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 7 of 35)
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uneven. The color generally varies from
green to brown. The luster is vitreous.
Hardness, 6.5. Specific gravity about 3.4.
Subtransparent to subtranslucent. The
chemical composition of vesuvianite is not exactly known;
it is a silicate of calcium and aluminum containing hy-
droxyl and fluorine, but small amounts of ferric iron and
magnesium are generally present. The formula has been
written H 4 Ca 12 (AlFe) 6 Si 10 043 but this is probably not
correct. Before the blowpipe it fuses readily with intu-
mescence to a greenish or brownish glass, which gelatinizes
with hydrochloric acid. The fresh mineral is slightly
soluble in hydrochloric acid.

As a rock forming mineral vesuvianite characteristically
occurs in limestones which have become crystalline
through the contact action of igneous rocks and its forma-
tion is evidently conditioned by the pneumatolytic
emanations of water and fluorine from the igneous mag-
mas. In these occurrences it is commonly associated
with garnet, pyroxene, tourmaline, chondrodite, and other
contact minerals.

Vesuvianite may be confused with garnet, pyroxene,
epidote or hornblende, but the study of its crystal form,



its other physical characters and behavior before the blow-
pipe will generally serve to distinguish it from them.


Form. Staurolite is orthorhombic in crystallization
and usually in distinct crystals of the form shown in Fig.
34. They are often stout and thick, sometimes long and
more slender but not strikingly so. The angle of the

Fig. 34

Fig. 35

Fig. 36

faces m on m is 50 40'. They are terminated by flat
bases c, though it often happens these cannot be seen
in the rock. Staurolite is very apt to form compound
twinned crystals as shown in Figs. 35, 36. From this
fact its name is derived from the Greek, meaning a cross.

Physical Properties. The mineral has a moderate but
distinct cleavage parallel to the face b; the fracture is sub-
conchoidal. The color varies from a dark reddish or
yellowish brown to almost black, the light transmitted
through thin splinters appears almost blood-red. The
streak is white to gray. The hardness is 7-7.5, the specific
gravity 3.75.

Chemical Composition. The formula is rather complex,
(AlO) 4 (A10H)Fe(Si0 4 ) 2 , the alumina may be partly
replaced by ferric iron and the ferrous iron by magnesia,
as seen in the included analysis of a crystal from Franklin,
North Carolina. The percentage of silica is very low;

Si0 2 A1 2 3 Fe 2 3 FeO MgO H 2 O Total
27.91 52.92 6.87 7.80 3.28 1.59 = 100.37


staurolite is one of the rock-forming silicates containing
the least silica and this fact with the high alumina is
significant of its place and mode of origin in metamor-
phosed clay rocks.

Blowpipe and Chemical Characters. Staurolite is prac-
tically infusible before the blowpipe. It is almost in-
soluble in acids. It may be fused with carbonate of soda
and the resulting fusion after solution in hydrochloric
acid may be tested for alumina, iron and magnesia. It is
easily recognized by its color, crystal form, hardness,
method of twinning and mode of occurrence.

Occurrence. Staurolite occurs in the metamorphic
rocks; it is a highly characteristic mineral of the crystal-
line schists. It is found in mica schists, in certain slates
and sometimes in gneiss. Frequently it is associated
with dark red garnets in these rocks.


Andalusite is orthorhombic in crystallization and is usually seen
in rough prisms, nearly square in cross section. Sometimes the
prisms are collected in radiated groups. The cleavage parallel to
the prism is good in other directions poor. Fracture uneven to
subconchoidal. The normal color is white to pink or red to brown,
but the mineral is very apt to contain impurities, especially particles
of carbonaceous matter, which may color it dark or even black.
Often these are arranged in a symmetrical manner in the crystal so
that the cross section, when it is broken or cut, displays a definite
pattern, such as a white cross in a black square. This may help to
identify the mineral. It is usually subtranslucent in thin splinters.
Brittle. Hardness, 7.5. Specific gravity, 3.2. Streak, whitish.
The chemical composition is Al 2 SiO 5 = A1 2 O 3 . SiO 2 . It is insoluble
in acids but decomposed by fusion with carbonate of soda. Before
the blowpipe it is infusible; after moistening with cobalt nitrate
solution it turns a blue color upon intense ignition (as do also cyanite
and some other alumina minerals).

