Page 100 - Petrology of Sedimentary Rocks

degraded illite (stripped of most of its K+) enters sea water and regains
much of its original K+ content.

Montmorillonite forms in a Mg-rich environment, most of it by alteration of
volcanic ash, some by temperate weathering especially of basic rocks. On
incipient metamorphism it may convert to illite or chlorite. Marine
diagenesis has little effect on volcanic montmorillonite, except to change
adsorbed cations.

Chlorite forms on marine diagenesis, particularly in lagoonal or near-shore marine
environments where rivers bearing iron derived from weathering enter the
SW. it is also common in marine sands that have received contributions
from basic igneous rocks, and forms on diagenesis, deep burial and metamor-
phism. Much of it is simply detrital, derived from older schists, phyllites,
etc. It is possibly formed to a small extent in soils by weathering of micas
and montmorillonite.

Talc, pyrophyllite, vermiculite, sepiolite, corrensite, stevensite and other clay
minerals occur locally.

Heavy Minerals

Heavy minerals are defined operationally as those with a specific gravity greater
than 2.85, the specific gravity of the bromoform liquid used to separate them from
lighter quartz, feldspar or calcite. Although over 100 different minerals have been
recorded from sediments, they probably form no more than 0.1 - 0.5% of the terrige-
nous fraction of sediments. Despite their small amount, they are of great value in
studying provenance, transportation and weathering history of a sediment and in
correlation and paleogeographic studies. They represent the accessory and varietal
minerals of igneous and metamorphic rocks, which are much reduced in quantity (except
for zircon and tourmaline) as they pass into sediments because they are chemically
unstable and also considerably softer than quartz. It is estimated that the average
quartzarenite contains 0.05 - 0.2% heavies; phyllarenite 0.2 - 0.8%; and average arkose
I - 2%, chiefly because the more abrasion or weathering a sediment undergoes, the less
unstable minerals it contains.

Common heavy minerals range from 3 to 5 in specific gravity. Because of their
heaviness, they usually travel with quartz averaging 0.5 to l.O+ size larger--this
difference is known as the “hydraulic ratio” (Rittenhouse) and varies for each mineral
species; the value is affected chiefly by the specific gravity but to some extent also by
the shape, also by the original size of the mineral grains in the parent rock. Thus if a
sand has a median of 2.54, tourmaline may have a median 2.9$ and zircon 3.54. When a
sand such as this is sieved, the heavy minerals fall in the finer size grades, and the
heaviest heavies occur at the very finest end; thus when the heavy minerals are
mounted one may find that the 4.0 - 4.5$ grade has 90% zircon and 10% tourmaline, and
the 3.5-4.0@ grade has 10% zircon and 90% tourmaline. Sands of different size within
the very same bed may consequently have radically different percentages of the several
heavy minerals; hence mineral ratios between minerals of different specific gravity or
shape are chiefly a function of grain size. One has to use minerals of the same shape
and specific gravity-- in practice this boils down to making varietal studies of one
mineral, such as tourmaline or zircon. Then when comparing ratios between the
different varieties one knows that the specific gravity and shape are essentially the
same hence there is no hydraulic factor; differences in varietal ratios will then reflect
differences in source area Iithology.

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