MICRITE

MICRITE

By Adam Murfet, Amal Nadiah, Liam Palmer, Nicolle Vanderputt, Rahul Munavalli

Introduction

What is Micrite?

• Microcrystalline calcite (less than 4 microns)

• Occurs as the matrix in limestone

• Appears as fine-grained carbonate mud and crystalline cement of calcite precipitated during diagenesis

• 10 to 50% of micrite - matrix supported called micritic limestone

• Gives the dull opaque appearance of most limestones

References

Chapter 6: Carbonate Sedimentary Rocks [PowerPoint Slides]. Retrieved from faculty.uml.edu/lweeden/Lectures/SedStrat/Chapter%206.ppt

Flugel, E. (2010). Microfacies of Carbonate Rocks: Analysis, Interpretation and Application. Heidelberg, Berlin: Springer Berlin Heidelberg

Friedman, G. M. (1985). Meaning and Usage of Micrite Cement and Matrix. Journal of Sedimentary Petrology, 55(5), 777-784

Figure 1: Micrite under microscope

Nichols, G. (2009). Sedimentology and Stratigraphy. Oxford, UK: Wiley-Blackwell

Stephen, A. N. (2013). Earth and Environmental Sciences 2120: Petrology. Retrieved from http://www.tulane.edu/~sanelson/eens212/carbonates.htm

Process of Formation

Chemical composition

Factors that can affect CHEMICAL precipitation of CaCO3 in water:

1. In-situ formation of fine-grained carbonate as a result of biochemical and physiochemical factors

2. Post-mortem disintegration of calcareous algae

• Temperature

Calcite (CaCO3)

Carbonates

• A soluble carbonate mineral that has the same chemical composition as aragonite (less stable) - a polymorph

• Group of minerals found mostly in limestones

3. Physical or biological abrasion of skeletal material

• S.G. = 2.7, Hardness = 3, Trigonal

• Pressure

• Calcite/aragonite - one of the main categories of those minerals

• Colourless or white with white streak

• Vitreous luster and has fibrous aggregates

• Microcrystalline calcite = Micrite

• Often tabular and prismatic or rhombohedral

4. Accumulation of pelagic calcareous plankton

• Micrite, a carbonate mineral and the most abundant in limestones

• Salinity

The Folk Classification scheme divides carbonates into two groups:

1. Allochemical rocks - rocks containing grains brought from elsewhere and it is grain supported

2. Orthochemical rocks - rocks in which the carbonate crystallizes in place (in situ) and it is matrix supported

5. Diagenesis process - cementation and recrystallisation of carbonate mud

Figure : Folk Classification of allochemical and orthochemical rocks

Figure 2: Micritic calcite under a microscope

Conclusion

"The term micrite is both a descriptive and genetic term. It relates to lithified, micron-size particles of lime mud which formed in a low energy setting" (Friedman, 1985)

• Micrite is : microcrystalline consisting of calcium carbonate grains of 4um in diameter

• It is the matrix to that of bioclasts limestones and is the main constituent of fine-grained limestones (micritic limestones)

• Sources of micrite include;
• disintegration of calcareous algae
• skeletal carbonate material
• Accumulation of calcareous plankton

Depositional Environments

• Process of Micrite formation include:
• Bio-erosion and physical erosion
• Inorganic precipitation from carbonate saturated waters
• Diagenesis (recrystallization of carbonate mud)

Protected tidal flats

• Carbonate sands are carried in by the high tides

Protected lagoons

• Cemented together by carbonate producing organisms

• Ideal places for carbonate deposition

• Reef building organisms causes the carbonate buildups

Deep ocean

• Carbonate compensation depth- it is the depth at which there is a limit for calcite to form
• Factors that influence solubility rate:
• Temperature of the water,
• Pressure due to depth
• The amount of carbon dioxide in the water (chemical composition)

Figure : Depositional settings of micrite accumulation