Andalusite is a mineral characteristic of metamorphism, and es-
pecially of the contact zones of igneous rocks, such as granite. It
is produced by the alteration of clay slates and shales as described
on a later page. It occurs in mica schists and gneisses; sometimes
though rarely it is found in granite.



Cyanite usually occurs in long bladod crystals which rarely show
distinct end faces, or in coarsely bladed columnar masses. It is
triclinic. It has one very perfect cleavage (parallel to the face a)
and another less so (parallel to 6); the angle between these is about
74 degrees. The color is white to pure blue, sometimes the center
of the blade is blue with white margins; rarely gray, green to black.
Streak whitish. Transparent to translucent. Luster vitreous to
pearly. Hardness varies in different directions from 5-7; least on
face a (best cleavage) greatest on face 6 (second cleavage). Specific
gravity, 3.56-3.67. Chemical composition, Al 2 SiO 5 , and other
chemical and blowpipe properties similar to those of andalusite,
mentioned above.

Cyanite is a mineral characteristically developed in regions sub-
jected to intense regional metamorphism. It occurs in gneisses and
in mica schists. In the latter case the mica is sometimes muscovite
and sometimes the soda-bearing variety, paragonite. It is often
associated with garnet, sometimes with staurolite or corundum. It
alters to talc and steatite.

Cyanite is easily distinguished from other minerals, especially
andalusite which has the same chemical composition by its form,
color, variable hardness, specific gravity and other properties.

Sillimanite. With andalusite and cyanite may be mentioned
sillimanite, a mineral which has a chemical composition identical
with them, but which is separated mineralogically because it has a
different crystal form as shown by its angles. Its chief importance
is microscopical, but it may sometimes be seen with the eye or lens,
mostly in gneisses or quartzites, as slender white or light-colored,
four-sided prisms, or radiated aggregates of them forming brushes.
Its blowpipe and other chemical characters are like those described
for andalusite and cyanite.


Tourmaline is a mineral of which there are a number of
varieties based on the color, which in turn depends on the
chemical composition. The chief ones are black, green,
brown and red, but of these the black variety known also as
schorl is the only one which is of importance in mega-
scopic petrography.

Form. Tourmaline crystallizes in the rhombohedral
division of the hexagonal system. The faces therefore



are in threes or multiples of three. A simple form is
shown in Fig. 37 and its appearance looking down upon
the upper end in Fig. 39. It consists of the
three-cornered prism m its edges bevelled
by the prism faces a and terminated by
the rhombohedron r. The crystals if well
developed are apt to be more complicated
than this, other faces being present and if
both ends are perfect they have unlike faces.
Though sometimes short and thick the crys-
tals are commonly elongated prisms, often
extremely long and thin. Very often also
the faces a and m oscillate or repeat so
that the prism is striated or channeled as
shown in Fig. 38 and the outline and appear-
ance from above is that seen in Fig. 40. This
spherical triangle cross section is very characteristic of the
prisms of rock-making tourmaline. It is rarely in form-

Fig- 37

Fig. 40

less grains or large shapeless masses. The slender prisms
and needles are apt to be aggregated together into bundles,
sheaves and radiate groups. The section of the latter
in rocks furnishes the so-called " tourmaline suns."

General Properties. Tourmaline has no good cleavage
and its fracture is rather conchoidal to uneven. It is
brittle. The color is black, the luster glassy, sometimes
dull, streak uncolored, not characteristic. Opaque. The



hardness is 77.5, the specific gravity 3.13.2. It becomes
electrified by friction.

Chemical Composition. Tourmaline is a very complex
silicate of boron and aluminium with hydroxyl and some-
times fluorine and with magnesium, iron and sometimes
alkali metals. It may be said to be a salt of an alumin-
ium-borosilicic acid in which the hydrogens are re-
placed by iron, magnesium, alkalies and aluminium in
varying amounts. This acid has been formulated as
H 8 Al 3 (BOH) 2 Si 4 O 13 and in common black tourmaline
the hydrogens are replaced mostly by iron or iron and
magnesia as shown in the analyses here given.

Si0 2

A1 2 3

Fe 2 3



Na 2 O

B 2 3

H 2 O














II .











I, Paris, Maine. II, Pierrepont,
quantities of other oxides, etc.

