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 5 of 35)
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exposed to the atmosphere and to the action of fluids
circulating in the rocks at lower levels. They become
converted into kaolin or muscovite and also very com-
monly into zeolites. The latter case is very general; all
that is necessary is a rearrangement of the molecule and
the assumption of water and silica; hence when the feld-
spathoids are heated in a closed glass tube they are very
apt to yield water. Thus

Nephelite and silica and water yield analcite.
NaAlSiO 4 + SiO 2 + H 2 O = NaAl(SiO 3 ) 2 . H 2 O.

The determination of the feldspathoids in rocks is best done by
chemical means. With the exception of leucite, which is too rare a
mineral to be considered except in very unusual cases, they yield
gelatinous silica and may be tested for as described later under


mineral tests. Nephelite is easily confused with quartz which it
often closely resembles in rocks; its association with other minerals
and the appearances of those rocks in which it chiefly occurs and
which are described in their appropriate places, helps in arousing
suspicion of its presence and this is readily confirmed by its solubility
in acids. Fortunately for field determinations nephelite is a very
rare mineral, quartz an exceedingly common one ; thus the assump-
tion that the mineral is quartz in the vast majority of cases will be


The micas form a natural group of rock minerals, which
is characterized by great perfection of cleavage in one
direction, and by the thinness, toughness and flexibility
of the elastic plates or laminae into which this cleavage
permits them to be split. For practical purposes of
megascopic rock study and classification they can be
divided into two main groups, light colored micas or
muscovite and related varieties, and dark colored biotite
and related varieties.

Form. Micas crystallize in six-sided tablets with flat
bases; they appear to be short hexagonal prisms, (see
Fig. 17); in reality, as maybe shown by optical methods,
their crystallization is monoclinic. Their side faces are
rough and striated, the flat bases, which are usually cleav-

Fig. 17 Fig. x8

age faces, bright and glittering. Sometimes two of the
side faces are much elongated, as in Fig. 18. While
distinct crystal form is often observed in rocks, par-
ticularly the igneous ones, the micas are much more
commonly seen in shapeless flakes, scales or shreds, with
flat, shining, cleavage faces. Sometimes the foliae or
leaves are curled or bent.


Cleavage. This has been already mentioned. It is
perfect parallel to the base and it is this property combined
with its flexibility, transparency and toughness that
makes the large crystals and sheets of muscovite found in
pegmatite veins so useful in making stove windows, lamp
chimneys, etc., where ordinary glass is easily broken.
Sometimes when the mineral occurs in an aggregate of
minute scales, especially muscovite in the sericite form,
the cleavage is not so apparent, but can generally be seen
by close observation.

Color, Luster and Hardness. Muscovite is colorless,
white to gray or light brown, often with greenish tones.
The other light-colored micas are similar, except that
lithia mica or lepidolite, found in pegmatite veins, is
usually pink or lilac colored. These micas in thin sheets
are transparent.

Biotite and its congeners are black, in thin sheets
translucent with strong brown, red-brown or deep green
colors. The phlogopite variety is pale brown, sometimes
coppery. The luster of micas is splendent, on cleavage
faces sometimes pearly and in the sericite variety of
muscovite frequently silky. The hardness varies from
2-3; all are easily scratched with the knife.

Chemical Composition. Chemically the micas which
take part in rock-making may be divided into two main
groups, one containing iron and magnesia, of which the
dark-colored biotite is an example, the other devoid of
these oxides, of which muscovite is the most prominent
member. They are complex in composition, silicates of
alumina with alkalies and containing more or less hydroxyl
and fluorine. The two main varieties may be represented
as follows:

Muscovite = H 2 KAl 3 (SiO 4 )3.

Biotite = (HK) 2 (MgFe) 2 (AlFe) 2 (Si0 4 )3.

The other members of the muscovite group are, paragonite>
a rare mineral like muscovite, in which soda replaces



potash and lepidolite, in which the potash of muscovite is
partly replaced by lithia. In the biotite sub-group,
phlogopite is a variety nearly free from iron and thus a
magnesia mica; the lack of iron accounts for its lighter
color; lepidomelane, on the contrary, is very rich in iron,
especially ferric oxide, while another, zinnwaldite, contains
some lithia in place of part of the potash. The formulas
of these compounds are very complex and in part not
absolutely settled. The adjoining table of analyses
shows the chemical differences between the varieties.








