Page 68 - Petrology of Sedimentary Rocks

MINERAL COMPOSITION OF SEDIMENTARY ROCKS

In igneous rocks, many minerals occur commonly and are important rock-
formers. In sedimentary rocks, however, only a few minerals are important rock-
formers, but those few minerals may be present in a bewildering number of varieties.
For example in one piece of limestone the mineral calcite may be present as oolites,
reworked pebbles, fossils, pellets, microcrystalline ooze, several generations of pore-
filling cement, and veinlets. Furthermore, in normal igneous rocks essentially all the
minerals are direct chemical precipitates with no transported or abraded particles and
Ii ttle replacement; but in sediments one must distinguish between (I) terrigenous
minerals, and (2) chemical minerals, divided into (2a) allochemical minerals (trans-
ported) and (2b) orthochemical minerals (forming in place). Orthochemical minerals
furthermore may be formed either by replacement of previous minerals or by direct
precipitation. A given mineral species may be present in all of these categories even
within the same rock.

The abundance of a terrigenous mineral in a sedimentary rock depends on three
factors (Krynine):

(I) Availability. The mineral must be present in sufficient abundance in the
source area. One cannot get an arkose by eroding limestone, or chert
pebbles by eroding granite. Similarly lack of feldspar may be due not to a
humid climate but to the fact that the source was an older sandstone, or a
low-rank phylite or schist.

(2) Mechanical durability, favored by lack of or poor cleavage, and high
hardness. Long abrasion selectively eliminates soft or easily cleaved
minerals.

(3) Chemical stability. Minerals that form late in igneous rocks, when those
bodies are crystallizing under cooler and more hydrous conditions, are most
stable in sediments because they are more nearly adjusted to the relatively
cold and wet sedimentary environment. The order of chemical stability is
approximately the reverse of Bowen’s reaction series, but local chemical
conditions can alter the order (e.g. Todd, I968 JSP).

Stability Series for Terrigenous Minerals

Quartz, Zircon, Tourmaline
C hert
Muscovite
Microcline
Ort hoclase
Al ite
____________-----___-------------- _------

Hornblende, Biotite f
Pyroxene Anorthi te
Olivine

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