New York. XyO = small

Blowpipe and Chemical Characters. Difficultly fusible
before the blowpipe with swelling and bubbling. When
mixed with powdered fluor spar and bisulphate of potas-
sium momentarily colors the flame a fine green, showing
presence of boron. Decomposed on fusion with sodium
carbonate. Not acted on by acids but after fusion gelat-
inizes in hydrochloric acid.

Occurrence. Tourmaline is not a common megascopic
component of rocks, but it is of interest and importance
because it is perhaps the most common and typical
mineral which is produced in the pneumatolytic or
fumarole stage of igneous rock formation as described in
another place. This is shown by the boron, hydroxyl and
fluorine which it contains. Thus it is one of the most
common and characteristic accessory minerals found in the
pegmatite dikes associated with intrusions of granites; its



presence in granite indicates, as a rule, nearness to the
contact and in the rocks which have suffered contact
metamorphism it is very liable to appear. In this way
it is not infrequently found associated with certain ore
deposits. It appears at times also in gneisses, in schists
and in crystalline limestones of the metamorphic rocks
and its occurrence in these cases indicates that the meta-
morphism has been induced in part by the contact action
of igneous masses giving off water vapors and other
volatile substances. The beautiful red and green trans-
parent tourmalines which are valued as gem material
occur in pegmatite dikes of granite, often associated with
the common black variety. The red is usually found with
lepidolite the lithia mica.

Determination. The black color, crystalline form, and
mode of occurrence of common tourmaline are usually
sufficient to identify it. From black hornblende it is
easily distinguished by its lack of good cleavage, superior
hardness and the shape of the cross section of the prism and
this can be made certain by the blowpipe test for boron.


Topaz crystallizes in the orthorhombic system and the
form in which it is generally seen is in pointed prisms, as
illustrated in Fig. 41. There is a very
perfect cleavage parallel to the base c, at
right angles to the prism: the fracture is
uneven. The mineral is very hard = 8,
and brittle. The specific gravity is about
3.5. In color it is, while generally trans-
parent, often colorless, sometimes yellow
to brown - yellow, sometimes white and
translucent. The luster is vitreous. The
chemical composition as established by
Penfield is (AlF 2 )SiO 4 in which the fluorine may be
replaced in part by hydroxyl ( OH). Before the
blowpipe it is infusible. If fused in a closed tube with


previously fused and powdered phosphorus salt hydro-
fluoric acid will be given off which etches the glass and
deposits a ring of silica on the colder upper walls of the
tube. If the pulverized mineral be moistened with
cobalt nitrate solution and intensely heated before the
blowpipe on charcoal it assumes a fine blue color showing
presence of alumina.

Topaz, while not a common or important rock-forming mineral,
is a very interesting one as it is particularly characteristic of the
pneumatolytic stage in the formation of igneous rocks.

Thus it is found in crystals in the miarolitic cavities of granites
where the vapors have collected and in the same way in felsite lavas
(especially in rhyolite). It is also found in pegmatite dikes and in
the cracks and crevices of the surrounding rocks which have served
as channel ways for the escape of gases as explained under the
description of pegmatite dikes and of contact metamorphism. It
is apt to be associated in these occurrences with quartz, mica, tour-
maline and sometimes with cassiterite, tin ore.

The form, color, cleavage and great hardness of topaz, together
with its mode of occurrence, serve to readily distinguish it from
other minerals and the determination may be confirmed by the
chemical tests mentioned above.


Chondrodite is really one of a small group of minerals chon-
drodite, humite, clinohumite, etc. which are so closely allied in
all of their general properties that for practical megascopic rock
work they are indistinguishable and may all be comprised under
this heading. While the mineral is monoclinic it rarely shows, as
a rock component, any definite crystal form which is of value in
determining it, but appears as embedded grains and lumps. The
cleavage is not marked but is sometimes distinct in one direction.
Brittle; fracture subconchoidal. The color is yellow, honey yellow
to reddish yellow, to brown red. Luster vitreous. Hardness
6-6.5. Specific gravity 3.1-3.2. In chemical composition the
mineral is closely allied to olivine but differs in containing fluorine or
hydroxyl or both, as may be seen from the following formulas deduced
by Penfield.

Olivine = Mg 2 SiO 4 .

Chondrodite = Mgj;Mg(F,OH)] 2 (SiO 4 ) 2 .
Humite = Mg^Mg(F,OH)] 2 (Si0 4 ) 3 .
Clinohumite = Mg 7 [Mg(F,OH)] 2 (SiO 4 ) 4 .


