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

ROCK-MAKING MINERALS 63

or fibrous varieties, silky. Streak, white to gray-green or
brownish.

Hardness and Specific Gravity. The hardness varies
from 5-6; some specimens can be scratched with the
knife. The specific gravity varies, chiefly with the
amount of iron, from 2.9-3.5.

Chemical Composition. Amphiboles like the pyroxenes
are metasilicates, salts of H 2 SiO 3 , in which the hydrogen
atoms are replaced by calcium, magnesium, iron, soda
and also, as shown by Penfield, by radicals in which
alumina plays a prominent part and which contain
hydroxyl ( OH) and fluorine. Penfield has also shown
that when calcium is present it replaces one fourth of the
hydrogen atoms. Thus, while the amphiboles resemble
the pyroxenes in being metasilicates and composed of the
same elements, they differ from them in being much more
complex and in containing hydroxyl and fluorine. Their
compositions, as a rule, are too complicated to be repre-
sented by simple formulas, but in a general way, disregard-
ing the hydroxyl and fluorine, one may say that each
type of pyroxene has a corresponding amphibole and in
this connection the composition of the pyroxenes should
be studied.

Thus tremolite, if simply represented by CaMgs (8103)4
corresponds to diopside CaMg(SiOs)2: while actinolite,

Ca(MgFe)3(Si0 3 )4,

with variable amounts of ferrous iron replacing magnesium
corresponds to common pyroxene, Ca(MgFe) (8103)2;
common hornblende or hornblende for short which consists
of the actinolite molecule with others in which radicals
containing alumina or ferric iron and usually both, are
present and perhaps some alkalies, corresponds in general
to augite which is a variable mixture of pyroxene molecules
with alumina and ferric iron; arfvedsonite, which con-
tains chiefly soda, lime and ferrous iron, plays the part of
aegirite, the soda iron pyroxene, though a very rare



64



ROCKS AND ROCK MINERALS



variety, riebeckite, more nearly corresponds in com-
position.

Glaucophane is a rare variety, consisting of a mixture of a soda-
alumina molecule with a hypersthene molecule,

NaAl(SiO 3 ) 2 . (FeMg)SiO 3 .

It is distinguished from other hornblendes by its blue color, often a
rich sky-blue or lavender-blue. It occurs only in a rare variety of
hornblende-schists, called glaucophane-schists, which are described
under amphibolites.

The chemical composition is illustrated in the following
table of analyses.





Si0 2


A1 2 3


Fe 2 3


FeO


MgO


CaO


Na 2 O


H 2 O


F 2


XyO*


Total


I


57.5


1.3


0.2


0.2


24.9


12.8


0.7


1.3


0.8


0.6


100.3


II


56.1


1.2


0.8


5.5


21.2


12.1


0.2


1.9


0.1


0.6


99.7


III


41.9


11.7


2.5


14.3


11.2


11.5


2.7


0.7


0.8


2.6


99.9


IV


43.8


4.4


3.8


33.4


0.8


4.6


8.1


0.1




1.5


100.5


V


55.6


15.1


3.1


6.8


7.8


2.4


9.3






0.5


100.6



* XyO = small quantities of minor components.

I, Tremolite, Richville, Gouverneur, New York; II, Actinolite
Greiner, Tyrol; III, Hornblende, Edenville, Orange County, New
York: IV, Arfvedsonite, Kangerdluarsuk, Greenland; V, Glauco-
phane, Island of Syra, Greece.

Blowpipe and Chemical Characters. Tremolite, actino-
lite and common hornblende melt quietly or with little
intumescence before the blowpipe, fusing rather easily at
4. The color of the bead depends on the amount of iron,
tremolite nearly colorless, actinolite green or brown;
common hornblende dark and shining. Common horn-
blende sometimes colors the flame yellow, indicating soda.
Arfvedsonite fuses easily (3.5), colors the flame strong,
persistent yellow, intumesces decidedly (difference from
aegirite) and yields a black, shining, magnetic bead. The



ROCK-MAKING MINERALS 65

amphiboles are only slightly attacked by the ordinary
acids, those rich in iron more than those without.