SiO 2








A1 2 O 3








Fe, O,













11 6











Na 2 O







K 2 O







Li 2 O



H 2 O












2 2




1 2











* X represents small quantities of non-essential oxides present.

I, Muscovite, Auburn, Me. ; II, Paragonite, the Alps ; III, Lepid-
olite, Hebron. Me.; IV, Biotite, from granite, Yosemite, Cal.; V,
Phlogopite, Burgess, Ontario; VI, Lepidomelane, from nephelite
syenite, Litchfield, Me, ; VII, Zinnwaldite, Zinnwald, Erzgebirge.

Blowpipe and Chemical Characters. Usually the micas
whiten before the blowpipe and fuse on the edges, when
in thin scales. Lepidomelane fuses to a black magnetic
globule. Heated in the closed glass tube they yield
very little water, which helps to distinguish them from


chlorites and other micaceous rock minerals. When thin
scales are treated with a little boiling concentrated sul-
phuric acid in a test tube, muscovite and the re-
lated light-colored kinds are scarcely acted upon, but
biotite and its congeners are decomposed, the scales losing
their luster and transparency while the acid becomes

Lepidomelane is soluble in hydrochloric acid, depositing
silica in scales, an important character serving to dis-
tinguish it from the other micas. The lithia micas impart
a red color to the blowpipe flame, paragonite the yellow
color of sodium.

Alteration. Biotite under the action of weathering
changes to chlorite, loses its elasticity and becomes soft
and of a green color. Muscovite being itself often the
product of various alterations of other minerals, especially
of feldspars, appears well fitted to withstand the process
of weathering and its scales often occur in soils made of
broken-down rocks whose other constituents may be
greatly changed. It eventually changes, loses its trans-
parency and elasticity and perhaps becomes ultimately
converted into clay.

Occurrence. The common micas are minerals of wide
distribution as rock components. Biotite is a very
common and prominent ingredient of many igneous rocks,
especially of those rich in feldspar like granites and
syenites in ferro-magnesian rocks like gabbro it is less
prominent; it is also seen in many felsite lavas and por-
phyries. It occurs commonly in some metamorphic
rocks such as gneisses and schists and is frequently one of
the products of contact metamorphism of igneous rocks.
From its liability to alteration it does not figure as a
component of sedimentary beds. The phlogopite variety
containing little iron has been found in some rare cases in

* Care should be used in making this test not to bring the hot
acid in contact with water, or the mixture will take place with
explosive activity.


igneous rocks, but it chiefly occurs as a product of meta-
morphism in crystalline limestones or impure marbles
and dolomites. Lepidomelane and zinnwaldite appear to
occur chiefly in granites and syenites, especially in peg-
matitic varieties. Muscovite occurs in granites and
syenites, especially in pegmatite veins and in miarolitic
druses and in places where the igneous rocks have been
subjected to later fumarole actions furnishing water and
fluorine. It is sometimes seen in intrusive porphyries
and lavas of felsitic character. It is especially common
in the metamorphic rocks and is widely distributed in
gneisses and schists; sometimes, especially in the latter
rocks, it is in the form of an aggregate of minute scales
which have a silky luster and largely lack in appearance
the evident characters of the mineral, such as its cleavage;
this variety has been called sericite. When feldspars are
altered to muscovite, rather than to kaolin, this sericite
variety is the common product. In the sedimentary
rocks, such as conglomerates and sandstones, muscovite
is sometimes seen, an unchanged remnant of the original
rocks from which their material came. Lepidolite is
practically restricted to granite-pegmatite veins and is
constantly accompanied by tourmaline. Paragonite has
been found in only a few cases, in schists, playing the role
muscovite would ordinarily have.

