Page 156 - Petrology of Sedimentary Rocks

Is the size distribution continuous (unimodal) or discontin-
uous (bimodal)?
7. Idiomorphism, and range of idiomorphism. If the mineral is
nearly idiomorphic, describe the crystal habit if desirable.
Are grains truly crystals or merely cleaved pieces?
8. Average Sphericity (stated as W/L, e.g. .67), uniformity
and range of sphericity.
9. Average roundness, (Powers classes), uniformity and range
of roundness, and variation of roundness with grain size. Is
there a continuous gradation in roundness, or are there
rounded and angular grains with no intermediate roundness
va I ues?
IO. Etching, surface and contact features; is the mineral
indented by others, or does it indent its neighbors? (This is
important in paragenesis).
I I. For elastic minerals, are overgrowths present? For authi-
genie minerals, are elastic nuclei present?
12. Cleavage, fracture, or parting.
Zoning and internal structure (describe in detail).
12: Index and relief
I 5. Color, pleochroism, diaphaneity; luster and color in re-
flected light.
16. Twinning.
17. Birefringence; does it show undulose or straight extinction?
I 8. Optical elongation and orientation; extinction angle.
19. Interference figure: uniaxial or biaxial, plus or minus, 2V,
dispersion.
20. Inclusions
a. Identity of each inclusion
b. Properties of each, using above outline for descrip-
tion
C. Size of each inclusion
d. Distribution or zoning of inclusion within the host
grain, and orientation of inclusion within host
e. Abundance of each inclusion type
21. Freshness, alteration products, homogeneity of decomposi-
tion with the same species, and time of alteration (pre-
depositional, epigenetic etc.--give the evidence).
22. Definition and description of varieties within one mineral
species; differences in other properties that may occur
between varieties (for example, is quartz* with rutile
needles more round than quartz without rutile needles; is
purple zircon larger than colored zircon; etc. This is very
important in determining history of rock). What is the
relative abundance of each variety?
23. Affinities and antipathies of occurrence (e.g. does calcite
selectively occur in the coarser-grained layers; is dolomite
preferentially associated with organic matter, etc.).
24. Chemical and age relations with other minerals (i.e. is it
being replaced or attacked by other minerals, is it euhedral
against others, earlier or later than others, at what time
and how did it form; give evidence for these statements).
25. Derivation and probable source area for detrital constit-
utents.

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