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 3 of 35)
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pigment, as when, for example, quartz is colored dark red
by a reddish dust of ferric oxide particles in it; or the
inclusions may be so arranged in regular systems as to
act refractively upon light, breaking it up and producing
a play of prismatic colors, or opalescence in the substance.
Usually in the latter case, one color predominates and
gives its character to the mineral. A good example of


this is seen in the variety of feldspar called labradorite, a
constituent of the rocks called anorthosite and gabbro,
which often shows a fine play of colors, a rich dark blue
being usually predominant.

The white color which so many rock-making minerals
exhibit may be due to minute inclusions, as when feld-
spars are sometimes, through alteration, filled with scales
of kaolin or white mica, but more commonly it is due to
the reflection of light from the surfaces of innumerable
microscopic cracks and crevices which everywhere per-
meate the mineral substance. In such cases the material
is really colorless and transparent. The effect is the same
as if a piece of colorless glass should be ground to powder,
y/hich would of course be white. Hence white minerals
are not regarded as possessing any color, and they are
often free from such cracks and are then colorless and
transparent. Good examples of this are seen in such
common minerals as quartz, calcite and feldspars. As
explained under cleavage, these cracks in feldspar are
sometimes so regularly arranged as to produce a play of
colors, giving the mineral an opalescence or pearly luster
with a distinctly predominating color tone like that
mentioned above as produced by inclusions.

Streak. In addition to that color which minerals show
in the solid form, there is another way in which this
property may be often usefully employed in determining
them. This is the color which the substance presents
when reduced to a state of powder. The powder may be
obtained by grinding a small fragment in a mortar, but
it is more easily produced by scratching a sharp point
of the mineral across a plate of unglazed porcelain; the
color of the resultant streak is of course that of the
powdered mineral. While any piece of unglazed porce-
lain will answer fairly well, pmall plates are specially
made for this purpose and sold by dealers in chemical


The color shown by minerals in the powdered state is usually
much lighter than that which they exhibit in the mass and sometimes
very different. It is most useful in helping to discriminate the
dark colored minerals, especially the metallic oxides and sulphides
of the heavy metals used as ores, and hence its application with the
light colored silicates and carbonates that chiefly form the rock-
making minerals is much more limited and of lesser value. In the
case of these minerals it is sometimes useful in distinguishing exotic
from natural colors; for the color of the streak is generally that of
the mineral substance itself, and the pigment or other impurities
which produce an exotic color must be present in very large amount
to exert a definite influence. Thus calcite is colorless or white, but
sometimes yellow, brown or red, but the streak of all these colors
is white or barely tinted except in unusual instances. The feld-
spars are normally white or colorless, but in some rocks, such as
anorthosite, they sometimes are black and at first glance might be
mistaken for an iron-bearing mineral; the streak, however, is white
and helps to show their true character.

In the field, the bruised surface of the rock, where struck by the
hammer, often shows the powdered minerals, giving in a rough
manner the color of the streak; or a bit of the substance may be
ground between two hammer surfaces and the powder rubbed on
white paper.

Cleavage. When mineral bodies possess crystalline
structure, it frequently happens that the arrangement of
the physical molecules composing them is such, that the
force of cohesion among them is less in some particular
direction or directions than in others. Along such
directions, if suitable means be employed, such as placing
the edge of a knife upon the mineral and striking with a
hammer, the body will tend to split or cleave. The degree
of perfection with which minerals possess this property
is very variable; some, like mica, which is used for stove
windows and lamp chimneys, are capable of being almost
indefinitely split into thin leaves ; others, like the feldspars,
have a good cleavage; while some, like quartz, have no
apparent cleavage. When the cleavage is very good the
new surfaces are smooth and shining like the original ones
of a crystal and it is termed perfect. This property,
being then so distinctive, is a most useful one in helping


