does not
form independent rock-masses or play any role as a rock-
component as the two first mentioned do.


Form. Gypsum crystallizes in the monoclinic system,
and the common form of the crystals is shown in Fig. 59.
The same crystal is shown in Fig. 60 revolved so that the
side face b is parallel with the plane of the paper; such
crystals may be roughly tested by placing them on the

Fig. 60

Fig. 61

diagram and seeing if the angles coincide. Twin crystals
are common and they are apt to assume arrow-head
forms as shown in Fig. 61. More commonly as a rock
constituent, gypsum occurs massive, foliated often with
curved surfaces, or granular to compact and sometimes

Cleavage. Gypsum has a perfect cleavage parallel to
the side face 6; by means of it on good material very thin
sheets with perfect luster may be split off, almost as in
mica. Such sheets will be found to break in one direction



Fig. 62

in straight lines with a conchoidal fracture; this is due to
another cleavage parallel to the vertical edge between
mm. If such sheets be bent cracks will
appear in them making angles of 66 and
114 degrees with the straight fracture
edge mentioned above; if the bending
parallel to this direction is continued the
plate will break with a fibrous fracture,
and a cleavage rhomb like that shown in
Fig. 62 may be obtained. In massive
coarsely crystalline gypsum, these cleav-
ages can usually be readily obtained and
furnish one means of helping to identify
it; in fibrous material it simply cleaves parallel to the
fibers; in the compact massive forms it may happen that
no cleavage is seen.

General Properties. The natural color of gypsum is
colorless or white, and crystals are transparent to trans-
lucent, but it is frequently tinted reddish or yellowish or
in massive varieties may be even red, brown or black
through impurities, and translucent to opaque. The
luster of the cleavage face b is glassy to pearly, of fibrous
varieties satiny, while massive forms are glistening,
glimmering to dull. Streak, white. Hardness, 1.5-2.0,
easily scratched by the finger nail. Specific gravity of
pure crystals, 2.32. Chemical composition, hydrous sul-
phate of calcium, CaSO 4 + 2 H 2 O. CaO 32.5, S0 3 46.6,
H 2 O 20.9 = 100. Before the blowpipe fuses easily and
after ignition colors moistened turmeric paper brown.
Fused with carbonate of soda and charcoal dust on char-
coal and transferred to a moistened surface of silver
stains it dark. Finely powdered mineral is readily
soluble in boiling dilute hydrochloric acid. Heated
intensely in a closed glass tube gives off water and becomes
opaque. Heated moderately (not above 200 degrees)
it loses some water and becomes plaster of Paris and the
powder, if moistened, again takes up water and sets or


becomes solid turning back into gypsum. If heated too
highly it loses all its water, becomes anhydrite, CaSC>4,
and is then called dead burnt plaster and does not set as
described above.

The occurrence of gypsum is mentioned later in its
description as a stratified rock.


General Properties. Anhydrite crystallizes in the ortho-
rhombic system but in the rocks in which it occurs it is
seen in granular to compact masses, less commonly in
fibrous or foliated forms. It has a cleavage in three
directions at right angles, and if coarsely crystalline this
may be observed to produce cube-like forms. Usually
white but sometimes tinted as in gypsum; luster of
cleavage faces pearly to glassy; in massive varieties varies
to dull. Harder than gypsum = 3-3.5 but easily cut by
knife. Specific gravity, 2.95. Chemical composition,
CaSO 4 ; CaO 41.2, S0 3 58.8 = 100. Blowpipe and other
reactions as with gypsum, but it does not yield water in

Online LibraryLouis V. (Louis Valentine) PirssonRocks 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 text (page 9 of 35)