Alteration. The amphiboles have methods of alteration
similar to those of the pyroxenes. Under the action of
various agencies they may be changed into serpentine or
into chlorite or into both, accompanied by the formation
of carbonates, sometimes of epidote and also quartz.
Under the continued action of weathering they may
break down further in limonite, carbonates and quartz.
Thus on much weathered rock surfaces only rusty-looking
holes and spots may be left to show their former presence.

Occurrence. Amphiboles are common and widely
distributed minerals playing an important role in igneous
rocks and especially in the metamorphic ones. The
presence of water, hydroxyl and fluorine in them shows
that they are not formed by simple reactions like the
pyroxenes but require the presence of mineralizing
vapors; they are in some sense pneumatolytic minerals.
Thus they cannot be artificially formed by allowing simple
dry fusions containing their constituent silica and metallic
oxides to cool and crystallize; pyroxenes are produced
instead of them. And if hornblendes are fused and the
melt allowed to crystallize we obtain pyroxenes, iron ore,
etc., in their place; this is because the necessary water and
fluorine have escaped.

Tremolite is chiefly found in the impure crystalline limestones and
dolomites in the older schistose metamorphic rocks and in contact
zones. In such occurrences it not infrequently has an extra-
ordinarily fine fibrous structure and is capable of being split into
long, flexible fibers of great fineness and strength, forming the
greater part of what is known as asbestus. Sometimes actinolite
and other hornblendes are found in this asbestus form. Some so-
called asbestus is really a fibrous variety of serpentine.

Actinolite has its true home in the crystalline schists; it is the
characteristic light green to bright green amphibole of many horn-
blende-schists and greenstones : in many of these cases it is second-
ary after original pyroxene of former gabbro and trap rocks as
described under uralite.

Common hornblende occurs both in igneous and metamorphic



66 ROCKS AND ROCK MINERALS

rocks. It is found in granites, common syenites, and in the doleritic
types; is in diorite and some varieties of peridotite. It may also be
often observed in the phenocrysts of felsitic intrusive porphyries and
lavas. In dark traps and basalt lavas it is rare. In the meta-
morphic rocks it is found in gneisses and is the prominent mineral of
the hornblende schists.

Arjvedsonite occurs in nephelite syenites and in rare porphyries.

Uralite is a fibrous or fine needle-like, columnar hornblende,
secondary after pyroxene and as mentioned under that mineral
produced from it by metamorphic processes. Instances have been
found where the outward crystal form of the pyroxene is retained
but the substance composing it is this hornblende in parallel bundles
of needle-like prisms. Generally it is in aggregates, which may be
very fine and felt-like, lying in the plane of schistosity. It is espe-
cially apt to occur when basic, pyroxenic, igneous rocks have been
subjected to dynamic changes in the earth's crust attended with
squeezing and shearing. It varies in composition from actinolite
to common hornblende, depending on the kind of pyroxene from
which it was derived. It is clear that it cannot be a simple rearrange-
ment of the pyroxene molecule since the latter has twice as much
lime as the hornblende and is lacking in the necessary water or
fluorine. Lime is separated out in the process to form a carbonate
(calcite) or some other mineral and the presence of water, containing
often other substances in solution, is a necessary aid to the dynamic
processes of pressure and shearing which set up chemical activity
and the reactions which produce this mineral.

In this connection the reader should consult what is said under
metamorphism and the hornblende-schists.

Determination. Amphibole may be confused in mega-
scopic work with pyroxene, tourmaline and epidote. To
distinguish it from the last two use may be made of the
various physical properties mentioned under the deter-
mination of pyroxene; the good cleavage separates it at
once from tourmaline. The distinction from pyroxene
is much more difficult, owing to the fact that these two
minerals have similar chemical compositions and physical
properties. The following points will be found of service
in this connection. If the mineral appears in tolerably
distinct crystals the form should be carefully studied,
especially the outline of the section of the prism which
can often be observed on a fractured surface of the rock
and comparison made with Figs. 23 and 28.



