Chemistry Reference
In-Depth Information
- aluminous:
hornblendes
(Na, K)
0-1
Ca
2
(Fe, Mg, Al)
5
(Si, Al)
8
O
22
(OH)
2
Among the hornblendes, pargasite NaCa
2
Mg
4
Si
6
Al
2
O
22
(OH)
2
occurs in
the metamorphic dolomitic limestones and dolostones; its ferro-magnesian
equivalent, hastingsite occurs in skarns.
-
Pyroxenes
-
diopside
CaMgSi
2
O
6
-
hedenbergite
CaFe
2+
Si
2
O
6
-
johannsenite
CaMnSi
2
O
6
series
-
aluminous diopsides
3.4.2 Occurrences
3.4.2.1 Sedimentary rocks
Calcite and dolomite are the major constituents of sedimentary carbonate
rocks: limestones and dolostones, and carbonate rocks containing a greater
or lesser proportion of clay, quartz, etc. (marls, calcareous sandstone, etc.
Figure 1.10).
Minerals that precipitate from seawater are magnesian calcite and
aragonite. Aragonite may form ooliths and carbonate muds; it is associated
with gypsum and celestine in evaporitic environment; it also may form the
cement of intertidal deposits.
Calcite may also be formed through living organisms (protists, stroma-
tolites, algae, sponges, corals, echinoderms, etc.). Aragonite also forms the
shell or skeleton of some animals (mollusks, and some corals). Bioclast thus
form an important part of the carbonate deposits.
Since aragonite is metastable and recrystallizes into calcite, and magne-
sian calcite loses its Mg, most carbonate sediments are low-Mg calcite.
Calcite and aragonite form also karstic cave formations (stalagmites,
stalactites, etc.) Aragonite, in particular, forms cave pearls, cave flowers and
frostwork.
Dolomite may precipitate directly from seawater in so far that its Mg/
Ca ratio is increased: supersaturated anaerobic lagoons and deep seated
sediments rich in organic material. In a lagoonal environment, the order of
precipitation follows the order of increasing solubility in seawater: calcite,
dolomite, gypsum/anhydrite, halite, potassium and magnesium salts.
Most dolomites do not seem to have been formed in such extreme envi-
ronments. The process is probably a diagenetic evolution caused mixing of
freshwater and saltwater by subsurface conditions. This mixing lowers the
salinity of seawater and/or allows the dissolution of magnesium either from
magnesian calcite either from clay, which increases the Mg/Ca ration of
the waters. Sub-saturation of these waters in calcite and supersaturation in
dolomite occurs then: dolomite precipitates and therefore replaces calcite.
