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

crystal form is imperfect or wanting. Carlsbad twins
of the character described are found of all the different
varieties of feldspar; they are most perfectly developed
in the phenocrysts of the porphyritic igneous rocks,
especially in the large orthoclase phenocrysts of some
granite porphyries.

In the Carlsbad twin the plane of division of the two
parts is one parallel to the face b; the axis
on which one part is revolved is the vertical
line parallel to the edge ab of Fig. 10 and
not one perpendicular to b or parallel to the
edge ac which is usually the case in twinning,
as already mentioned. The face c in ortho-
clase makes a right angle with 6; the outline
of the face a is, therefore, a rectangle, and if the crystal
were divided along the dotted line by a plane parallel to 6
and one of the halves revolved 180 degrees on an




Fig. xo



38



ROCKS AND ROCK MINERALS



parallel to the edge ac, that is, perpendicular to 6, it would
appear precisely as before and no twinning would occur.
The crystallographic reason for this is that & is a symme-
try plane, since the crystal is monoclinic, and a symmetry
plane cannot be a twinning plane.

In the plagioclase group, in albite, anorthite and their
admixtures, the face c makes an oblique angle with the
face 6; the face a is, therefore, a rhomboid and not a
rectangle as shown in Fig. 11: if this crystal is divided
along the dotted line and one of the halves revolved 180
degrees it will present the appearance seen in Fig, 12;
the face c and the lower c now brought on top slope toward





M



Fig. ii



Fig. 12



Fig. i 3



each other, forming a re-entrant angle, while below they
produce a salient angle. A twin crystal is, therefore,
produced, and this kind of twinning is known as the albite
method because it is so generally found in that variety
of feldspar. A complete crystal of this kind is seen
in Fig. 13. The crystallographic reason that this can
occur is because these feldspars are triclinic; they have,
therefore, no symmetry plane, and any one of the faces
might serve as a twinning plane.

Multiple Twinning. In nature, in actual practice, we
rarely find a single albite twin of the kind described above.
In the rock-making plagioclases the crystals are divided
again and again into thin slices, and these are alternately
twinned upon one another, producing the effect seen in
Fig. 14. Indeed, this albite twinning descends to such
a remarkable degree of fineness that the twin layers are



ROCK-MAKING MINERALS



39



less than the one hundred thousandth of an inch in thick-
ness and are scarcely to be perceived in thin sections in
polarized light under the highest powers of the micro-
scope. It frequently happens, however, especially in
those feldspars containing much lime, like labradorite,
that it is coarse enough to be readily seen by the naked
eye; one cleavage surface of such a feldspar appears as if





Fig.



Fig. 15



ruled by fine parallel lines or striations as illustrated in Fig.
15. Even when very fine and on a small cleavage surface of
a feldspar grain embedded in the rock, by a proper adjust-
ment of the light reflected from the surface and the use of
a good lens this multiple twinning may be distinctly seen.
Sometimes feldspars are twinned both according to
the Carlsbad and the albite laws; they
may be seen divided into the Carlsbad
halves by. the reflection of light from
the cleavage and each of these ruled
by the fine lines of albite twinning.
An illustration of the combination of
these two, each Carlsbad half divided
into albite halves, is seen in Fig. 16.
The practical use of the twinning of feld-
spars is explained in the paragraph on methods for their
determination. Other methods of twinning beside those




Fig. 16



40 ROCKS AND ROCK MINERALS

mentioned occur in the feldspars, but in the megascopic
study of rocks they are not of importance.

Cleavage. All the different varieties of feldspar are
alike in possessing a good cleavage in two directions, one
parallel to the face c and another parallel to b (see Fig.
7). Since in orthoclase these two faces intersect at a
right angle, so also do the cleavages, and from this fact its
name is derived (Greek, op0o<s } straight, right -f- K\dv,
to break) ; in the lime-soda feldspars, albite to anorthite,
these faces are slightly oblique, and so are the cleavage
planes; hence the name plagioclase (Greek, 7r\ayios }
oblique + K\av } to break) has been given to the group.

In rocks, if the feldspar grains are of good size, the
cleavages are readily seen by reflected light; they are com-
monly interrupted, giving rise to steplike appearances.
Even when the grains are small the cleavage can usually be
detected with a lens in good light. Sometimes when the
feldspars are more or less altered, as described under
alteration, they lose more or less completely their capacity
for showing good cleavage faces on a broken surface of
the rock, and this fact must be taken into account in
making determinations. As in the crystals which show
distinct faces, so in cleavage pieces, the amount of obliquity
of the plagioclases is too small to be used in distinguishing
them from right angled orthoclases by the eye or lens.

