Monday, August 22, 2016

Photomicrograph- Marine, Meteoric And Burial Carbonate Cements

JSR Paper Clips in their "A Look Back" series highlights an influential paper by J.A.D. Dickson on the use of staining of carbonate rocks to differentiate in a thin section the different mineral phases of calcium carbonate.

A staining procedure consisting of preliminary etching with dilute hydrochloric acid, treatment with a mixed solution of alizarin red-S and potassium ferricyanide, and a final treatment with alizarin red-S alone (Dickson, 1965) permits the distinction of orthorhombic carbonates and of calcite from other trigonal carbonates. The potassium ferricyanide stain reveals the distribution of iron in both calcite and dolomite. The use of the stains is illustrated by a discussion of the petrography of selected specimens and interpretations of the origin of various petrographic entities.

I am heartily thankful for this technique. I stained literally hundreds of thin sections of Ordovician carbonates for my PhD work. It helped me understand the changes in cement types and their chemical composition as the limestones passed from a marine setting to becoming a freshwater aquifer during sea level drops to their ultimate burial to depths of hundreds of feet where they encountered Mg rich brines from which precipitated the mineral dolomite.

Here is that sequence brought out so clearly by a mix of Alizarin Red S and Potassium Ferricyanide.

1) Bladed crystals of non ferroan marine calcite nucleated on a brachiopod shell (stained pink)
2) Equant crystals of ferroan calcite precipitated in a confined fresh water aquifer that formed during a late Ordovician sea level drop (stained purple)
3) Rhombic crystals of a non-ferroan dolomite precipitated during deep burial (not stained). This dolomite cuts across the early marine and later ferroan calcite cements.

... my series on photomicrographs of carbonates will continue...

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