to determine minerals, especially in rocks where the
mineral grains on the surface, broken by the hammer, if
they possess it, everywhere show shining cleavage faces.
It must not be imagined that the directions of cleavage
occur at random in a mineral; on the contrary, they always
bear a definite relation to the special crystal form that
characterizes a particular mineral. If the latter has two
directions in which it may be cleaved, like feldspar, for
example, the angle between the two surfaces is, for a
feldspar of a certain definite chemical composition, always
the same. Some minerals, like mica, have only one
direction in which there is good cleavage; others have two
directions, and sometimes the two are exactly alike and
sometimes unlike, one being better than the other; again
there may be three directions in which cleavage can be
produced, all alike as in calcite or unlike as in barite
(heavy spar, BaS0 4 ), or there may even be four or more.
Whether the cleavages are alike or unlike, when there is
more than one, depends not only on their direction in the
crystal, but also on the geometric form or system of
crystallization the latter exhibits. A description of these
relations would involve too much of the principles of
crystallography for discussion in this place, but the
following will be helpful in understanding certain terms
frequently used.

A. Good cleavage in one direction only: the mineral
grains in the rock in this case are apt to be developed in
tables, folia, or scales, whose surface is parallel to the
cleavage. This is well shown in such minerals as the
micas and chlorite.

B. Good cleavage in two directions and both alike:
the minerals are apt to be developed in elongated forms
parallel to the cleavage, and the latter is spoken of as being
prismatic. This is shown by the minerals hornblende
and pyroxene. If the two cleavages are not exactly
alike, the mineral still is often elongated in the direction
of the edge produced by the meeting of the two cleavage


planes. It may be sometimes columnar and sometimes
tabular parallel to the better cleavage. The feldspars,
which form the free developed crystals in porphyry, often
show such relations.

C. Good cleavage in three directions alike: if the three
planes are at right angles to each other the mineral will
break up into cubes and the cleavage is cubic or apparently
so; if they are at some other angle, rhombs will be produced
and the cleavage is called rhombohedral. Cubic cleavage
is well shown by galena, PbS, the common ore of lead and
by rock salt; it is not exhibited by any common rock-
making mineral. Rhombohedral cleavage is character-
istic of the common rock-making carbonates, calcite,
CaCOs, and dolomite (MgCa)CO3. Three unlike directions
have the same practical effect as two unlike, and four
directions are not of importance in megascopic petro-
graphy, as no common rock-making mineral exhibits

If a rock with component mineral grains sufficiently coarse so
that they can be readily studied by the pocket-lens, the size of peas,
for example, be carefully examined, it will be found that almost
without exception, where a mineral shows a cleavage face, 'it will be
full of minute cracks and fissures. These cracks are paral'el some-
times to one cleavage and sometimes to all the cleavage directions
the mineral has. In addition to the cleavage cracks there are also
irregular lines of fracture which do not correspond to any definite
direction. Commonly, the mineral grains of rocks contain not only
these large cleavage cracks and irregular fractures which can be
perceived with the eye or with the aid of the lens, but, in addition,
they are everywhere rifted by similar ones so minute that they can
only be detected in thin sections of rocks under high powers of the
microscope. It is the reflection of light from these minute micro-
scopic cracks that renders so many minerals opaque and white in
color that would otherwise be colorless and transparent. These
cracks and fissures have been produced in the rocks by the various
forces to which they have been subjected; sometimes they are due
to the contraction following a heated stage as in metamorphic and
igneous rocks, and sometimes and more generally to the intense
pressures and strains to which the rocks of the earth's crust are and
have been subjected. Minute as the rifts in the mineral grains are,



they are of great importance in geologic processes, for by means of
them, and drawn by capillary action with great force, water con-
taining CO 2 in solution penetrates not only the rocks but the indi-
vidual grains as well, to their very interiors, and alters and changes
them into other minerals and the rocks into soil.

