MICRITE
Ian G. Macintyre
Smithsonian Institution, Washington, DC, USA
Synonyms
Microcrystalline calcite
Definition
Micrite is a textural term for microcrystalline carbonates
less that 4 mm in size.
Introduction
The term “micrite” is a contraction of the words “microcrystalline calcite” and was introduced by Folk (1959,
p. 8) as an aid in the classification of limestones. Initially,
Folk applied the term to microcrystalline calcite ooze in
the size range of 1–4 mm. Adding a prefix, he coined terms
such as “biomicrite” to refer to fossils in a microcrystalline
matrix. In 1974, Folk expanded the meaning of the term
“micrite” to encompass all carbonate minerals measuring
1–4 mm. He also added a new term “minimicrite” for sizes
less than 1 mm.
Bathurst (1966) applied Folk’s terminology to refer to
the process of grain surface alteration to micrite caused
by microborings filled with micrite precipitates as
“micritization”. However, Friedman (1985) thought that
Bathurst and others who used his terminology (e.g.,
Lloyd, 1971; Gunatilska, 1976; Kobluk and Risk, 1977)
were misusing Folk’s terminology that simply referred to
a microcrystalline ooze matrix.
It was Alexandersson (1972) who suggested that
“micrite” should as a textural term to cover all microcrystalline carbonates. Most subsequent studies of grain alteration
or submarine lithification use both “micrite” and
“minimicrite” as textural terms (e.g., Land and Moore,
1980; Macintyre and Marshall, 1988; Reid et al., 1992; Reid
and Macintyre, 1998; Macintyre and Reid, 1995, 1998).
Conclusion
In carbonate petrographic studies, the terms micrite
(4–1 mm) and minimicrite (<1 mm) should be used as textural terms to refer to all microcrystalline carbonates.
Bibliography
Alexandersson, E. T., 1972. Micritization of carbonate particles:
process of precipitation and dissolution in modern shallow
marine sediments. Universitet Uppsala, Geologiska Institut Bulletin, 7, 201–236.
Bathurst, R. G. C., 1966. Boring algae, micrite envelopes, and
lithification of molluscan biosparites. Geological Journal, 5,
15–32.
Folk, R. L., 1959. Practical petrographic classification of limestones.
American Association of Petroleum Geologists, 43, 1–38.
Folk, R. L., 1974. The natural history of crystalline calcium carbonate:effect of magnesium content and salinity. Journal of Sedimentary Petrology, 44, 40–53.
Friedman, G. M., 1985. The problem of submarine cement in classifying reefrock: an experience in frustration. In Schneidermann,
N., and Harris, P. M. (eds.), Society of Economic Paleontologists
and Mineralogists, pp. 117–121.
Gunatilska, A., 1976. Thallophyte boring and micritization within
skeletal sands from Connemara, western Ireland. Journal of Sedimentary Petrology, 46, 548–554.
Kobluk, D. R., and Risk, M. J., 1977. Micritization and carbonate
grain binding by endolithic algae. American Association of
Petroleum Geologists Bulletin, 61, 1069–1082.
Land, L. S., and Moore, C. H., 1980. Lithyification, micritization
and syndepositional diagenesis of biolithites on the Jamaican
island slope. Journal of Sedimentary Petrology, 50, 357–370.
Lloyd, R. M., 1971. Some observations on recent sediment alteration (“micritization”) and the possible role of algae in submarine cementation. In Bricker, O. P. (ed.), Carbonate Cements.
The Johns Hopkins Press, pp. 72–79.
Macintyre, I. G., and Marshall, J. F., 1988. Submarine lithification
in coral reefs: some facts and misconceptions. Proceedings of
the 6th International Coral Reef Symposium, Australia. Vol. 1,
pp. 263–272.
Macintyre, I. G., and Reid, R. P., 1995. Crystal alteration in a living
calcareous alga (Halimeda): implications for studies in skeletal
diagenesis. Journal of Sedimentary Research, A65, 143–153.
Macintyre, I. G., and reid, R. P., 1998. Recrystallization in a living
porcelaneous foraminifera (Archaias angulatis): textural
changes without mineralogic alteration. Journal of Sedimentary
Research, 68, 11–19.
Reid, R. P., and Macintyre, I. G., 1998. Carbonate recrystallization in
shallow marine environments: a widespread diagenetic process
forming micritized grains. Journal of Sedimentary Research, 68,
928–946.
Reid, R. P., Macintyre, I. G., and Post, J. F., 1992. Micritized skeletal grains in northern Belize lagoon: a major source of Mg-calcite
mud. Journal of Sedimentary petrology, 62, 145–156.
Cross-references
Aragonite
Calcite
Platforms (Cemented)
Porosity Variability In Limestone Sequences
Submarine Lithification
MICROATOLL
Scott Smithers
James Cook University, Townsville, QLD, Australia
Definition
Microatolls are intertidal coral colonies with dead, relatively flat, upper surfaces surrounded by an annular rim
of living coral (Figure 1a,b). They form when upward
coral growth is constrained by prolonged exposure near
to shallow water, but polyps on the colony sides remain
alive and continue to grow laterally (Scoffin and Stoddart,
1978; Stoddart and Scoffin, 1979; Woodroffe and McLean,
1990). Fossil microatolls are microatolls in which no living
polyps survive but the colony morphology is preserved
(Figure 1c).
