Micrite
Carbonate mud composed of very fine size calcite crystals is present in many ancient limestones in addition to sand-size carbonate grains. [1]
Composition and Appearance
Modern Carbonate Mud
Carbonate mud or lime mud occurs also in modern environments where it consists dominantly of needle-shaped crystals of aragonite about 1-5 microns (0.001-0.005 mm) long. [1]
Ancient Carbonate Mud
The carbonate mud in ancient limestones is composed of similar-size crystals of calcite. [1] Lime muds may also contain small amounts of fine-grained detrital minerals such as clay minerals, quartz, feldspar, and fine-size organic matter. [1]
They have a grayish to brownish, subtranslucent appearance under the microscope, and they are easily distinguished from carbonate grains and sparry calcite crystals by their extremely small crystal size. [1]
Terminology
Folk (1959) proposed the contraction micrite for microcrystalline calcite, a term that has been universally adopted to signify very fine-grained carbonate sediments. [1]
Occurrence in Limestones
Micrite may be present as matrix among carbonate grains, or it may make up most or all of a limestone. [1] A limestone composed mostly of micrite is analogous texturally to a siliciclastic mudstone or shale. [1]
Environmental Significance
The presence of micrite in an ancient limestone is commonly interpreted to indicate deposition under quiet-water conditions where little winnowing of fine mud took place. [1] By contrast, carbonate sediments deposited in environments where bottom currents or wave energy are strong are commonly mud-free because carbonate mud is selectively removed in these environments. [1]
Origin
On the basis of purely chemical considerations, carbonate mud or micrite can theoretically form by inorganic precipitation of aragonite, later converted to calcite, from surface waters supersaturated with calcium bicarbonate. [1] Geologists are uncertain, however, about how much aragonite is actually being generated by inorganic processes in the modern ocean. [1]
Much modern carbonate mud appears to originate through organic processes. [1] These processes include breakdown of calcareous algae in shallow water to yield aragonite mud, and deposition of carbonate nannofossils (<35 μm in size) such as coccoliths in deeper water to yield calcite muds (chalks). [1]
Related Topics
Crystal
The word crystal is used in two distinct ways in mineralogy, and the difference between them matters more than it might first appear.{/* SRC: Nesse p.5: "grains. It might be better to use the term...
Clay Minerals
Clay minerals are the dominant component of matrix in sandstones and the principal mineral group in shales. They belong to the phyllosilicate mineral group, characterised by two-dimensional layer...
Sparry Calcite
Many limestones contain large crystals of calcite, commonly on the order of 0.02-0.1 mm, that appear clear or white when viewed with a hand lens or in plain light under a polarizing microscope.{/*...
References & Citations
- 1.Principles of Sedimentology and Stratigraphy Boggs
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