On a fractured rock surface if the crystal grains are of
sufficient size the cleavages frequently permit one to
recognize that they are twinned according to the Carlsbad
method. The grain or broken crystal appears divided
into two parts by a distinct line; on one side of this, if the
line points away from the observer, the cleavage sur-
faces slope or step away in one direction ; in the other half
they slope towards the observer at an equal angle, like
the two c faces in Fig. 8, to which indeed they are
parallel. This can usually be readily seen by shifting the
position of the surface in a good light until the cleavages
reflect it. At the same time if examined with a good



ROCK-MAKING MINERALS 41

lens they may often be seen to be ruled by the fine parallel
striations of the albite twinning, which indicates that the
feldspar grain is a plagioclase.

Fracture. In directions in which they do not cleave
the fracture of feldspars is uneven and sometimes some-
what conchoidal. They are brittle.

Color, Luster and Streak. Feldspars do not possess
any natural color, hence, as explained under the color of
minerals, they should normally be either limpid and color-
less or white. Transparent, colorless, glassy feldspars in
rocks are confined to fresh and recent lavas in which they
may be frequently seen in the phenocrysts; they practi-
cally never occur in massive granular rocks like granites,
gneisses, etc. In such lavas the luster may be strongly
vitreous. More commonly they are semi-translucent or
opaque and white, grayish white or yellowish and of a
somewhat porcelain-like appearance. Orthoclase and
the alkalic group of feldspars in general are very apt to
have a tinge of red; this color varies from a pale flesh
color to a strong brick-red or brownish red; a distinct
flesh color is the shade most common. It is this which
gives many granites used for building stones their color.
It is most probable that this variety of color is caused by
finely disseminated ferric oxide dust which acts as a pig-
ment, and it must be considered as exotic and not a natural
color. The plagioclases or lime-soda feldspars more rarely
show this; they are commonly gray, and the difference
between the two classes of feldspars is apparently due to
a difference in the chemical behavior of iron towards soda
and potash; soda enters readily into combination with
iron in silicate minerals, while potash does not. Thus
in the potash feldspars the iron would tend to be present
as free oxide and color them. Therefore rocks with
potassic feldspars often tend to be of reddish color, those
with sodic feldspars tend to be gray. This distinction
may be used to some extent as an indicator of the kinds
of feldspar, but it must never be taken as an absolute



42 ROCKS AND ROCK MINERALS

rule, because many potassic feldspars are white or gray,
and conversely many instances occur where rocks with
soda-lime feldspars are red. In general one may say
that if the rock contains two feldspars one of which is red
while the other is not, it is almost certain that the red
feldspar is a potassic one or orthoclase.

The potassic feldspars, especially the variety called microcline
when occurring in distinct crystals in the miarolitic druses of granitic
rocks, have sometimes a green color, pale to bright grass-green.
This is also an exotic coloration and is supposed to be due to some
organic substance acting as a pigment, since it disappears on heating.

Sometimes the rock feldspars are gray, dark, smoky or bluish-
gray or even black. While this may happen with alkalic varieties^
it is much more common with the soda-lime ones, especially lab-
radorite. It is caused by a fine black dust disseminated through
them which acts as a pigment and which may sometimes be mag-
netite dust, but is much more often ilmenite, titanic iron ore.
Pine examples of these are seen in the labradorite rocks from Canada,
the Adirondack region in New York State and from Labrador which
have been called anorthosites. Sometimes these inclusions are of
sufficient size and so regularly arranged in the feldspar that, by the
interference of light, they produce an opalescence or play of colors
in the mineral as seen in the beautiful examples from St. Paul's
Island on the coast of Labrador and from Kiev in Russia.

In other cases feldspars have a pearly bluish opalescence from
innumerable minute cracks regularly arranged which reflect light
with interference colors.

The luster is vitreous and on cleavages often pearly.
Feldspars which are more or less altered often have a
waxlike appearance and a waxy, glimmering luster; if
completely altered they may look earthy and have no
luster.

The streak is white and not characteristic.

Hardness. This is 6. Scratched by quartz, scratches
glass, but is not scratched by the knife.