MICROATOLL
691
Ian G. Macintyre
Smithsonian Institution, Washington, DC, USA
Synonyms
Microcrystalline calcite
Definition
Micrite is a textural term for microcrystalline carbonates
less that 4 mm in size.
Introduction
The term “micrite” is a contraction of the words “microcrystalline calcite” and was introduced by Folk (1959,
p. 8) as an aid in the classification of limestones. Initially,
Folk applied the term to microcrystalline calcite ooze in
the size range of 1–4 mm. Adding a prefix, he coined terms
such as “biomicrite” to refer to fossils in a microcrystalline
matrix. In 1974, Folk expanded the meaning of the term
“micrite” to encompass all carbonate minerals measuring
1–4 mm. He also added a new term “minimicrite” for sizes
less than 1 mm.
Bathurst (1966) applied Folk’s terminology to refer to
the process of grain surface alteration to micrite caused
by microborings filled with micrite precipitates as
“micritization”. However, Friedman (1985) thought that
Bathurst and others who used his terminology (e.g.,
Lloyd, 1971; Gunatilska, 1976; Kobluk and Risk, 1977)
were misusing Folk’s terminology that simply referred to
a microcrystalline ooze matrix.
It was Alexandersson (1972) who suggested that
“micrite” should as a textural term to cover all microcrystalline carbonates. Most subsequent studies of grain alteration
or submarine lithification use both “micrite” and
“minimicrite” as textural terms (e.g., Land and Moore,
1980; Macintyre and Marshall, 1988; Reid et al., 1992; Reid
and Macintyre, 1998; Macintyre and Reid, 1995, 1998).
Conclusion
In carbonate petrographic studies, the terms micrite
(4–1 mm) and minimicrite (<1 mm) should be used as textural terms to refer to all microcrystalline carbonates.
Bibliography
Alexandersson, E. T., 1972. Micritization of carbonate particles:
process of precipitation and dissolution in modern shallow
marine sediments. Universitet Uppsala, Geologiska Institut Bulletin, 7, 201–236.
Bathurst, R. G. C., 1966. Boring algae, micrite envelopes, and
lithification of molluscan biosparites. Geological Journal, 5,
15–32.
Folk, R. L., 1959. Practical petrographic classification of limestones.
American Association of Petroleum Geologists, 43, 1–38.
Folk, R. L., 1974. The natural history of crystalline calcium carbonate:effect of magnesium content and salinity. Journal of Sedimentary Petrology, 44, 40–53.
Friedman, G. M., 1985. The problem of submarine cement in classifying reefrock: an experience in frustration. In Schneidermann,
N., and Harris, P. M. (eds.), Society of Economic Paleontologists
and Mineralogists, pp. 117–121.
Gunatilska, A., 1976. Thallophyte boring and micritization within
skeletal sands from Connemara, western Ireland. Journal of Sedimentary Petrology, 46, 548–554.
Kobluk, D. R., and Risk, M. J., 1977. Micritization and carbonate
grain binding by endolithic algae. American Association of
Petroleum Geologists Bulletin, 61, 1069–1082.
Land, L. S., and Moore, C. H., 1980. Lithyification, micritization
and syndepositional diagenesis of biolithites on the Jamaican
island slope. Journal of Sedimentary Petrology, 50, 357–370.
Lloyd, R. M., 1971. Some observations on recent sediment alteration (“micritization”) and the possible role of algae in submarine cementation. In Bricker, O. P. (ed.), Carbonate Cements.
The Johns Hopkins Press, pp. 72–79.
Macintyre, I. G., and Marshall, J. F., 1988. Submarine lithification
in coral reefs: some facts and misconceptions. Proceedings of
the 6th International Coral Reef Symposium, Australia. Vol. 1,
pp. 263–272.
Macintyre, I. G., and Reid, R. P., 1995. Crystal alteration in a living
calcareous alga (Halimeda): implications for studies in skeletal
diagenesis. Journal of Sedimentary Research, A65, 143–153.
Macintyre, I. G., and reid, R. P., 1998. Recrystallization in a living
porcelaneous foraminifera (Archaias angulatis): textural
changes without mineralogic alteration. Journal of Sedimentary
Research, 68, 11–19.
Reid, R. P., and Macintyre, I. G., 1998. Carbonate recrystallization in
shallow marine environments: a widespread diagenetic process
forming micritized grains. Journal of Sedimentary Research, 68,
928–946.
Reid, R. P., Macintyre, I. G., and Post, J. F., 1992. Micritized skeletal grains in northern Belize lagoon: a major source of Mg-calcite
mud. Journal of Sedimentary petrology, 62, 145–156.
Cross-references
Aragonite
Calcite
Platforms (Cemented)
Porosity Variability In Limestone Sequences
Submarine Lithification
MICROATOLL
Scott Smithers
James Cook University, Townsville, QLD, Australia
Definition
Microatolls are intertidal coral colonies with dead, relatively flat, upper surfaces surrounded by an annular rim
of living coral (Figure 1a,b). They form when upward
coral growth is constrained by prolonged exposure near
to shallow water, but polyps on the colony sides remain
alive and continue to grow laterally (Scoffin and Stoddart,
1978; Stoddart and Scoffin, 1979; Woodroffe and McLean,
1990). Fossil microatolls are microatolls in which no living
polyps survive but the colony morphology is preserved
(Figure 1c).
MICROATOLL
691