As in olivine part of the magnesium is usually replaced by some
ferrous iron. The powdered mineral is slowly dissolved by hydro-
chloric acid, yielding gelatinous silica. The solution evaporated to
dryness, then moistened with acid and taken up in water, after the
silica is filtered off, yields tests for iron with excess of ammonia and
after its separation by filtering, for magnesia with sodium phosphate
solution. From olivine it may be distinguished by a test for fluorine
as described under topaz. Before the blowpipe it is nearly infusible.

The characteristic mode of occurrence of chondrodite is in lime-
stone, especially dolomite, which has been subjected to contact
action of igneous rocks. In them it forms yellowish or reddish
embedded grains or lumps usually associated with other contact
minerals such as pyroxene, vesuvianite, magnetite, spinel, phlo-
gopite, etc. The presence of the hydroxyl and fluorine shows its
derivation by pneumatolytic processes.

The appearance, color, mode of occurrence and associations are
usually sufficient to identify the mineral, and these may be con-
firmed by the chemical tests mentioned.

6. Oxides, etc.

The list of important rock-making oxides includes first,
silica, Si0 2 and then corundum, A1 2 O 3 . Then come the
oxides of iron which are of importance as nearly constant
accessory minerals in rocks and therefore have a wide
distribution. For this reason two other minerals not
oxides, pyrite, the sulphide of iron, and apatite, the phos-
phate of lime, are also here included. Limonite, the
hydrated oxide of iron, which is always secondary, is
placed with the other iron ores for the sake of convenience.


Form. Quartz crystallizes in the hexagonal system,
the ordinary form being a hexagonal prism terminated by
a six-sided pyramid. This form, which is the common one
for the crystals of veins and is illustrated in Fig. 42, is not
often seen in the quartzes of rocks, except in igneous rocks
which possess miarolitic or drusy cavities; into them the
rock-making quartzes project with free ends which show
crystal form. The large crystals seen in pegmatite veins
and which sometimes attain huge dimensions are only a



Fig. 4*

Fig. 43

manifestation of the same thing on a larger scale, as
explained under the pegmatite formation of igneous rocks.
In porphyries where quartz may have crys-
tallized free as phenocrysts it tends to take
the form shown in Fig. 43; the two pyramids
are present and the prism is very
short or even wanting. Since the
crystals are usually poorly devel-
oped, with rough faces, they appear
as spherical objects, like shot or
peas, embedded in the rock, with
round cross sections where broken
across on a fracture face. In general, quartz has no
definite form in rocks, especially in igneous ones like
granite, where, being usually the last substance to crys-
tallize, its shape is conditioned by the other minerals
which have already formed. In granites, therefore,
it commonly appears in small shapeless lumps and
masses, but in some of the fine-textured varieties the
quartz tends to appear in granules like those composing
lump sugar. In pegmatite dikes it appears on fracture
surfaces in curious script-like figures intergrown with
feldspar, forming the substance known as graphic granite.
Cleavage and Fracture. The cleavage of quartz is so
poor that for practical petrographic purposes it may be
regarded as not possessing any. It has commonly a good
conchoidal fracture which is a great help in distinguishing
it in granitic rocks but in some massive forms it is uneven
and splintery. The mineral is brittle to tough.

Color and Luster. Rock-making quartz varies in color
from white through shades of gray and dark smoky gray
or brown to black. The gray and smoky tones are most
common in igneous rocks and the white color in the sedi-
mentary and metamorphic ones but there is no absolute
rule about this. The black color is rare and mostly con-
fined to igneous rocks; sometimes in them it has a strong
bluish tone. The colorless, limpid quartz, so characteristic


of the crystals found in veins and geodes and deposited by
solution, is rare as a rock-making component but some-
times occurs as in some very fresh lavas. The mineral
may also at times possess an exotic color given it by some
substance acting as a pigment; thus it may be red from
included ferric oxide dust or green from scales of chlorite,
and in the sedimentary and metamorphic rocks, such as
quartzite,it may be very dark from included organic matter
or charcoal-like substance.

The luster varies from glassy to oily or greasy. The
streak is white or very pale colored and not a prominent
character. Hardness, 7. Scratches feldspar and glass
but is not touched by the knife. Specific Gravity = 2.66.

