The diagram above illustrates the Folk classification scheme as applied to fossiliferous rocks. If other grain types dominate, their names are substituted for "fossiliferous", e.g., oolitic, intraclastic, or peloidal. Assessment of the abundance of mud or cement is pertains only to primary intergranular pores; cement or mud in other pore types is ignored. Sorting and rounding are qualitatively evaluated and subject to observer bias and experience.
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Type I - Sparry Allochemical Rocks
Sparry allochemical rocks, or Type I limestones, consist mainly of allochemical constituents that are cemented together by sparry calcite. Because there is a limit to the tightness with which allochems may be packed, there will always be some amount of pore space to fill with sparry calcite. Type I limestones indicate strong or persistent currents.
Type II - Microcrystalline Allochemical Rocks
Microcrystalline allochemical rocks, or Type II limestones, consist of allochems surrounded by a microcrystalline mud matrix. Allochem percentages in Type II limestones have a wide range, from 10% allochems (an arbitrary boundary, set at 10% to distinguish from micrites) all the way up to 80% allochems (a maximum amount based on restrictions of packing). Type II limestones indicate weak, short-lived currents or a rapid rate of formation of carbonate mud.
Type III - Micrite
Under the Folk classification scheme, micrite is the Type III limestone (microcrystalline rock). These consist almost entirely of microcrystalline calcite with little or no allochem material, and no sparry calcite. Micrites are indicative of a depositional environment lacking strong currents. They are equivalent to claystones in terrigenous rocks. Micrites with up to 10% allochems are classified according to the type of allochem (Intraclast-bearing micrite, ooid-bearing micrite, fossiliferous micrite, pelletiferous micrite).
Type IV - Biolithite
Biolithites (Type IV) consist of organic structures growing in situ (bioherm rocks), that have become lithified in place. Biolithites should always be classified by the specific organism that is dominant in the rock (i.e., coral biolithite, bryozoan biolithite, blue-green algal biolithite, etc.).
Dismicrite
When a microcrystalline limestone has been disturbed by boring organisms or soft-sediment deformation, it is termed a dismicrite (meaning "disturbed microcrystalline" rock). Openings caused by these disturbances are filled with irregular "eyes" of sparry calcite. Dismicrites also form when beds of microcrystalline ooze are partially torn up by bottom currents and redeposited rapidly without the production of distinct intraclasts.