Determination. From the ordinary rock minerals the
micas are at once distinguished by their appearance, high
luster and eminent cleavage, the latter quality and their
hardness being readily tested in the field by the knife
point. From the chlorite group and from talc, which
resemble them, they are told by the elasticity of their
split-off laminae, those of the chlorites and talc being
flexible but not elastic. From chloritoid a micaceous
appearing mineral of a gray or green color, a hydrated
silicate of alumina, magnesia and iron, which is sometimes
seen in distinct crystals in certain metamorphic rocks, they
are readily distinguished by its superior hardness = 6.5


and brittieness. The different varieties of mica are
best discriminated by the chemical and blowpipe tests
already mentioned.


The pyroxene group embraces a number of important
minerals which have in common the fact that they are
metasilicates, salts of metasilicic acid, H 2 Si0 3 , in which
the hydrogen is replaced by various metals as shown
later, and although they may differ in the system in which
they crystallize, in having closely related crystal form,
notably a prismatic cleavage of 87 and 93 degrees. As
rock minerals they are of greatest importance in the
igneous rocks though they may be prominent at times in
some of the metamorphic ones. Some igneous rocks are
composed almost entirely of pyroxene.

It is often difficult to recognize pyroxene in the rocks
and distinguish it from several other minerals purely by
simple megascopic methods and largely impossible to
tell from one another by such means the many varieties
recognized by mineralogists and petrographers. The
differences between these varieties are chiefly in chemical
composition and optical properties and these must be
determined by chemical and optical methods.

For practical megascopic petrography the pyroxenes
may be divided into the following sub-groups dependent
on their color, behavior before the blowpipe and chemical
reaction for lime as described later: hypersthene, diopside,
common pyroxene, augite and aegirite.

Form. Hypersthene crystallizes in the orthorhombic,
the others in the monoclinic systems, but this distinction
is not a matter of practical importance in megascopic
work, since the former is rarely well enough crystallized
to determine the system. The common form, in which
the monoclinic rock pyroxenes crystallize, is a prism,
usually short and thick though sometimes longer and
more slender. Such a prism is shown in Fig. 19, the



ends modified by pyramidal faces. Generally, however,
the edges of the prism mm are truncated by a front face
a and a side face b sometimes these truncations are

Fig. 19

Fig. 20

Fig. ax

small so that a and b are slender (Fig. 20); often they
are very broad and mm narrow. While these faces are
commonly well developed and often lustrous the pyra-
midal faces are often very imperfect or wanting, the
crystal being rounded at the ends; rarely other pyramidal
faces are present and the ends much more complex than
in the figures. The augites which occur in igneous rocks,
especially porphyries and lavas, very often have the
appearance and development shown in Fig. 21. The most
important thing in the crystallization is that the angle m
on TO is nearly a right angle, 87 degrees, so that the prism
is nearly square in cross section or when truncated by a

Fig. 22

and b, octagonal as shown in Fig. 22. Besides occurring
in prismatic crystals the pyroxenes also are very common
in grains, or in more or less shapeless masses; this is


usually the case in certain massive igneous rocks such as
gabbros and peridotites.

Cleavage and Fracture. As previously mentioned the
pyroxenes have a cleavage parallel to the faces mm, nearly
at right angles as shown in Fig. 23; this is
fundamental and serves to distinguish the
mineral from hornblendes. This cleavage is
usually very good but not perfect. Some
varieties often have a good parting in other
directions resembling cleavage which causes
the mineral to appear lamellar, perhaps even
somewhat micaceous, as seen in the pyroxenes of some
gabbros. Fracture uneven; the mineral is brittle.

Color and Luster. The color varies from white through
various shades of green to black, according to the amount
of iron present. The pure diopsides are white, rarely
colorless and transparent, often pale green, and more or
less translucent; common pyroxenes are dull green of
various shades; augite and aegirite are black; these are
opaque. The luster, which is often wanting, is glassy to
resinous. Streak varies from white to gray-green.

Hardness and Specific Gravity. The hardness varies
from 5-6. Some varieties can be just scratched by the
knife. The specific gravity varies, chiefly with the iron
present, from 3.2-3.6.

