Page 104 - Petrology of Sedimentary Rocks

shells without forming any dolomite; more MgCO is taken up in warmer sea waters.
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(Chave). In recent sediments, magnesian calcite and calcian dolomite is not uncommon,
because rapid crystallization catches the “wrong” ions. To a lesser extent, Mn is also
easily substituted for Mg or Fe, giving rhodochrosite (MnCo3) and kutnahorite (Mn-
dolomi tel.

All these minerals are uniaxial negative with extreme birefringence, rhombic
cleavage and hardness 3-4. Calcite and dolomite have one index well below balsam, the
other well above so that they “twinkle” on rotating the stage (except for near basal
sections, which always give the higher index); siderite has both indices well above
balsam, therefore does not twinkle. Furthermore, both ankerite and siderite often are
brownish due to iron staining and partial alteration to limonite pseudomorphs.

Calcite probably forms over half by volume of the carbonates, occurring as
directly-precipitated microcrystalline ooze (forming irregularly rounded grains l-4
microns in diameter); as mosaic, xenomorphic crystals which are clear and often
twinned, and as fibrous coatings and crusts. In some fresh water environments (caliche,
streams) calcite forms tiny euhedral rhombs (Folk, 1974). It forms “lithographic” ooze,
cement in sandstones and limestones, oolites, fecal pellets, fossils, vein fillings, and
occasionally is a replacement mineral. In some sandstones, it occurs only as scattered
patches while the rest of the rock is noncalcitic. These patches usually consist of one
large crystal engulfing numerous sand grains, and the rocks are known as “lustre-
mottled” sandstones or sand crystals. Aragonite has the same composition as calcite
but is orthorhombic with slightly higher indices. It occurs in fossil shells and recent
oolites and carbonate oozes; it inverts to calcite eventually, but some Pennsylvania
shells are still aragonitic, while some Pleistocene aragonite has already changed to
calcite.
Dolomite may occur as a directly-precipitated (?) ooze in crystals 2-20 microns in
diameter; as coarser, idiomorphic to xenomorphic crystals replacing limestone; and
occasionally as veinfillings or cement in sandstones. It may be distinguished from
calcite by the fact that it nearly always shows some evidence of rhombic crystal form,
even in grains as small as a few microns; also, it is almost never twinned. When
replacing limestone, the allochem structure is sometimes preserved, especially if the
dolomite crystals are coarse; but just as commonly it is obliterated. Even in one thin
section there may be areas where this structure is nicely shown, and adjoining areas
where it is destroyed. “Limpid” dolomite (perfect, exceptionally clear crystals)
probably indicate fresh-water mixing, and “Baroque” dolomite (coarse, white, twisted
crystals--Pedone 1978) probably indicates S or SO4 association and higher temperature.

Siderite and ankerite may, like dolomite, form as a replacement, as a directly-
precipitated ooze, or as concretions. Ankerite may form beds, reworked pellets or
intraclasts in some swampy, deltaic sediments (Rizvi).

Miscellaneous Chemical Minerals

Magnetite is usually a detrital heavy mineral, but it may form authigenically
under mildly reducing conditions, especially when associated with glauconite. Under
most reducing conditions, magnetite is dissolved. Hematite may be dull red and earthy,
or else form blood-red translucent scales; it is the pigment of most redbeds. Oxidizing
conditions are necessary both for its formation and for its survival. May form in soils
under humid, tropical conditions, or may be precipitated from solution in the sea. In

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