ROCK-MAKING MINERALS 67

In case the crystal form is imperfect or wanting, if it is
possible, the angle at which the cleavage surfaces meet
should be carefully studied, as this is a fundamental
character, the cleavage prism as already described being
nearly square in pyroxene and much more oblique in
amphibole. Further, the perfection of the cleavage in
amphibole and the bright glittering surfaces it yields
furnish indications not commonly seen in pyroxene whose
cleavage is only fairly good. Amphibole also is apt to
occur in needles or long bladed prisms; pyroxene is com-
monly in short prismoids or grains. Before the blow-
pipe amphibole, on account of the combined water
(hydroxyl), is more apt to intumesce than pyroxene
(arfvedsonite from aegirite) but this cannot be relied on as
a general definite test. If fluorine is obtained by a
qualitative test this is also indicative of amphibole, but
many do not contain this element and it is not a method
which is ordinarily in one's power to make. Finally, in
many cases, especially in fine grained igneous rocks, it is
impossible by purely megascopic means to tell if the dark
ferromagnesian mineral present is hornblende or pyroxene
or, as often happens, a mixture of both. Only in a thin
section under the microscope can this be certainly deter-
mined. This is a limitation which the megascopic method
for the study and determination of rocks and rock-minerals
imposes.

OLIVINE.

Form. Olivine crystallizes in the orthorhombic system ;
the crystals are rather complex as illustrated by a common
form shown in Fig. 29. The form is not, however, a matter
of importance, as the mineral very rarely shows well
developed crystals in rocks but occurs in grains or small
formless masses composed of grains.

General Properties. There is a cleavage parallel to the
face b but it is not a very perfect or noticeable property
in rock grains. The fracture is conchoidal. The color is



68



ROCKS AND ROCK MINERALS



green, generally of a medium shade and varying from
olive-green to a yellow-green; a bottle-green is very
common. It is often transparent varying
to translucent but becomes brown to dark
red on oxidation of the iron and more or
less opaque; this is frequently noticed in
lavas which have been exposed to the action
of steam. Luster vitreous; streak white
to yellowish. Hardness 6.5-7.0. Specific
gravity varies with the iron from 3.3-3.5.
Chemical Composition. Olivine is magnesium ortho-
silicate, Mg2Si04, and ferrous orthosilicate, Fe2SiO 4 , which
mingle isomorphously in all proportions. The nearly
pure magnesium compound is called forsterite, the nearly
pure iron compound faijalite', these occur in rocks but are
rare. Much more common are variable mixtures of the
two which make common olivine or chrysolite as it is
often called. These variations may be seen in the fol-
lowing table of analyses.



Fig. 29





SiO 2


MgO


FeO


XyO*


Total.


I .


41.8


56.2


1.1


0.7


99.8


II ....
Ill ....

IV ....
V


39.9
37.2
41.9
33.6


49.2
39.7
28.5
16.7


10.5
22.5
29.2
44.4


5.0


99.6
99.4
99.6
99.7


VI ....


30.1




68.2


1.5


99.8



* XyO = small quantities of other oxides, chiefly MnO.

I, Forsterite, Monte Somma, Italy; II, Olivine, Mt. Vesuvius,
Italy; III, Olivine, Montarville, Canada; IV, Olivine, Hochbohl,
Germany; V, Hortonolite, Monroe, Orange Co., N. Y.; VI, Fayalite,
Rockport, Mass.

Blowpipe and Chemical Characters. Before the blow-
pipe nearly infusible; varieties very rich in iron fuse and
yield magnetic globules these are apt to turn red on
heating. The powdered mineral dissolves in hydrochloric



ROCK-MAKING MINERALS 69

or nitric acid, yielding gelatinous silica on evaporation.
The solution may be tested for iron and magnesium as
directed under mineral tests.

