Page 102 - Petrology of Sedimentary Rocks

4. Metastable Group.

a. Olivine is very rare in sediments, occurring only under dry climatic
conditions or rapid erosion. Chiefly from basic igneous rocks.

b. Apatite is moderately stable; commonly it occurs sporadically (abun-
dant in a few specimens, sparse in others), then indicates a volcanic
source. Otherwise it can occur in basic to acid plutonic rocks.

C. Hornblende and pyroxene are moderately unstable; may come from
either igneous or metamorphic rocks, but when present in abundance
indicate volcanics or metamorphic rocks, such as hornblende schist.
Brown Oxyhornblende is diagnostic of a basaltic source; Glaucophane
and tremolite are other less common amphiboles, indicating almost
certainly a metamorphic source. Pyroxenes are etched and dissolved
rapidly by solution after deposition, hence are rare in porous sands.

d. Garnet may come from plutonic rocks, pegmatites, or metamorphics
but in abundance indicates a metamorphic source. Many varieties
present, based chiefly on color. Stability variable, depending on the
variety; often corroded intrastratally to produce fantastic etch fig-
ures. Rapidly dissolved in many porous sands, especially those flushed
by fresh water. Garnets bearing chattermark trails are diagnostic of
glacial environments (Folk, I975 Geology).

e. Epidote, Clinozoisite, and Zoisite indicate a metamorphic or hydro-
thermal source; they are moderately stable.

f. Kyanite, sillimanite, andalusite, staurolite are highly diagnostic of a
metamorphic source. They are moderately stable, but usually rather
soft.

54. Over 100 others have been reported. For origin and properties see
standard works like Krumbein and Pettijohn, or Milner.

Evaluation - of Heavy Mineral Content

The heavy mineral content of a sediment is a function of five very complex
variables. Usually the aim of the petrographer is to eliminate all variables except one:
the lithology of the source area at a given time and place. This he can use as a tool in
correlation, and can interpret geologic history by following the source area as it
becomes unroofed by erosion and exposes different types of minerals in different
amounts. The five variables are:

(I) Lithology of the source area-- the ultimate aim of the petrographer. It may
be simple (one rock type or assemblage) but more commonly is complex (e.g. sandstone
with rounded tourmaline and zircon, overlying schists with angular kyanite).

(2) Differential stability of minerals as they are subjected to weathering in the
soils of the source area--to investigate this factor compare the chemical stability of
the minerals with their abundance in your sediment.

(3) Durability of the mineral to long-continued abrasion--check hardness against
abundance to evaluate this.

96