Page 93 - Petrology of Sedimentary Rocks

source for the micas is, however, in the metamorphic rocks where shales are built up
into phyllites and schists consisting of biotite, muscovite and chlorite. Chlorite is also a
frequenty hydrothermal mineral, forming by the alteration of ferro-magnesian minerals.
Chlorite also forms by low-temperature subaqueous weathering of mafics.

Volcanic biotite is a very important variety. In general, although biotite is probably the
most important mica in the source rocks, muscovite is far more abundant in sediments
by a ratio of about 4:l. This is a result of the chemical instability of biotite. Hence,
when in a sedimentary rock biotite is more abundant than muscovite, two causes may be
responsible: either (I) erosion rate far overbalanced the rate of weathering in the
source area, or (2) the sediment has received contributions from volcanic rocks or
volcanic ash.

Volcanic biotite usually occurs in flakes showing partial hexagonal outlines, and
occurs usually in sporadic beds through the stratigraphic section, corresponding to
sporadic ash falls. It commonly appears thus in the insoluble residues from limestones.

Durability. Despite its low hardness of 2-3 and the good cleavage, mica is a very
durable mineral, extremely difficult to round and almost impossible to fracture. This is
becasue it is so elastic; also because of the flaky shape it tends to dodge around grains
in transport. Rarely, formations contain rounded flakes of mica but this requires
exceptional conditions of prolonged gentle abrasion. Modern dunes of southern
California contain coin-shaped rounded micas (Sims).

Stability. Moscovite is stable except under very warm and humid conditions.
Biotite is unstable, first becomes bleached through loss of iron, then if under reducing
conditions below the water table may lose alkalies and pass into chlorite or vermiculite.
Under oxidizing conditions both biotite and chlorite are believed to pass into Iimonite
and clay minerals.

Depositional Characteristics. Because of its flaky shape mica behaves hydrauli-
cally as a much smaller particle, therefore large mica flakes are usually washed out of
the coarser sands and deposited with fine silts.- The percentage of mica varies radically
from bed to bed depending on grain size. Flakes are very well oriented parallel to the
bedding in most cases. Randomly-oriented micas indicate slumping or some disturbance
of bedding.
Authigenic Micas. On metamorphism, clay minerals may be built up into
muscovi te, chlorite, or bi otite. Similar but not so profound changes take place on the
sea floor or during deep burial of the sediments. Chlorite is the chief mineral built up
in this way (see section on clays), but the flakes are seldom larger than IO microns. On
deep burial, during incipient metamorphism, or in faulted or hydrothermally-soaked
zones chlorite commonly forms as tiny flakes, sometimes replacing quartz and feldspar;
biotite and sometimes muscovite form when alteration becomes more intense. Biotite
and chlorite may also form authigenically as crusts or small flakes in ordinary
sediments, but these occurrences are quite rare,

Metamorphic Rock Fragments (MRF’s)

Metamorphic rock fragments consist of sand- or gravel-sized pieces of slate,
phyllite, schist, metaquartzite, and occasionally gneiss (gneiss is usually too coarse-
grained to be recognized as such when broken into sand-size particles). They are the

87