Alteration. In one case this takes place through oxida-
tion of the iron, the mineral turns reddish or brownish,
and eventually a mass of limonite replaces it, accompanied
with carbonates and some form of silica. The rusty iron
product is the most noticeable feature of the process.

A most important mode of alteration is that by which
the olivine becomes converted into serpentine. This
appears to take place through the agents of weathering
near the surface and deeper down through the action of
heated waters. This is more fully discussed under the
head of serpentine. Other substances such as carbonate
of magnesia, iron ores, free silica, etc., are also liable to
occur as by-products in the process. Other kinds of
alteration of olivine are known but are of less importance
in this connection.

Occurrence. Olivine is a quite characteristic mineral of
igneous rocks, especially the ferromagnesian ones. It so
rarely occurs in those composed chiefly of alkalic feldspars
in the granite-syenite rocks, feldspathic porphyries
and felsite lavas that for practical purposes it need not
be sought in them. Anorthosite is the only feldspathic
rock in which it may become of importance. Thus its
true home is in the gabbros, peridotites and basaltic
lavas. In the later it usually occurs in bottle-green grains;
in the former it is sometimes colored dark by inclusions.
It also forms masses of igneous rock known as dunite
which consist almost wholly of olivine. Fine trans-
parent crystals of olivine from basaltic lavas are fre-
quently cut for gems, commonly called peridotes. The
mineral is also often found in meteorites.

Olivine also occurs in metamorphic rocks, in crystalline lime-
stones of dolomitic character and in other rocks found in such
associations, composed of varying quantities of other magnesian
(and lime) silicates, such as amphibole, pyroxene and talc. Its



70



ROCKS AND ROCK MINERALS



origin may be ascribed to a reaction between the magnesium car-
bonate of the dolomite and quartz sand or silica-bearing solutions.

2 MgCO 3 + Si0 2 = Mg 2 SiO 4 + 2 C0 2

But in many such cases of its occurrence in the crystalline schists.
mixed more or less with other silicate minerals, its presence is prob-
ably due to the fact that the masses containing it were originally of
igneous origin, rather than metamorphosed sedimentary beds.

Determination. The appearance, associations and
characters described above are usually sufficient to readily
identify the mineral. It may be confused with greenish,
more or less transparent grains of pyroxene, but the lack of
pronounced cleavage, the superior hardness and easy
gelatinization in acid enable one to distinguish it from that
mineral.

GARNET.

Form. Garnets crystallize in the isometric system in
the simple form of the rhombic dodecahedron shown in

Fig. 30 or in the trapezo-

hedron shown in Fig. 31.

Sometimes, they show

these forms well developed

and are then excellent

crystals, which may be

more complicated by bev-

ellings or truncations of
the edges of the dodecahedron. Very commonly how-
ever the faces are not well developed and the mineral
then appears as a spherical mass or grain.

Cleavage and Fracture. The cleavage is generally poor
and not a prominent feature; sometimes a parting, in
garnets occurring in sheared rocks, may be seen which
suggests a lamellar structure. The fracture is uneven.
The mineral is very brittle but some rocks composed
largely of massive garnet are very tough.

Hardness and Specific Gravity. The hardness varies
from 6.5-7.5; the specific gravity from 3.55 in grossularite
to 4.2 in almandite, common garnet being about 4.0.





Fig. 30



Fig. 31



ROCK-MAKING MINERALS



71



Color, Luster and Streak. The color depends upon the
composition; grossularite is sometimes white but usually
tinted pale tones of green, pink or yellow, sometimes
yellowish or reddish-brown to brown; pyrope is deep red
to black; almandite and most common garnet is deep red
to brownish-red; melanite is black. Streak, light-colored,
not important. The luster is glassy, sometimes rather
resinous. The light-colored garnets are transparent to
translucent, the darker ones translucent or opaque.