Specific Gravity. Orthoclase = 2.55, albite = 2.62,
anorthite = 2. 7 6. That of the various mixtures varies



ROCK -MAKING MINERALS



43



between these limits; thus the alkalic feldspars which con-
sist of a mixture of orthoclase and albite average about
2.57, while the plagioclases vary regularly with the relative
amounts of soda and lime, that of labradorite being 2.67.
If the specific gravity of a fragment of feldspar can be
taken with accuracy to the second place of decimals it
affords a fairly good rough method of ascertaining its
composition.

Chemical Composition. This is shown in the following
table.





SiO 2


A1 2 3


CaO


Na 2 O


K 2 O


Total.


I


64.7


18.4






16.9


100


II


68.7


19.5




11.8




100


III


43.2


36.7


20.1






100


IV


55.6


28.3


10.4


5.7




100


V


66.7


18.9




5.7


8.7


100



I, Orthoclase (and microcline); II, Albite; III, Anorthite; IV,
Labradorite (equal mixture of albite and anorthite); V, Alkalic
feldspar (equal mixture of orthoclase and albite).

The mixtures vary naturally with the proportions
of the pure products; examples of equal parts are given
in IV and V. The other substances, such as iron oxide,
etc., shown in feldspars by chemical analyses, are due
to impurities.

Blowpipe and Chemical Characters. A fine splinter
fuses before the blowpipe with difficulty to a globular
ending, more easily with anorthite and the varieties rich
in lime than with albite and orthoclase. The flame shows
the persistent yellow coloration of soda; only occasionally
in the rock feldspars does orthoclase occur, which is pure
enough to give the violet flame of potash. Orthoclase
and albite are not acted upon by ordinary acids to an
appreciable extent; as the feldspars increase in lime they



44 ROCKS AND ROCK MINERALS

become more soluble, thus labradorite is very slowlj
dissolved while anorthite is slowly dissolved and affords
gelatinous silica.

Alteration. Under the action of various agencies the
feldspars are prone to alter into other substances, which
depend in part on the nature of the agents and in part on
the composition of the feldspar attacked. Some of these
changes and products are quite complex and their nature
and significance have not as yet been sufficiently studied
for us to understand them, but some of the simpler and
more important ones are as follows.

When the feldspars are acted upon by water carrying
carbonic acid gas in solution, which may be the case in
surface waters leaching downward or in hot waters rising
from depths below, they may be turned into kaolin or
muscovite with separation of free silica and alkaline
carbonates. These changes may be expressed chemically
as follows.

Orthoclase 4- Water + Carb. diox. = Kaolin + Quartz +Potas. Carb.

2KAlSi 3 O 8 +2H 2 O + CO 2 = H 4 AI 2 Si 2 O9 + 4SiO 2 + K 2 CO 3

Orthoclase + Water + Carb. diox. = Muscovite + Quartz. + Potas. Carb.

3KAlSi 3 O 8 + HaO + CO 2 =H 2 K(AlSiO 4 ) 3 + 6 SiO 2 + K 2 CO 3

What determines whether the removal of the potash from the
feldspar will be complete so that kaolin is formed or only partial
so that muscovite is the resultant product is not clearly understood.
In a general way one may say that weathering from the action of
surface waters generally forms kaolin while the change to muscovite
is more apt to be a deep-seated affair and is especially noted in
processes of metamorphism. In mines it is often seen that the
solutions which deposited the ores have altered the rocks enclosing
them, sometimes to kaolin, sometimes to a form of muscovite (sericite)
and sometimes to other products. It is due to this in great part
that such rocks are so often changed from their original fresh con-
dition.

All feldspars undergo similar changes to those men-
tioned, but in those which contain lime they are more
complex, as calcite, the carbonate of lime is also formed.
Accordingly, as this change to muscovite or kaolin is more



ROCK-MAKING MINERALS 45

or less complete, the feldspars lose their original bright
appearance and become dull and earthy in character; if it
is pronounced they are soft and may be cut or scratched
with the knife or even with the finger nail. In certain
changes in the lime-soda feldspars they have a faint,
glimmering luster, are semi-translucent, often of a pale
bluish or grayish tone, lose to a great extent their property
of cleavage and resemble wax or paraffin as mentioned
under cleavage. Often these changes do not take place
regularly through the whole mass of the crystal, some-
times the border is altered, sometimes the center only is
attacked and sometimes, especially in the lime-soda ones,
like labradorite, zones between the two are altered. If
the feldspars of a rock do not show bright, glistening
cleavage surfaces it may be considered practically certain
that they are more or less altered. These alterations of
the feldspars are of great importance in geologic processes
and especially in the formation of soils.