Chemical Composition. Pyroxenes are composed of
the metasilicate molecules MgSiOs, FeSiOs, CaMg(SiC>3) 2 ,
CaFe(Si0 3 ) 2 , NaFe(SiO 3 ) 2 and RR 2 Si0 6 in which last
R = MgFe and R = Al and Fe. These molecules are
isomorphous, that is, capable of crystallizing in various
mixtures which produce the same crystal form and many
similar physical properties. The hypersthene sub-group
contains mixtures of MgSiOg and FeSiOs without lime;
diopside is CaMg(SiOs)2 with little or none of the iron
molecule, common pyroxene contains variable mixtures of
CaMg(SiO 3 ) 2 (diopside) and CaFe(SiO 3 ) 2 (hedenbergite)
with small portions of the other molecules ; augite contains



the same but in addition a large amount of HR 2 SiO e ;
aegirite is mostly NaFe(SiO 3 ) 2 and is thus a soda

Blowpipe and Chemical Characters. Hypersthene varies
from almost infusible in the blowpipe flame when contain-
ing little iron (variety enstatite) to difficultly so with much
iron; in the latter case it becomes black and slightly
magnetic. The other pyroxenes are much more fusible
= 4 and melt quietly or with little intumescence to glassy
globules whose color depends on the amount of iron, diop-
side nearly colorless, common pyroxene green or brown,
augite and aegirite black; the last two magnetic. Aegirite
fuses quietly and colors the flame yellow. They are but
slightly acted upon by acids, those with iron more so
than those without.

These differences in the chemical composition are shown
in the table of analyses.

SiO a

A1 2 3

Fe 2 3




Na 2



I .

































V .


















* X = small quantities of other oxides.

I, Hypersthene, Romsaas, Norway; II, Hypersthene (var. ensta-
tite), Bamle, Norway; III, Diopside, DeKalb, N. Y. ; IV, Common
pyroxene, Edenville, N. Y. ; V, Black augite, Vesuvius lava; VI,
Aegirite, from syenite, Hot Springs, Ark.

Alteration. The pyroxenes are prone to alter into
other substances whose nature depends partly on the
kind of process to which they are subjected and partly
on their own composition. Thus under the action of
weathering they may be converted, if containing much
magnesia, into serpentine, or into chlorite, if containing


iron, or Into both and often carbonates are also formed,
such as calcite. Those containing much iron may com-
pletely break down into hydrated iron oxides, such as
limonite, and carbonates.

Another very important change is one which they
suffer under metamorphic processes, especially regional
ones. In this they become altered to masses of fibrous,
felty or stringy hornblende needles and prisms, usually
of distinct but variable green colors. This process is of
great geologic importance for by means of it whole masses
of pyroxenic rocks, generally of igneous origin, such as
gabbros, peridotites, basalts, etc., have been changed
into hornblendic ones to which a. variety of names, such
as greenstone, greenstone schist, hornblende schist, etc.,
have been applied. The process is further mentioned
under metamorphism, and under gabbro, dolerite, green-
stone and amphibolite.

Occurrence. The pyroxenes are chiefly found in
igneous rocks, especially those which are formed from
magmas rich in lime, iron and magnesia. Therefore, in
the dark colored rocks of this class they should always be
looked for. They are not often found in igneous rocks
which contain much quartz, hence in granites, felsite
porphyries and felsite lavas they are rare. Augite is
found in basaltic lavas and dark, trap-like intrusives,
often in well formed crystals; when it occurs in gabbros
and peridotites it is commonly in grains and lumps.
Hypersthene is prominent in masses and grains in some
varieties of gabbro and peridotite. Aegirite occurs chiefly
in nephelite syenites and the phonolite variety of felsite
lava. Some normal syenites and related rocks contain
diopside-like or common pyroxene. In the metamorphic
rocks common pyroxene and diopside, the latter some-
times white or pale greenish and transparent, are found
in impure recrystallized limestones and dolomites, some-
times in well formed scattered crystals, sometimes
aggregated into large masses. Common pyroxene also


occurs in some gneisses. Being readily decomposed by
weathering they play no part in sedimentary beds.

