Nepheline crystal phases

Figure 7.9A shows the NaAlSi04-Si02 (nepheline-silica) system, after Schairer and Bowen (1956). Let us first examine the Si02-rich side of the join. At P = 1 bar, the pure component Si02 crystallizes in the cristobalite form (Cr) at r = 1713 °C (cf figure 2.6). At P = 1470 °C, there is a phase transition to tridymite (Tr), which does not appreciably affect the form of the liquidus curve, which reaches the eutectic point at P = 1062 °C. 

Several other anhydrous calcium aluminosilicates are known, including grossular or garnet (C3AS3), which is a high-pressure phase, various dehydration products of zeolites, and various products formed metastably by crystallization from melts or glasses. Most are too acid in composition to be of clear relevance to cement chemistry, but some of the devitrification products, especially those with compositions near to CA and structures similar to those of nepheline (Na3KAl4Si40i6) or kalsilite (KAlSiOj (Y4), are of possible interest in relation to the formation of calcium aluminate cements. 

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