Chemical Composition. Garnets are orthosilicates of
the general formula R'sRgCSiO^a, in which the radical R
may be calcium, magnesium, ferrous iron and other
bivalent metals, while R may be aluminum, ferric iron or
chromium, trivalent elements. There is therefore oppor-
tunity for a number of combinations which are isomor-
phous. The most common ones which are of importance
as rock minerals are grossularite , CasA^CSiO^s, pyrope,
, almandite, FesA^SiO^s and andradite,
These compounds, however, rarely, if
ever, occur pure, generally there are variable amounts of
the other molecules present and the mineral is named
from the one predominating. Common garnet is chiefly
almandite with more or less of the others present, especially
the andradite molecule, and at times this may predom-
inate. Melanite, the black garnet found in some rocks,
is chiefly andradite. These facts are illustrated in the
following analyses of typical specimens.





SiO 2


A1 2 3


Fe 2 3


FeO


MgO


CaO


XyO


Total.


I


39.8


22.1


1.1




0.7


36.3




100.0


II . .


40.4


19.7


4.0


6.9


20.8


5.8


2.6


100.2


Ill


39.3


21.7




30.8


5.3


2.0


1.5


100.6


IV. .


35.9


19.2


4.9


29.5


3.7


2.4


4.8


100.4


V . .


35.7


0.1


30.0


1.2


0.1


32.3


0.9


100.3



I, Grossularite, Hull, Ontario. II, Pyrope, Krems, Bohemia,
XyO = Cr 2 O 3 . Ill, Almandite, Fort Wrangell, Alaska, XyO = MnO.
IV, Common Garnet (mostly almandite), Shimerville, Perm.

XyO = MnO; V, Andradite, Sisersk. Ural Mts.



72 ROCKS AND ROCK MINERALS

Blowpipe and Chemical Characters. The garnets fuse
readily before the blowpipe and in the reducing flame
those containing much iron become magnetic. After
fusion and grinding of the bead to powder they dissolve
in hydrochloric acid with gelatinization on boiling. They
are slightly attacked by acids, andradite quite strongly.
Give little or no water by heating in closed glass tube.
Decomposed by fusion with sodium carbonate.

Alteration. Garnets change into other substances,
commonly chlorite, serpentine, etc., and those containing
iron oxides may alter into rusty spots of limonite and
other products of weathering.

Occurrence. Common garnet is a widely distributed
mineral as an accessory component of metamorphic and
sometimes igneous rocks. Its most striking occurrence
is in schists, especially in many mica-schists though it is
also found in other kinds, in many hornblende-schists and
in gneisses for example. It is apt to occur in the ferro-
magnesian igneous rocks which have been squeezed and
sheared. It is sometimes seen in granite-pegmatites,
rarely in granite itself, in occasional scattered crystals. It
also occurs in the contact zone of igneous rocks where
mixed beds containing clay, calcareous matter and
limonite have been metamorphosed. Pyrope, which
chiefly furnishes the garnet used as a jewel, is an acces-
sory component of some peridotites and the serpentines
derived from them. Grossularite is especially found in re-
crystallized limestone beds both in contact and regional
metamorphism. Melanite occurs mostly in certain
igneous rocks and is not an important megascopic
mineral.

Determination. The crystal form of garnets, the
appearance, color and hardness are generally sufficient
to enable one to easily recognize them and in case of
doubt the blowpipe tests will furnish sufficient confir-
mation.



ROCK-MAKING MINERALS



73



Fig. 33



EPIDOTE.

Form. Epidote crystallizes in the monoclinic system,
the simplest form being that shown in Fig. 32, the crystals
are apt to be more complex with
other faces. Well-developed crystals >

usually occur only in druses in seams V

and cavities and the form is there- \ r \
fore not generally a character which
can be of much use in megascopic rock
determination. Commonly seen in bladed prisms extended
in the direction of the edge ac and sometimes passing
into slender, needle-like forms. Often in bundles or
aggregates of prisms or needles. Terminations of prisms
often rounded. Also occurs in spherical and angular
grains and in aggregates of such grains.