Fracture. The appearance of the breakage of minerals
in directions which are not those of cleavage or in cases
where the mineral does not
possess cleavage is called its
fracture. If the mineral is
fibrous in structure, the
fracture may be fibrous; or
it may be rough and un-
even or hackly; if . the
mineral is dense, compact
and homogeneous it will be
conchoidal, that is, it will
present a sort of shelly
appearance such as is shown
on surfaces of broken glass
which recall the inside or
outside of a clam shell.
Rocks which are extremely dense and homogeneous, like
some flints, or glassy lavas or fine-grained compact ones,
have also a conchoidal fracture more or less pronounced.
Quartz is the most common mineral which gives a good
example of conchoidal fracture.

Associations of Minerals. The facts that certain kinds
of minerals are apt to be found together in the same kind
of rock and that the presence of one mineral excludes
the presence of some other mineral are of great value in
petrography but of much greater use in microscopic work,
where the distinguishing characters of minerals are easily
made out, than in field determinations. Even in mega-
scopic petrography, however, these facts are at times of
practical use; thus the fact that the two minerals, quartz
and nephelite, cannot occur naturally together as rock-

Fig. 4. Conchoidal fracture in obsidian
volcanic glass.


making components is of value in discriminating between
certain rocks. The various relations of this kind that are
of importance will be mentioned in their appropriate places.
Hardness. This property is of great value in helping
to make determinations of minerals, and it is likewise very
useful in the field in making rough tests of rocks. The
hardness of minerals is determined by comparing them
with the following scale:

Scale of Hardness

1. Talc. 6. Feldspar.

2. Gypsum. 7. Quartz.

3. Calcite. 8. Topaz.

4. Fluorite. 9. Corundum.

5. Apatite. 10. Diamond.

This means that each mineral, using a sharp point, will
scratch smooth surfaces of all the minerals in the list above
it but of none below it. If, for example, a fragment of an
unknown mineral is found to scratch calcite its hardness
is greater than 3; if it will not scratch fluorite, but, on the
contrary, is scratched by it, its hardness is not so great
as 4, but must be between 3 and 4 or approximately 3^.

The point of a pocket-knife blade as ordinarily tempered
with a hardness of a little over 5 and pieces of common
window glass with hardness of about 5^ are very useful
for testing the hardness of common minerals and of the
rocks made up of them. A common brass pin point is a
little over 3 and will scratch calcite; the finger nail is a
little over 2 and will scratch gypsum.

Specific Gravity. The specific gravity of a substance
is its density compared with water or the number of times
heavier a given volume of the substance is than an equal
volume of water. It is obtained by weighing a piece of
the mineral or rock in air and then in water; the difference



between the two is equal to the weight of an equal volume
of water (the volume displaced) and we have

wt. in air

wt. in air wt. in water

= Sp. Gr,

The operation may be carried out with one of the special forms
of apparatus devised for determining specific gravity and described
in the manuals of determinative mineralogy, or it may be done with
a chemical, an assay or a jeweler's balance. It is first weighed in
the pan and then suspended from it by a hair and weighed in water.






















Labradori te.

































































A piece about one-half inch in diameter is convenient both for
minerals and rocks, but in the case of minerals it is frequently
necessary to select a fragment smaller than this to obtain pure
homogeneous material, without which it is perhaps needless to say
the determination is of little value. Adherent air bubbles and air
in cracks are best gotten rid of by boiling the fragment in water and
then allowing it to cool before weighing. If the mineral has an


invariable chemical composition and crystal form, as for example,
quartz (SiO 2 ), calcite (CaCO 3 ), etc., the specific gravity is an invari-
able quantity, and departures from it must be due to the presence
of impurities. Many minerals, however, while they retain the same
crystal form, vary considerably in chemical composition in that one
metallic oxide may be more or less replaced by another similar oxide
or oxides. Thus we find minerals which at one end of a series con-
tain magnesia, MgO, and at the other end ferrous oxide, FeO, and
between these extremes all degrees of mixtures of these two oxides.
In accordance with such variations the specific gravity of the mineral
varies. The pyroxenes, amphiboles, garnets, olivines, etc., are
examples of this, and it accounts for most of the variations in specific
gravity which may be observed in the annexed table.