Determination. If the mineral under examination is
in well formed crystals careful observation will usually
show if it is a pyroxene by its possession of the forms
previously described. The outline of the section presented
by the prisms, especially when broken across, should be
noted in this connection. The common minerals in rocks
with which pyroxenes may be confused are hornblende,
epidote and tourmaline. The lack of good cleavage, the
superior hardness, the high luster, dense black color and
triangular shape of the prism cross section of tourmaline
readily distinguish it from pyroxene. Epidote has one
perfect cleavage, one poor one; it is much harder, 6-7;
while green it commonly has a yellow tone, giving a yel-
lowish green; before the blowpipe it intumesces when
fusing. The distinction of pyroxene from hornblende is
more difficult and is treated under the head of that

To distinguish the different varieties of pyroxene from one another
the blowpipe tests previously mentioned should be used in conjunc-
tion with the natural color of the mineral. The hypersthenes are
most certainly told from other pyroxenes by making a chemical test
to prove the absence of lime or at least its presence in only minute
quantity. This is best done by making a small fusion with soda as
described in the chapter treating of mineral tests.


The amphiboles, or hornblendes, names which are used
interchangeably, are a natural group of silicate minerals
which like the pyroxenes are salts of metasilicic acid
H 2 SiO 3 , in which the hydrogen is replaced by various
metals or radicals. They have in common a certain
crystal form, a prismatic cleavage of about 55 degrees,
and are nearly related in many physical properties. As
in the pyroxene group, to which the amphiboles are closely
allied in several ways, there are many varieties recognized



by petrographers, dependent upon differences in chemi-
cal composition and physical properties, especially optical
ones, which are impossible to distinguish by the eye and
many of them indeed by ordinary megascopic tests.

For practical work in megascopic petrography the
amphiboles may be divided into the following sub-groups:
Tremolite, Actinolite, Common Hornblende, and Arfved-
sonite. These may be distinguished by their colors, asso-
ciations and behavior before the blowpipe.

Form. Amphiboles crystallize in the monoclinic sys-
tem. The crystals are usually long and bladed, formed
by two prisms mm which meet at angles of 55 and 125
degrees. Sometimes there are terminal faces rr as in Fig.
24, sometimes the crystals are imperfect at the ends and
no terminal faces are seen; this latter is common in rocks.
Very often the side face 6 is present truncating the prism


Fig. 24

Fig. a 5

Fig. a6

Fig. 27

edge and the crystal has a nearly hexagonal cross section
as in Fig. 25. More rarely the front face a is present as in
Fig. 26. The black hornblendes found as phenocrysts in
some basaltic rocks have often a not very short prism
and appear as in Fig. 27; these are the hornblendes which
most often have distinct terminal planes. The prismatic
faces mm and the face b, if it is present, are apt to be
shining, the ends are frequently dull. It is not common


for amphibole to present itself in rocks in crystals whose
planes can be distinctly seen; when this occurs it is mostly
with the black hornblendes found in lavas as phenocrysts
and in those which occur in limestones and dolomites
which have been subjected to metamorphism. The
common appearance is in long slender blades with irregu-
lar, rough ends; this is usual in the hornblende schists
where the crystals are aggregated together in more or less
parallel position; they may dwindle in size to shining
needles, becoming so fine that the minute prisms can
hardly be seen with the lens; the aggregate then has a
silky appearance. In the felsitic lavas and porphyries
the prisms of the hornblende phenocrysts vary from
rather short, like those in the figures, to slender needles;
in the massive doleritic rocks like diorite the amphibole is
apt to occur in irregular grains and small masses. Some-
times as in asbestus the mineral has a highly developed
columnar, fibrous form.

Cleavage. Amphiboles have a highly perfect cleavage
parallel to the prism faces mm as illustrated in the cross
section, Fig. 28. Like the faces mm
these cleavages meet at angles of 125
and 55 degrees, a fact of great import-
ance in distinguishing the mineral . The
glittering prismatic faces seen on the
blades and needles of fractured rock
Pig. as surfaces are commonly due to this

good cleavage. The fracture is uneven.
Color and Luster. The color varies with the amount of
iron from white or gray in tremolite to gray-green or
bright green in actinolite to darker greens and black in
common hornblende. Arfvedsonite is black. Some var-
ieties found in igneous rocks which appear black are
really deep brown. The mineral varies from opaque in the
deeper colored varieties to translucent in the lighter
ones. The luster is bright and vitreous to somewhat
pearly on the cleavage surfaces; in very fine needle-like

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 5 of 35)