micrite

micrite

[′mī‚krīt]
(petrology)
A semiopaque crystalline limestone matrix that consists of chemically precipitated calcite mud, whose crystals are generally 1-4 micrometers in diameter.
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The aggrading neomorphism involves the conversion of fine grained micrite to coarse grained spar.
In MF3, some dolomitization were seen as replacement of fine grained micrite. The dolomite crystals are mostly dispersed and rusty brownish in colour (Fig.
Indeed micrite grains can be viewed as spheres whose radius decreases during dissolution while the sparite crystals can be viewed as volumes surrounding spherical pores whose radii are increasing (Noiriel et al., 2009).
Dolomite ghosts measure 50 to 150 [micro]m wide, and their porosity is either preserved or partially filled by micrite. Cathodoluminescence images show growth zonations within certain sparry calcite crystals, but dull to nonluminescent sector zonations are mostly observed (Figure 4).
The dominating biogenic compounds are: (i) calcite as < 5 m[micro] micrite forming the groundmass mudstone, and both micrite of the matrix and > 5 m[micro] to 1 mm grains--skeletal particles (shells and their broken fragments) in wackestone, (ii) silica as < 5 m[micro] particles belonging to the groundmass of mudstone and wackestone matrix, (iii) organic matter (kerogen) in the mudstone and matrix of wackestone, (iv) phosphate skeletal fragments in certain interbeds of wackestone and very fine apatite in groundmass.
Table 1: Main mineral composition Main compositions (%) Rock sample Opaque number Quartz Feldspar Debris Dolomite mineral 1# 35 20 5 <1 <1 2# 30 15 8 -- <1 3# 3 -- -- -- -- 4# 40 15 5 <1 <1 5# 40 10 5 1 1 6# 38 12 5 1 <1 7# 50 10 5 8# 55 10 1 1 3 9# 4 -- -- -- -- 10# 50 10 -- 1 3 11# 38 8 19 3 -- 12# 40 12 23 <1 -- Main compositions (%) Rock Iron-stain sample Micrite Calcite clay mineral Calcareous number gravel + sericite 1# -- 3 30 7 2# 3 10 33 -- 3# -- -- 97 -- 4# -- 8 31 -- 5# -- -- 30 13 6# -- -- 30 13 7# -- 20 15 -- 8# -- 15 15 -- 9# -- 40 56 -- 10# 1 20 15 -- 11# -- 20 10 -- 12# -- 15 10 -- Table 2: Uniformity coefficient [C.sub.u] and gradation coefficient [C.sub.c].
The effect of eo or telodiagenetic processes by meteoric water in the different carbonate components commonly is reflected by increasing textural alteration of the matrix that change successively from micrite to microsparite, to minor pseudosparite and/or to precipitation of interparticle sparite cement.
As lower layers contain substantial micrite and other hydrologically significant carbonate morphologies, it is possible that this system is related to a currently active, albeit distant (>1 km), local spring that may have experienced intermittent flows of varying rates and emergence locations.
These facies pass laterally northward into heavily bioturbated, pale green, argillaceous, micrite facies that carry in situ lingulid brachiopods and mollusks; these are inferred to record shallow lagoonal settings.
By contrast, the presence of soft and small-sized nodules with loose micrite crystals suggests limited development of pedogenic calcite (Khormeli et al.
This microfacies is characterized by the abundant occurrence of small rotaliids (i.e., Neorotalia and Ammonia), bryozoa (i.e., Tubucellaria, Memberanipora and Onychocella) and abundant micrite matrix.