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9 AUTOCHTHONOUS SEDIMENTS
Table 9.3
Summary of the Common Carbonate Minerals
Mineral
Formula
Crystal system
Occurrence
Aragonite
Calcite
Magnesite
Dolomite
Orthorhombic
CaCO 3
Hexagonal
MgCO3
Hexagonal
CaMg(CO3)2
Hexagonal
Ankerite (ferroan
Ca(MgFe)(CO3)2
Hexagonal
dolomite)
Siderite
FeCO3
Hexagonal
Present in certain carbonate
skeletons
Unstable and reverts to
stable polymorph calcite
Present in certain carbonate
skeletons, as mud
(micrite) and cement
(sparite)
Present in minor amounts
within the lattices of
skeletal aragonites and
calcites
Largely as a crystalline
diagenetic rock, also
penecontemporaneously
associated with evaporites
A minor variety of dolomite
Found as concretions and
ooliths (spherosiderites)
monominerallic, are texturally diverse and polygenetic. The various grain types, matrix,
and cement are described next. They are tabulated in Table 9.4.
9.2.3.1 Grains
Grains are the particles that support the framework of a sediment. They are thus generally of sand grade or larger. As Table 9.4 and Fig. 9.1 show, carbonate grains are of
Table 9.4
Main Components of Carbonate Rock
I. Grains
(a) Detrital grains
(b) Skeletal grains
(c) Peloids (including
fecal pellets)
(d) Lumps
(e) Coated grains
II. Matrix
{ Micrite
Clay
III. Cement
Sparite
IV. Pores
Lithoclasts
Intraclasts
Composite grains
Algal lumps
Ooliths
Pisoliths
Algally encrusted grains
After Leighton and Pendexter (1962).