In addition to these alterations others are also known, thus under
some circumstances the feldspars are changed into zeolites and in
metamorphic processes those containing lime may take part with
other minerals in forming epidote, garnet, etc., changes which are
mentioned elsewhere.

Occurrence. The feldspars are of wide distribution,
perhaps more so than any other group of minerals. They
are found in all classes of rocks, in most of the igneous
ones, such as granites, syenites, porphyries and felsite
lavas; in the sedimentary ones in certain kinds of sand-
stones and conglomerates and in the metamorphic rocks
in gneisses. Since, so far as our knowledge extends,the
crust of the earth, underlying all the sedimentary beds of
all ages deposited upon it, is composed chiefly of granites,
gneisses, etc., in which feldspars are the main minerals,
it is not too much, perhaps, to say that there is more
feldspar in the world than any other substance of whose
occurrence we have knowledge.



46 ROCKS AND ROCK MINERALS

Determination. In general, the two cleavages at right
angles or nearly so, the vitreous luster, light color and
hardness, which resists the point of the knife, enable one
in the field to recognize the feldspar grains of rocks and to
distinguish them from the other common minerals,
especially quartz, with which they are usually associated.
Sometimes the crystal form may also be of assistance,
especially in porphyries. In addition one or more of the
various chemical and physical properties enumerated
above may be determined on separated fragments, if the
feldspar grains or masses are of sufficient size.

The determining of the different varieties of feldspar which may be
present in a rock is, however, a much more difficult task when only
megascopic means are employed. Sometimes the remarks made
under the heading of color will be of assistance. If the cleavage
surfaces are closely examined with a lens and the fine lines of stria-
tion of the albite twinning are found then one knows that a plagio-
clase feldspar is present, since orthoclase cannot have this twinning
as previously explained. The only practical exception to this rule
is that the large, often huge, crystals of potash feldspar found in
granite-pegmatite dikes are often not really orthoclase but micro-
cline, a tricJinic variety and a good cleavage surface of this ex-
amined in a strong light with a powerful lens frequently shows a
minute, scarcely perceptible, multiple twinning like the albite
twinning.

If no multiple twinning is seen it would not be, therefore, safe
to conclude that the feldspar is necessarily an orthoclase or alkalic
variety and not a plagioclase because this twinning, as already
stated, is often so fine that it cannot be detected with the lens and is
sometimes wanting. As the grain of rocks grows finer it becomes
increasingly difficult to detect, but a good training of the eye by
studying a series of rocks in which it is present in the feldspars is a
great help and eventually enables one to perceive it clearly in cases
where at first it could not be seen. The modern tendency on the
part of geologists to refer all difficulties in rocks to microscopic
examination of thin sections has led to a great neglect in the training
of the eye for megascopic determination of minerals in rocks with a
corresponding loss of efficiency in the field.

If the albite twinning is clearly seen in several of the feldspar
grains of a rock it may be quite safely concluded that a considerable
proportion of plagioclase is present and this may indeed be prac-



ROCK-MAKING MINERALS 47

tically the only feldspar present. If it cannot be seen plagioclase
may or may not be present.

Other means which may be resorted to are the determination of
the specific gravity, the behavior before the blowpipe, and with
acids, as previously mentioned, and the chemical tests for soda,
potash and lime, which suggest themselves to those experienced in
analytical chemistry. Further information in the subject should be
sought in the special manuals devoted to determinative mineralogy.

THE FELDSPATHOID GROUP.

The feldspathoid group owes its name to the fact, that,
like the feldspars, it is composed of minerals which are
silicates of alumina with soda, potash and lime and that
they are found in the same associations, accompanying
or replacing feldspars and playing a similar function in
the making of rocks. Unlike feldspars they are com-
paratively rare and are restricted entirely to certain kinds
of igneous rocks such as nephelite syenite. Thus in
treating of the occurrence of common rocks they are,
compared with the feldspars, of relatively much less
importance, but, in dealing with questions regarding the
origin of igneous rocks, they are of great significance.
The more important members of the group are nephelite
and sodalite, less common ones are noselite and hauynite,
cancrinite and leucite.

