Page 92 - Petrology of Sedimentary Rocks

rather soft and highly susceptible to weathering so usually do not last long in sediments.
However, “eugeosynclinal” sandstones are full of them.

Volcanic Glass. These particles are formed usually by explosive eruption of
siliceous magmas. The glass, if unaltered, is isotropic and has an index well below
balsam (more basic glasses have higher indices, and exact measurement of the index can
be used to determine the silica content of the lava). Altered glass may be
recrystallized to very fine-grained feldspar or microcrystalline silica, or may be
changed to clays (usually montmorillonite) or zeolites or may be dissolved and partially
re-filled. Glass usually shows characteristic shard-like forms (representing the curved
surfaced of the bubble-filled viscous lava) which may be preserved even after total
alteration of clays. Hence, Bentonites sometimes show shard ghosts.

Large Micas (> 20 microns)

Muscovite and biotite are hydrous potassium aluminum silicates, with biotite
containing ferrous iron and magnesium in addition. Chlorite contains less silica and
lacks potassium, and is an alteration product chiefly of hydrothermal or metamorphic
origin. Biotite may become leached to a pale golden color through loss of iron on
weathering (“leached biotite”), or under hydrothermal attack or weathering may turn
from brown to green (“green biotite”), an intermediate stage in the passage of biotite to
chlorite probably under reducing conditions. All these species are monoclinic, flaky
minerals with biaxial, usually negative figures and small 2V. Sections cut across the
flakes are pleochroic with the dark direction N-S, and show parallel extinction, length-
slow, with maximum birefringence in 2d order reds, blues and greens. Basal sections
are very deceptive inasmuch as they show no cleavage, are almost isotropic, and show
no pleochroism, always giving the darkest color.

Color Br
NY
/duscovi te colorless 1.58 .036
Bioti te brown I .60- I .66 .040-.060
Leached bi oti te pale brown I .60- I .66 .040-.060
Green bioti te green I .60- I .66 .040-.060
Chlorite green I .57- I .60* .ooo-.o I o**
*Less common forms of chlorite have NY up to I .67.

**Chlorite commonly shows anomalous birefringence--peculiar rich blues,
gray-blues, russet browns, or sickly yellowish khaki color.

Mica in sediments rarely contains inclusions of zircon or rutile; usually it is free
of inclusions.

Availability. Large micas, although often conspicuous in hand specimens, rarely
form more than 2% of the rock even in the most micaceous specimens; altogether, it
forms probably less than l/2% of the terrigenous fraction of sediments. The boundary
line between micas and the coarser clay minerals such as illite and sericite is arbitrarily
set at 20 microns, so this percentage is subject to considerable revision depending on
the size limit adopted. Granites contribute chiefly biotite, with some muscovite in the
more acid types. In basic rocks and volcanics biotite is almost the only primary mica
present. Pegmatites contain chiefly muscovite, with a little biotite. The greatest

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