Aragonite, CaCC

SAQ 3.10 Calcite and aragonite are both forms of calcium carbonate, CaCC>3. Calcite converts to form aragonite. If A"f°(calcite) = -1206.92 kJ... 

Polymorph Two or more solid crystalline substances with the same chemistry, but different crystalline structures. Calcite (rhombohedral CaCCE) and aragonite (orthorhombic CaCCE) are polymorphs. 

Figure 2.1. Generalized phase relations in the unary system CaCC>3. "A" -aragonite "I" through "V - calcite polymorphs with metastable fields indicated by (). Dash-dot line at 800°C represents transition encountered on cooling runs solid line at lower T represents transition encountered on heating runs. (After Carlson, 1980.)...

This model has the attractive characteristics of not requiring biotic or chemical formation of CaCC>3 directly on odid surfaces and offering an explanation for the observation that odids appear to require "rest" between depositional cycles. It also provides a reasonable explanation for the concentric organic-rich layers generally observed in laminated aragonitic odids. Researchers such as Oppenheimer (1961), Cloud (1962a), Wilson (1967), and Folk (1973) have accepted the general principles put forth in this model. 

Kinsman J.J. and Holland H. D. (1969) The coprecipitation of cations with CaCC>3-IV. The coprecipitation of Sr2+ with aragonite between 16° and 96°C. Geochim. Cosmochim. Acta 33, 1-17. 

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