Nephelite. This mineral crystallizes in short, thick,
hexagonal prisms or tables with a flat base and top but it
rarely shows distinct crystal form in rocks. Most com-
monly it occurs in shapeless masses and grains like quartz.
Its normal color is white, but it is usually gray, varying
from light smoky to dark in tone, sometimes it is flesh
colored or brick-red. The white color may shade into
yellowish, the gray into bluish or greenish. Streak, light
not characteristic. Translucent. Its luster, when
fresh, is oily or greasy and much like that of quartz and,
like this mineral, it has no good cleavage and its fracture
is somewhat conchoidal. Brittle. Hardness, nearly that
of feldspar = 6. Specific gravity, 2.55-2.61. Its com-



48 ROCKS AND ROCK MINERALS

position is chiefly NaAlSiO 4 with a small varying amount
of potash replacing soda. Before the blowpipe a fine
splinter fuses quite readily to a globule tingeing the flame
deep yellow. Readily soluble in dilute acid with forma-
tion of gelatinous silica.

Sodalite. The form of crystallization is the isometric
dodecahedron, so often seen in garnet, but this rarely
occurs in rocks, the mineral commonly occurring in form-
less grains and lumps. It is sometimes white, pink, or
greenish gray, but the usual color is a blue of some shade,
often a bright sky-blue to dark rich blue. The blue color
may be due to a slight admixture of the lapis-lazuli
molecule acting as a pigment. Usually translucent.
Cleavage dodecahedral but not striking as a megascopic
property; fracture uneven to poorly conchoidal. Luster
vitreous to greasy. Streak, white. Hardness nearly
that of feldspar, 5.5-6. Specific gravity, 2.15-2.30. Its
composition is Na 4 (AlCl)Al2(Si04)3 and this may also be
expressed 3 NaAlSi0 4 . NaCl, but it should not be under-
stood from this that it consists of a mixture of nephelite
and common salt molecules; it is a definite chemical com-
pound into which the chlorine enters. Fuses rather
easily before the blowpipe with bubbling, coloring the
flame yellow. Easily soluble in dilute acids with forma-
tion of gelatinous silica; in the nitric acid solution chlorine
may be tested for with silver nitrate.

The other feldspathoids are less common and in their
general properties, modes of occurrence and functions as
rock minerals are similar to nephelite and sodalite, which
they are usually found associated with or in part replacing
in those rocks in which they occur.

Hauynlte and Noselite. These show characters like sodalite but
they differ from it in containing the radical SO 3 of sulphuric acid in
the place of chlorine and the best method of detecting them is by
the test for sulphuric acid with barium chloride in their nitric acid
solution. They differ from one another only that in hauynite a
part of the soda is replaced by lime while noselite is the pure soda
compound. Cancrinite is much like nephelite in its genera! prop-



ROCK-MAKING MINERALS 49

erties, it contains CO 2 in combination, which affords aid in detecting
it as explained later in testing minerals and rocks; its formula might
be written 8 NaAlSiO 4 . CaCO 3 . CO 2 . 3 H 2 O, but as in sodalite it
is not a mixture of molecules but a definite compound. The color
is variable but frequently a bright yellow to orange which may also
help in detecting it. It is supposed at times to be caused by the
alteration of nephelite, but in most cases, if not always, it is an
original mineral crystallizing from a molten magma, like nephelite
and feldspar.

Leucite is a rare feldspathoid crystallizing in isometric trape-
zohedrons, a form illustrated in garnet; the crystals when imperfect
appear spherical. Its cleavage is imperfect; fracture conchoidal;
color white to gray; luster vitreous. Hardness is 5.5-6; specific
gravity, 2.5. Before the blowpipe it is infusible and when mixed
with powdered gypsum gives the flame the violet color of potassium.
It dissolves in acids without gelatinizing. Its composition is
KAl(SiO 3 ) 2 . It occurs almost wholly in lavas and is nowhere
common except in those of central Italy, where the magmas are
characterized by a high content of potash. The most noted occur-
rence is in the lavas of Vesuvius, in some of which it is found in good-
sized, well-shaped crystals of the form illustrated in Fig. Blunder
garnet. Large crystals, altered, however, to other minerals, have
been found in certain syenites and related rocks in Arkansas, Mon-
tana, Brazil and elsewhere.

Alteration. The feldspathoids, like the feldspars, are
liable to alteration from the processes of weathering when