Blowpipe Reactions. The rock-making minerals, which
are chiefly carbonates and silicates, do not as a rule
exhibit before the blowpipe very characteristic reactions
by which they may be readily determined, as do so many
of the ores, the oxides and sulphides of the heavy metals.
Still, however, the relative degree of fusibility shown by
thin splinters, the coloration of the flame and the characters
of the melted bead which may result are properties which
may be of great service in helping to determine these
minerals, and so far as they have value in this direction
they are mentioned in the description of the minerals.
If instruction in the use of the blowpipe is desired it should
be sought in one of the manuals devoted to that purpose.

Chemical Reactions with Reagents. Certain qualitative
chemical tests which can generally be made with a few
reagents and simple apparatus are of great service in
mineral determination and in aiding to classify rocks.
In Chapter V, in which the methods for the identification
of minerals are given, these tests and the proper ways of
making them are fully described.


SEC. 1. Primary Anhydrous Silicates and Oxides.

THESE minerals from the geological standpoint are th^
most important in forming rocks. They are the most
abundant and the most widely diffused. They are the
chief minerals which are formed by the cooling and
crystallization of the molten fluids of the earth's interior,
and hence they are the main components of the igneous
rocks. The greater part of the metamorphic rocks are
also made up of them, and in the sedimentary beds they
are also important constituents in many cases.

It is difficult to draw a sharp line between the absolutely
anhydrous minerals and those containing considerable
quantities of combined water. Thus, most hornblendes,
micas and epidotes contain small amounts of hydroxyl
and yet are ordinarily considered as anhydrous, compared,
for instance, with kaolin, serpentine and chlorite. In
the same way feldspar, hornblende and pyroxene are
thought of as primary minerals although we know that in
some cases they are of secondary origin, that is, they have
been formed at the expense of previously existent minerals.
The grouping as given is largely a matter of convenience;
it includes those which are always anhydrous and always
primary and which thus give a certain distinctive charac-
ter to the group, which it is well to enforce, but it also
includes many which are at times secondary and some
which are hydrous, because on account of their mineralogic
positions and affinities it is more convenient and natural
to consider these minerals in this connection.

In the following section only such silicates and oxides
are treated as are both hydrous and secondary.



a. Silicates.

The silicates are salts of various silicic acids, in which
the hydrogen atoms have been replaced by various metals
or radicals composed of metals in combination with
oxygen, hydroxyl, fluorine, etc. The three important
silicic acids which in this group form rock minerals are
poly silicic acid, EUSisOg; metasilicic acid, H 2 SiO 3 , and
orthosilicic acid, EUSiO^ The list of those treated as of
importance on account of the functions which they have
as rock-making minerals includes the feldspar, felds-
pathoid, mica, pyroxene, amphibole, olivine, garnet,
tourmaline and epidote groups, and a few other less
common ones.


The term feldspar is not the name of a single mineral
of a definite chemical composition like quartz, Si02, but
is the designation of a group of minerals which have a
general similarity in chemical and physical properties.
They are indeed so much alike in general characters and
appearance that in determining rocks by megascopic
features they cannot be told apart except in special cases,
and it is, therefore, best to treat them as a group, and at
the same time mention those characters by which, when
possible, they may be distinguished.

The rock-making feldspars are composed of three kinds
and their mixtures as follows :

a. Orthoclase, KAlSisOg, silicate of potash and alumina;

b. Albite, NaAlSisOg, silicate of soda and alumina;

c. Anorthite, CaAl2Si20g, silicate of lime and alumina;
Alkalic feldspar, (KNa)AlSi3Og, mixtures of a and 6;
Plagioclase feldspar, (NaAlSiaOg)* + (CaAl 2 Si 2 O 8 ) y ,

mixtures of b and c.

