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Dolomite

Dolomilc, CaMg(C0₃)₂, is present in many scdimentary carbonaic rocks, usual-ly replacing pre-existing calcium carbonate. Optically, dolomite is similar to cal-cite, but in sediments, dolomite usually occurs as rhombohedra with distinctive parallelogram-shaped cross-sections. Dołomitic carbonate sediments arc classified according to their dolomite content:

0-10% dolomite: limestone 10-50% dolomite: dołomitic limestone 50-90% dolomite: calcitic dolomite 90-100% dolomite: dolomite

The term ‘dolomite’ is thus used for both the minerał and a rock madę up large-ly of that minerał. This can lead to confusion and the term chłostane is sometimes used for the rock.

130    shows a dołomitic limestone. The original sediment is a peloidal packstone, but parts of the rock. especially the matrix have been rcplaced by dolomite, here showing its characteristic euhedral rhombic shape.

It is not always easy to distinguish dolomite front calcite under the microscope. Whcn individual dolomite crystals growing in a sediment meet one another, con-tinued growth as rhombohedra is not possible and the euhedral shape is lost. A simple Chemical technique is often used to distinguish calcite from dolomite. A thin scclion is immersed in a solution of a slain called Aliz.arin Red S in weak hydrochloric acid. Calcite reacts with the acid and a reddish-coloured precipitate forms. Dolomite does not react so readily with weak acid and remains unchanged.

131    shows a limestone section which has been trcalcd with Alizarin Red S. lt is a dołomitic limestone, calcite showing shades of pink, red and brown. In this case the shell fragmenls are pink and the micritic intraclasls dark red-brown. The dolomite is colourlcss to grey and appears to be preferenlially replacing matrix and/or cement.

132    is a dolomite rock or dolostone in which no tracę of the original sediment remains. The different shades of grey exhibilcd by the dolomite result from its variable relief depending on orientation (as with calcite, page 62). This rock has significanl porosity. The sediment was impregnated with blue-staincd araldite before mounting on the slide and hence pores are blue. Dolomite crystals growing into these pores retain their euhedral shape but elsewhcrc crystals are subhedral or anhedral.

Dolomite

130    Dołomitic limestone in plane-polar-ized light. Locality: Jurassic, Grecce (x 20).

131    Dołomitic limestone treated with Alizariri Red S in planc-polarized light. Locality: Carbonifcrous Limestone, South Wales (x 7).

132    Porous dolomite rock in plane-polar-ized light. Locality: Carbonilerous Limestone. Derbyshirc, Rngland (x 10).

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