General Properties. The cleavage is perfect parallel to
c, parallel to a imperfect. Fracture uneven. Brittle.
Hardness is 6-7. Specific gravity is 3.3-3.5. The
color in general is green, usually of a peculiar yellowish,
oily green; varying from pistache-green to olive, some-
times very dark green; rarely brownish. Luster vitreous.
Streak whitish. Translucent to opaque.

Chemical Composition. Epidote is really the name of a
group of complex silicates, salts of orthosilicic acid whose
hydrogen atoms are replaced by calcium and by a set of
isomorphous radicals composed of variable amounts of
alumina, ferric iron and sometimes other oxides and of
hydroxyl. Of these only common rock-making epidote
is described in this section and its formula may be repre-
sented as being mixtures of Ca 2 (AlOH)Al 2 (Si0 4 )3 and
Ca 2 (FeOH)Fe2(Si0 4 )3. The composition may be seen
in these two specimen analyses.





SiO 2


A1 2 0,


Fe 2 3


FeO


CaO


H 2 O


XyO


Total.


I


37.8


22.6


14.0


0.9


23.3


2.1




100.7


II ....


37.0


25.8


10.0


1.3


21.9


3.0


1.0


100.0



I, Untersulzbach, Pinzgau. II, Macon Co., North Carolina,
XyO = MnO and MgO = 0.5 each.



74 ROCKS AND ROCK MINERALS

Blowpipe and Chemical Characters. Before the blow-
pipe epidote fuses easily with intumescence to a black
slaggy mass. Intense heating in the closed glass tube
causes the finely powdered mineral to give off water.
Only slightly acted on by hydrochloric acid but after
fusion dissolves and gelatinizes. Reacts with fluxes
for iron and decomposes on fusion with sodium car-
bonate.

Occurrence. Epidote is characteristic as a product of
alteration of other minerals. It appears through the
weathering of igneous rocks which contain largely original
lime, iron and alumina silicates and is then usually with
chlorite. When igneous rocks of this character also
suffer regional metamorphism epidote is apt to form.
The occurrences in which it appears most notable from the
megascopic view point are those in which mixed sedimen-
tary beds containing calcareous matter, with sand clay
and limonite (impure limestones) are subjected either to
general or contact metamorphism. Then epidote is apt
to be formed, usually in company with other silicates,
but sometimes so extensively as to form masses which
consist almost entirely of this mineral.

Determination. The peculiar yellow-green color, superior
hardness, perfect cleavage in one direction only and
the blowpipe characters described above generally suf-
fice to distinguish epidote from hornblende, pyroxene
and possibly tourmaline with which it might be con-
fused. The hardness distinguishes it at once from
some varieties of serpentine which resemble it in color.
This may be confirmed by a chemical test showing the
absence of magnesia as described in the section on
mineral testing.

Zoisite. This is a mineral which has the same chemical
composition as epidote and is closely related to it. It
consists almost wholly of the lime-alumina molecule pre-
viously mentioned and contains little or no iron oxides.
It is orthorhombic in crystallization but in the crystals




ROCK-MAKING MINERALS 75

seen in rocks this can generally only be told by optical
methods : it occurs in aggregated blades or prisms, parallel
or divergent or in grains and masses. Its color is usually
gray of varying shades. From epidotes lacking in iron it
can only be told by crystallographic investigations.

VESUVIANITE.

Vesuvianite is a tetragonal mineral which generally
crystallizes in short thick square prisms terminated by a
pyramid commonly cut off by a basal plane
as illustrated in Fig. 33. It also occurs
in lumps or grains. The cleavage is poor,
best parallel to the prism faces m; fracture