Geology Reference
In-Depth Information
Fig. 7.20.
Crystallization fabrics of recrystallized carbonates and marbles
can be described according to equant or non-
equant crystal size and crystal fabric depending on crystal shape (see Box 7.12 for definitions). From Friedman (1965).
Sect. 4.1.3) and continuing diagenesis may produce
pseudospar
with crystals measuring tens to even hun-
dreds of microns in diameter (Sect. 7.7).
Burial diagenesis and changes in temperature, pres-
sure, and volatile content will favor the formation of
carbonate rocks with crystal sizes up to several tens of
millimeters. Some of these rocks already correspond
to marbles, characterized by fine- to coarse-grained
granoblastic textures (Fig. 7.20).
Recrystallized carbonate rocks are not loved by ge-
ologists interested in facies analysis. These rocks show
only a few characteristics that can be used in paleo-
environmental interpretations and they are poor in cri-
teria allowing a reproducible
classification
.
What to do?
If no relicts of the primary depositional textures are
preserved, a classification can be based only on the at-
tributes of the diagenetic calcite. These features include
crystal sizes, crystal shapes, mutual relationships be-
tween crystals, types of boundaries between crystals,
and crystal foundations.
7.6.2 How to Describe
Recrystallized Carbonate Rocks?
Box 7.12.
Descriptive terms for crystallization textures
and fabric
s after Friedman (1964). Compare Fig. 7.20.
The scheme can also be used to describe calcite and do-
lomite marbles.
Textures and fabrics of recrystallized limestones and
dolomites as well as carbonate cements can be described
using the classification developed by Friedman (1964,
1965) and summarized in Box 7.12.
Hibbard (1994) drew attention to the limited value
of the terms anhedral, subhedral, and euhedral where
crystal morphology is considered an expression of the
conditions under which crystals and rocks are formed.
Crystallization textures:
Refer to the shape of mineral
crystals and the type of crystal faces at crystal bound-
aries. Descriptive terms are
Anhedral:
Characterized by the absence of crystal fac-
ing bounding the mineral grains. Pl. 38/3.
Subhedral:
Characterized by partly developed crys-
tal faces.
Euhedral:
Characterized by crystals that are bounded
by crystal faces.
Crystallization fabrics
: Refer to the size and mutual
relations of crystals that can be differentiated into
Equigranular fabrics:
Consist of crystals of approxi-
mately the same size (e.g. Pl. 38/6). Fabrics subdi-
vided into
Xenotopic:
Predominantly anhedral crystals.
Hypidiotopic:
Predominantly subhedral crystals.
Idiotopic:
Predominantly euhedral crystals.
7.7 Sparite: Recrystallization
Product or Carbonate Cement?
The non-genetical term sparite (Fig. 7.21) designates a
fabric consisting of calcite spar. Genetically, sparite is
carbonate cement (corresponding to
orthosparit
e) or a
product of recrystallization. Distinguishing the prod-
ucts of aggrading neomorphism from sparite represent-
ing carbonate cement is often difficult. Criteria have
been discussed by Folk (1965) and Bathurst (1975).
Sparite representing cement
occurs within pores.
Boundaries between sparite and pore limits are sharp.
Crystal boundaries are straight and have enfacial junc-
tions.
Inequigranular fabrics:
Consist of crystals of vari-
ous sizes (e.g. Pl. 38/4, 5, 7). Fabrics are subdivided
into
xenotopic
hypidiotopic
idiotopic.
Each of these inequigranular fabrics is in turn subdi-
vided into
Porphyrotopic:
Larger crystals are enclosed in a
fine-grained matrix.
Poikilotopic:
Crystals are of more than one size;
larger crystals enclose smaller crystals of another
mineral.
Neomorphic fabrics
often exhibit irregular crystals
with curved and embayed boundaries, variable crystal
size with remnants of micrite, and the presence of grains
