Important carbonate information
- or -
A little help with carbonate petrology



Carbonate thin section photomicrographs (much improved offerings since late 2004)

http://plaza.snu.ac.kr/~lee2602/atlas/carb_photo.html
(many are high magnification, so not that helpful with depositional textures)

http://web.umr.edu/~greggjay/Carbonate_Page/LSGallery/#Skeletal%20allochems
(a wide variety of subjects, good quality, high resolution images)

http://web.umr.edu/~greggjay/Carbonate_Page/DoloGallery/
(ditto but of dolomite)

http://strata.geol.sc.edu/thinsections/thinsections.html
( a huge collection of very high quality images; unfortunately, the menu system is a little confusing:
1) at the top of main page, click on the large thumbnail photo of the allochem or diagenetic feature you want to examine
2) when the page loads, you'll note a detailed discussion of the allochem or feature
3) once you are done browsing this information, go to the pulldown menu at the top labeled "Topic" and choose the menu item [photo subject] you are interested in)

http://people.uncw.edu/dockal/gly312/co3allochems/carbonate.htm
(simple collection of allochem photomicrographs; accompanying text as well)

http://wwwalt.uni-wuerzburg.de/mineralogie/links/petrology/thinsection.html
(extensive set of web links for mineralogists; includes a variety of subjects related to petrology and petrography)

http://www.earthscienceworld.org/images/search/results.html?Keyword=Thin%20Sections
(sedimentary rock thin sections at the Earth Science World Image Bank)

http://www.ees.nmt.edu/Geol/classes/geol524/Diag/CarbCmts.html
(Carbonate cements; a rudimentary collection with discussion)

http://www.ees.nmt.edu/Geol/classes/geol524/Diag/CarbDiag.html
(same site as above, but the main "Carbonate Diagenesis" web page)
 
Traits of carbonate minerals: http://rbmason.ca/databank/mineral/carbonate.html
Mineralogic traits of calcite: http://rbmason.ca/databank/mineral/calcite.html
Mineralogic traits of dolomite: 
(currently lacks photo)
http://rbmason.ca/databank/mineral/dolomite.html

Want to understand better what you are seeing through the microscope?
Visit "Features in Thin section: Plane polarized light" for a brief, informative discussion
http://www.uwgb.edu/dutchs/petrolgy/Tsecplp.htm

Rocks under the microscope
(only a few pertinent slides, but very good quality; in addition, if you still don't "get", for example, what a greywacke is, you can see a photomicrograph with explanation)
http://www.earth.ox.ac.uk/~oesis/micro/index.html



The University of South Carolina geology dept maintains a HUGE website all about Carbonate Petrology
(unfortunately, all pages load slowly unless you have a very fast connection)

Carbonate petrology index page:
http://strata.geol.sc.edu/caco3-petrology.html

Extensive thin section gallery:
http://strata.geol.sc.edu/thinsections/thinsections.html
( a huge collection of very high quality images; unfortunately, the menu system is a little confusing:
1) at the top of main page, click on the large thumbnail photo of the allochem or diagenetic feature you want to examine
2) when the page loads, you'll note a detailed discussion of the allochem or feature
3) once you are done browsing this information, go to the pulldown menu at the top labeled "Topic" and choose the menu item [photo subject] you are interested in)

Glossary of terms: (very helpful)
http://strata.geol.sc.edu/thinsections/Carbonate-glossary.html


- Essential terms -

Abbreviated vocabulary list with simplified definitions (translation: "terms and meanings you should know")
 
