Name: Dolomite in encrinite
Classification: Carbonate rock, grainstone
Mineral composition: Calcite, Dolomite
Fossils: crinoids and other echinoderms, molluscs, brachiopods (not in the photo)
Location: La Varella Refuge, Fanes Alps (46°36'48.35"N 12° 0'24.42"E)
Formation: Fanes encrinite
Era: Lower Jurassic (approximately 175 million years ago)
Depositional environment*:Submerged carbonate substructure

*) The environment refers to the encasing rock and not the dolomite that is the subject of the photo

dolomite

Dolomite is the most characteristic mineral of dolomite rocks, and it is a very common mineral in rocks that are formed in a number of ways. The coloured rhombi in this photograph, measuring up to one tenth of a millimetre, represent one of the many facets of dolomite.
In this example dolomite replaced an existing rock, a limestone (therefore made of calcite, calcium carbonate). What remains of the limestone can be seen in the dark areas  

surrounding the dolomite crystals. In this case the substitution most probably took place because magnesium-rich waters flowed over a calcitic sediment in the subsoil for a very long time, slowly dissolving the calcite and depositing dolomite, which is less soluble, in its place.
Other ways in which dolomite can be formed are through precipitation from very hot waters (hydrothermal dolomite) and in seawater (sedimentary dolomite).

http://en.wikipedia.org/wiki/Dolomite

 

GALLERY