Burnt Carbonates

Abstract

This chapter examines four major calcium carbonate materials – chalk, mollusc shell, earthworm granules and eggshell under a variety of temperature regimes in order to show the range of morphologies presented. Two basic types of alteration occur in burnt carbonates. These are scorching, and calcining. At moderate temperatures between 300 °C and about 700 °C, any organic matter within the carbonate structure starts to burn. If oxidation is incomplete, brown scorching is preserved, making the microscopic view browner and darker in PPL and XPL equally, but more noticeably in PPL. The main organic matter in carbonates is the protein content of biominerals, so browning is most likely to be found in eggshell, mollusc shell and earthworm granules. All materials were heated in oxidizing conditions in a muffle furnace in ceramic containers of about 1 cm in height. Temperature was brought up to the target as quickly as possible without cracking, and then cooled naturally.

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