Page 105 - Petrology of Sedimentary Rocks

most redbeds, hematite acts only as a stain (i.e., very finely divided, uniformly
distributed small particles) and forms a coating on the clay minerals comprising only a
few percent of the rock; in other specimens, however, solid masses of pure hematite
form as pore-filling cement or replacement of carbonate. Limonite is frequent as a
weathering product of other iron minerals. It requires conditions less hot and humid
than hematite in order to form in the soil, thus occurs chiefly in temperate soils. Aging
converts limonite to hematite, also. In many recent or continental sediments it forms a
solid cement, probably as the result of weathering. Walker (I 967) thinks red sandstones
may develop under semi-arid conditions by alteration of heavy minerals, intrastratally.
Mature desert sands are red (Folk, 1976); with aging the red darkens to dull maroon.

Pyrite forms under reducing conditions where sulfur is present, and is commonly
associated with organic or petroliferous matter, fossils, etc. It is a very “strong”
mineral and can replace anything, even quartz. It may form isolated tiny crystals or
huge concretions, the latter usually around an organic nucleus. Melni kovi te or
hydrotroilite is a very finely-divided black iron sulfide responsible for much of the dark
color of recent marine muds.

Glauconite, KA I SiOH with considerable Fe and Mg, varies from an Fe-rich type of
illite to an Fe-rich type of montmorillonite. In thin section it is usually some shade of
green with a microcrystalline or scaly birefringence. It is supposed to form under
marine, mildly reducing to mildly oxidizing conditons on the continental shelf, and is
abundant in modern offshore muds. Much of it occurs as fecal pellets; rarely it forms
as a pore-filling cement, or replaces calcite. It is considered very diagnostic of marine
beds, and the type of glauconite varies with environment, geologic age, etc. (Burst).
Collophane (CaP04) occurs as bones, teeth, scales, shells, conodonts etc.; as turds; and
as inorganic replacements of limestone beds, as nodules and oolites.

Evaporites sometimes occur as beds, sometimes as cement in sandstones or as
replacement crystals in limestone. Gypsum has low birefringence with index below
balsam, and is usually fibrous; anhydrite has indices above balsam, with high birefrin-
gence and often forms good crystals; and halite is isotropic, with very low index. Barite
and celestite may occur as cement in sandstones or as replacement in limestones; index
is above balsam, gray birefringence. Evaporites are frequently replaced by length-slow
chalcedony or quartz. Zeolites occur as common cements in volcanic sediments, and
may also result from burial metamorphism.

As mentioned previously, zircon, tourmaline, rutile, sphene, anatase and other odd
minerals may form authigenically. These often form good stratigraphic markers.

Organic matter, though not a mineral, deserves mention. It occurs as black or
deep brownish material, and represents the most resistant humic and bituminous
materials such as waxes, lignins, etc., left after decay. It is most abundant where
formation is rapid (flourishing life), where burial is fast, where bacteria are not active
(bacteria eat it up and destroy it)--stagnant, reducing waters are regarded as most
favorable. Modern marine muds run l-2% organic matter; there is very little in sands
because organic matter is light and fluffy and travels with the clay minerals. Liquid
hydrocarbons--paraffins, naphthenic and aromatic-- are now forming in recent marine
muds.

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