The simple feldspars are mostly confined to the crystals
found in veins, druses, etc.; they sometimes occur as the



component grains of rocks, but are comparatively rare;
in the great majority of cases the feldspars are either
mixtures of orthoclase and albite in varying proportions
but usually with a considerable excess of the potash
compound and are then called alkalic feldspar, or they are
mixtures of albite and anorthite and are then known as
soda-lime feldspar or plagioclase. All transitions from
pure albite to pure anorthite occur, and the series has been
divided into groups according to the different proportions
of the soda and lime molecules; one of the most important
of these is called labradorite in which there are about
equal amounts of the two kinds.

Mixtures of a and c, of the potash and lime feldspars, have been
found to occur but are so rare that for practical purposes they may
be neglected.

Form. Orthoclase is monoclinic in symmetry, and when
in distinct well-made crystals it commonly takes the

Fig. 5.

Fig. e.

Fig. 7.

forms shown in the accompanying figures. Sometimes
the crystals are stout and thick in their habit or appear-
ance as in Fig. 5, sometimes they are thin and tabular
parallel to the face 6 as in Fig. 6, and again they may
be rather long and columnar as in Fig. 7. In or-
thoclase the face c is always at right angles with the
face 6. In albite and anorthite, whose crystallization is


triclinic, these faces c and 6 are not at right angles but ara
slightly oblique; this is also true for all of their mixtures
or the plagioclase group in general. Some mixtures of
orthoclase and albite, as well as certain varieties of the
pure potash compound KAlSiaOg called microcline, are
also slightly oblique, but in all these cases mentioned the
amount of departure from a right angle is only a few
degrees which, even under favorable conditions, can
scarcely be perceived by the eye and must be measured
by a goniometer to be appreciated. It cannot, therefore,
under ordinary circumstances, be used as a means of
discrimination between the alkalic and plagioclase feld-
spars. The forms of the crystals in which the plagioclase
feldspars appear in rocks when they have the opportunity
to crystallize freely are similar to those mentioned above
for orthoclase in Figs. 5-7.

It is only in the phenocrysts of porphyritic igneous
rocks and in the miarolitic druses of the massive igneous
ones that these minerals have an opportunity to assume
the free crystal forms described; in ordinary cases their
crystallization is interfered with by other minerals or by
other crystals of feldspar and they are thus seen in shape-
less masses or grains. Nevertheless there is a tendency
to assume these forms, and in some rocks, such, for instance,
as the syenites, which are mainly composed of feldspar,
it may be observed that they have more or less perfectly
the shape of flat tables or rude laths as they approximate
to Figs. 6 or 7.

Twinning. Crystals frequently appear compound, as
if cut through parallel to some prominent plane on them
and one of the halves revolved 180 degrees, usually on an
axis perpendicular to the plane of division which is called
the twinning plane, and the two parts grown together.
Such an arrangement is called a twin crystal. Feldspars
very commonly occur in twin crystals, one of the most
frequent arrangements being that illustrated in Fig. 8
and known as the Carlsbad twinning from the town of that



Fig. 8

Fig 9

name in Bohemia where excellent examples have been
found. It is as if a crystal like that shown in Fig. 5
were cut through parallel to the face b, one of the parts
revolved 180 degrees around a
vertical axis parallel to the edge
mb and then joined and the two
parts pushed together so that
they mutually penetrate. In
Fig. 9 the same arrangement is
seen looking down on the face b
of the crystal; acya is the outline
of the original crystal; if this is
cut out in a piece of paper and then turned over 180
degrees or upside down and laid on acya so that the edges
aa are brought together, it will give the result seen in
the figure. In the twin crystal illustrated in Fig. 8 the
face c slopes toward the observer, the face y slopes away
behind; in the twinned half this is reversed; as explained
under the cleavage of feldspars this fact is of importance
in helping to recognize these twins when the outward

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 3 of 35)