Composition. Pure silica, Si0 2 . This is the composi-
tion of the crystallized common rock-making quartz, but
certain massive varieties of silica, which are not crystallized
or not apparently so, and are of common occurrence and
sometimes take part in forming rocks, such as jasper,
opal, chert, etc., contain in addition more or less com-
bined water, while impurities like clay, oxides of iron,
etc., are usually present and give them distinctive colors.

Blowpipe and Chemical Characters. Quartz is infusible
before the blowpipe varieties dark from organic matter
whiten but do not fuse. Fused with carbonate of soda,
it dissolves with effervescence of C0 2 gas. In the sodium
metaphosphate bead a fragment floats without dissolv-
ing. It is insoluble in acids except hydrofluoric, HF.

Occurrence. Quartz is one of the commonest of all
minerals, and is universally distributed, occurring in
igneous, sedimentary and metamorphic rocks alike. Not
only does it form rocks in company with other minerals,
chiefly feldspar, but in pure sandstones and quartzites
it may be the only one present in the rock-mass. It is
indeed so common that, with the exception of the lime-
stones and marbles and dark heavy igneous rocks like
dolerite and basalt, its presence in rocks should at least
always be suspected.


Determination. The hardness of quartz, its lack of
cleavage, its conchoidal fracture and generally greasy
luster are characters which help to distinguish it, especially
from the feldspars with which it is so often associated.
The gray and smoky color it often has in granites and
other igneous rocks helps in the same way. It may be
confused with nephelite but this mineral is readily soluble
in acids with gelatinization and moreover is very rare.
These characters, with the blowpipe and chemical ones
mentioned above, will readily confirm its determination.

Opal, Jasper, Flint, Chalcedony, etc. Silica, in addition to
forming the crystallized anhydrous mineral quartz, occurs in non-
crystalline, amorphous masses which contain varying amounts of
water. Accordingly as the color, structure and other properties
vary, a great number of different varieties are produced which have
received particular names. For a description of them the larger
manuals of mineralogy should be consulted. They seem to have
been formed, in large part at least, by the evaporation of liquids
containing soluble silica, which on the drying down has been
deposited in an amorphous, more or less hydrated condition instead
of as crystalline quartz. Sometimes they are a mixture of quartz
particles or fibers mixed with amorphous material. This form of
silica is illustrated by the gelatinous product obtained when a
silicate like nephelite is dissolved in an acid and the resulting solution
evaporated. It is also formed in nature as a secretion from water
by various living organisms.

Amorphous silica is not a rock component of any megascopic
importance in igneous or metamorphic rocks, but in the sedimentary
ones it forms accompanying masses and sometimes beds which, al-
though not of wide general importance, may be of considerable local
interest and value. These are further noticed in their appropriate
places. It may also act as a cementing substance of the grains of
some rocks.


Form. The crystallization is hexagonal and the form
assumed is either a thick six-sided prism often swelling out
in the middle into barrel-like shape or in thinner six-sided
tables; also commonly in grains or shapeless lumps.
The thick and barrel forms are most common when it


occurs in massive rocks like the syenites, and they are
associated with the grains and lumps. Sometimes on
parting faces a multiple twinning resembling that illus-
trated as occurring on feldspars may be observed, pro-
duced however by another method.

Cleavage. Corundum does not have a good cleavage
but possesses a parting that appears like perfect cleavage
parallel to the base of the prism and also in three other
directions at an angle to it (parallel to the unit rhombo-
hedron). In large pieces these partings or pseudo-
cleavages may appear nearly at right angles and the
mineral has a laminated structure.

Color, Luster, and Hardness. Rock-making corundum
is usually dark gray to bluish gray or smoky. It is very
rarely blue forming the variety sapphire, while the red
variety or ruby is excessively rare. The luster is adaman-
tine to vitreous, sometimes dull and greasy in rock grains.
Translucent to opaque. It is the hardest of rock minerals
= 9. Brittle, though sometimes very tough. Specific
Gravity = 4.

Blowpipe and Chemical Characters. Before the blow-
pipe it is infusible. The powder moistened with cobalt
solution and intensely ignited turns bright blue showing
alumina. It is insoluble in acids. Its composition is
pure alumina, A1 2 O 3 . By the action of weathering and
alteration it is apt to change into muscovite.

Occurrence. In recent years corundum has been
recognized as an important primary mineral in the igneous
rocks of a number of regions, in syenites in Canada,

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 7 of 35)