Allochem - a carbonate grain; i.e. ooids, fossils, intraclasts, or pellets/peloids; one of the three primary components of carbonate rocks (see also micrite and spar).
Aragonite - a variety of calcium carbonate that is common in marine sediments but is metastable. Therefore, aragonite is usually converted to calcite overtime by inversion
Carbonate mud - fine-grained sediment (< 4 microns?) typically composed of aragonite, calcite, or dolomite; synonymous with the term "lime mud."
Coated Grain - an allochem that has had thin, concentric layers of aragonite or calcite crystals precipitated upon it; these layers, called the cortex, enclose the nucleus; coated grains are similar to ooids, but a coated grain's nucleus is much larger than that of a true ooid. 
Diagenesis - general term for the changes that occur to sediments or rocks after deposition, due to chemical, biological (e.g. micritization), and physical processes (e.g. compaction); chemical diagenesis, or neomorphism, includes (but is not limited to): dissolution, dolomitization, inversion, recrystallization, replacement, and silicification.
Dolomitization - Pervasive chemical replacement of calcite by dolomite within what was originally calcium carbonate sediment or limestone; a type of diagenesis
Framework grains - allochems that are in grain-to-grain contact
Grain-to-Grain contact - a sediment texture in which most allochems are in physical contact with other grains
Grain support - more-or-less synonymous with grain-to-grain contact, however, not all grains will appear to be in contact within a grain-supported sample since cross-sections will not transect the maximum diameter of all grains
Intraclast - a flattened elongate carbonate grain, originally consisting of semi-consolidated lime mud; intraclasts form primarily by erosion of the substrate, typically during a storm event; sometimes called as a "rip-up clast". 
Inversion - conversion of aragonite to calcite over time, either by recrystallization in place, or by dissolution and reprecipitation (i.e. replacement); e.g. mollusks produce shells that are primarily aragonite; therefore, in thin sections, molluscan fossils consist of a cast filled with sparry calcite, due to inversion of aragonite (by replacement) to calcite.
Lime mud - fine-grained (< 4 microns?) carbonate sediment that is of both organic and inorganic origin; in shallow water, much organic lime mud results from the disaggregation of benthic (bottom-dwelling) calcareous algae; in deeper water, the tests (microscopic shells) of calcareous phytoplankton produce most lime mud; synonymous with carbonate mud
Matrix - finer grained sediment which composes the bulk of some rocks (e.g. wackestone or biomicrite) or fills the interstices between framework grains (e.g. packstone or packed biomicrite); essentially synonymous with lime mud (see above)
Matrix support - lack of a framework consisting of allochems; said of a rock or sediment in which matrix supports larger grains (if present); opposite of grain support
Micrite - microcrystalline calcite of either an organic or inorganic origin that is less than 4 microns in diameter; recrystallized lime, or carbonate, mud. Micrite is synonymous with the term matrix when applied to carbonate rocks; both Dunham's (e.g. "mudstone" and "packstone") and Folk's classification (e.g. "biomicrite") use the presence and abundance of micrite vs. allochems to classify carbonate rocks. One of the three primary components of carbonate rocks (see also spar and allochem)
Micritization - conversion of the microstructure of an allochem to micrite through the activity of boring algae. 
Microspar - micrite that has undergone neomorphic growth; microspar is between 4 and 30 microns in maximum diameter. 
Neomorphism - the alteration of one species or form of a mineral to another; literally changing to a "new form"; this includes any type of chemical diagenesis, e.g.: dolomitization, inversion, recrystallization, replacement; and silicification
Ooid - a carbonate grain formed by precipitation of microscopic crystals of aragonite or calcite in concentric layers around a nucleus; (see coated grain). 
Pellet - small, regular micritic grains presumed to be of fecal origin. 
Peloid - a relatively large micritic lump of uncertain origin; peloids may be irregularly shaped fecal pellets or intraclasts, or perhaps skeletal grains which have been pervasively micritized
Penecontemporaneous - referring to the period of time after deposition but before burial; i.e. used to refer to processes or products related to activities at the sediment-water interface; the very earliest stage in diagenesis ("eogenesis")
Poorly washed - antonym of "well-winnowed" (see winnow); denoting the presence of appreciable amounts of lime mud (or micrite) in the interstices between grains.
Pseudospar - the end product of neomorphism of micrite; during this type of neomorphosis, the grain size of micrite (< 4 microns) increases to microspar (4-30 microns) and then to pseudospar (> 30 microns). 
Recrystallization - changes in size or shape of carbonate grains in the solid state (i.e. without dissolution), while maintaining more-or-less the same chemical composition (e.g. neomorphism of micrite to microspar); see also replacement
Replacement - changes in crystal size or chemistry that occur by dissolution of mineral crystals, followed later by precipitation of the same or another mineral; see also recrystallization
Silicification - the replacement of minerals in sediment or rock by silica, especially chalcedony or chert
Skeletal grain - fossil grains or fragments of fossils within a carbonate sediment or rock
Spar - carbonate cement precipitated in the interstices between or within grains; both Dunham's and Folk's classification (e.g. "grainstone" or "biosparite") use the presence of spar between framework grains to classify carbonate rocks; one of the three primary components of carbonate rocks (see also micrite and allochem)
Well washed - essentially synonymous with well-winnowed. 
Winnow - to remove lime mud by the hydraulic action of waves or currents; "because it lacks micrite in its interstices, this skeletal grainstone, or biosparite, can be termed well-winnowed."