Calcite crystals from Illinois, USA.

Pale yellowish = calcite Pale bluish = fluorite

A mineral is a naturally-occurring, solid, inorganic, crystalline substance having a fairly definite chemical composition and having fairly definite physical properties. At its simplest, a mineral is a naturally-occurring solid chemical. Currently, there are over 5400 named and described minerals - about 200 of them are common and about 20 of them are very common. Mineral classification is based on anion chemistry. Major categories of minerals are: elements, sulfides, oxides, halides, carbonates, sulfates, phosphates, and silicates.

The carbonate minerals all contain one or more carbonate (CO3-2) anions.

Calcite is a common mineral. It is calcium carbonate (CaCO3). It has a nonmetallic luster, commonly clearish to whitish to yellowish to grayish in color, is moderately soft (H≡3), moderately light-weight, has hexagonal crystals, and rhombohedral cleavage (three cleavage planes at 75º & 105º angles - cleavage pieces look like lopsided boxes). The easiest way to identify calcite is to drop acid on it - it easily bubbles (effervesces) in acid. The bubbles are carbon dioxide gas. If the acid is dilute hydrochloric acid, the chemical reaction is:

2HCl(aq) + CaCO3(s) -->> CO2(g)↑ + H2O(l) + CaCl2(aq)

The most important & voluminous calcitic rocks in the world are limestone (sedimentary), marble (metamorphic), carbonatite (igneous), and travertine (speleothem, or "cave formations", and many hotspring deposits). Quite a few hydrothermal veins in the world are calcitic or have calcite as a principal component.

The calcite crystals seen here are scalenohedrons with low angle rhombohedron terminations. This specimen is from a Mississippi Valley-type deposit in southern Illinois. Commonly abbreviated "MVT", Mississippi Valley-type deposits are named for a series of mineral deposits that occur in non-deformed platform sedimentary rocks along the Upper Mississippi River Valley, USA. Many specific minerals occur in MVT deposits, but are dominated by galena, sphalerite, barite, and fluorite. These minerals occur in caves and karst, paleokarst structures, in collapse fabrics, in pull-apart structures, etc. MVT deposits in America are mined as important, large sources of lead ore and zinc ore. The classic areas for MVT deposits are southern Illinois, the tristate area of Oklahoma-Missouri-Kansas, northern Kentucky, southwestern Wisconsin, and southeastern Missouri. The minerals are hydrothermal in origin and were precipitated from basinal brines that were flushed out to the edges of large sedimentary basins (e.g., the Illinois Basin and the Black Warrior Basin). In basin edge areas, the brines came into contact with Mississippian-aged carbonate rocks (limestone and dolostone), which caused mineralization. The brines were 15% to 25% salinity with temperatures of 50 to 200 degrees Celsius (commonly 100 to 150 degrees C). MVT mineralization usually occurs in limestone and dolostone but can also be hosted in shales, siltstones, sandstones, and conglomerates. Gangue minerals include pyrite, marcasite, calcite, aragonite, dolomite, siderite, and quartz. Up to 40 or 50 pulses of brine fluids are recorded in banding of mineral suites in MVT deposits (for example, sphalerite coatings in veins have a stratigraphy - each layer represents a pulse event). Each pulse of water was probably expelled rapidly - overpressurization and friction likely caused the water to heat up. Some bitumen (crystallized organic matter) can occur, which is an indication of the basinal origin of the brines. The presence of asphalt-bitumen indicates some hydrocarbon migration occurred. Some petroleum inclusions are found within fluorite crystals and petroleum scum occurs on fluorite crystals. MVT deposits are associated with oil fields and the temperature of mineral precipitation matches the petroleum window. The brines may simply have accompanied hydrocarbon fluids as they migrated updip.

The high temperatures of these basin periphery deposits wasn't necessarily influenced by igneous hydrothermal activity. Hot fluids can occur in basins that are deep enough for the geothermal gradient to be ~100 to 150 degrees Celsius. If a permeable conduit horizon is present in a succession of interbedded siliciclastic sedimentary rocks, migration of hot, deep basinal brines may be quick enough to get MVT deposit conditions at basin margins.

MVT deposits occur in the Upper Mississippi Valley of America as well as in northern Africa, Scandinavia, northwestern Canada, at scattered sites in Europe, and at some sites in the American Cordillera. Some of these occurrences are in deformed host rocks. MVT deposits have little to no precious metals - maybe a little copper (Cu). Mineralization is usually associated with limestone or dolostone in fracture fillings and vugs. Little host rock alteration has occurred - usually only dolomitization of limestones.

The age of the host rocks in the Mississippi Valley area varies - it ranges from Cambrian to Mississippian. Dating of mineralization has been difficult, but published ages indicate a near-latest Paleozoic to Mesozoic timing.

MVT deposits in the Upper Mississippi River area are often divided into three subtypes based on the dominant mineral: 1) lead-rich (galena dominated); 2) zinc-rich (sphalerite dominated); and 3) fluorite-rich.

The crystalline calcite specimen shown here is from the Illinois-Kentucky Fluorspar District ("fluorspar" is a very old name for fluorite), which is an MVT fluoritic subtype. Fluorite and fluorite-rich rocks are mined for the fluorine, which is principally used by the chemical industry to make HF - hydrofluoric acid. Fluorite mineralization in this district occurred at about 277 Ma, during the Early Permian, according to one published study (Chesley et al., 1994). Another study concluded that fluorite mineralization was much later, during the Late Jurassic (see Symons, 1994).

Locality: Bethel Level, unrecorded/undisclosed mine in the Cave-in-Rock Mining District, southern Illinois, USA

Photo gallery of calcite: <a href="http://www.mindat.org/gallery.php?min=859" rel="nofollow">www.mindat.org/gallery.php?min=859</a>

Some info. on Mississippi Valley-type deposits was synthesized from:

Chesley et al. (1994) - Direct dating of Mississippi Valley-type mineralization: use of Sm-Nd in fluorite. Economic Geology 89: 1192-1199.

Symons (1994) - Paleomagnetism and the Late Jurassic genesis of the Illinois-Kentucky fluorspar deposits. Economic Geology 89: 438-449.

Rakovan (2006) - Mississippi Valley-type deposits. Rocks & Minerals 81(January/February 2006): 69-71.