The scapolite composition varies between that of the end members marialite Na4[Al3Si9O24]Cl and meionite Ca4[Al6Si6O24]CO3. Two binary solid-solution series exist between the end members: series (a) between 0 and 75% meionite is formed by a coupled replacement [Na4 · Cl]Si2 ⇌ [NaCa3 · CO3]Al2, and series (b) between 75 and 100% meionite is formed by a coupled replacement [NaCa3 · CO3]Si ⇌ [Ca4 · CO3]Al. These reactions behave independently. High-resolution transmission-electron microscopy indicates that series (a) scapolites belong to space group P4 or P4/m the result of ordering of the clusters (in the square brackets) and that series (b) scapolites belong to space group I4/m the result of disordered clusters. The ordering of the clusters in series (a) gives rise to antiphase domains; such domains are not observable in series (b). Although crystals from both series may be intergrown, the order-disorder relationships resulting from the net charge differences among the clusters indicate that they give rise to chemical domains, despite the fact that the framework is continuous. The observed compositional variations in scapolite can be explained adequately in terms of the two independent substitution schemes given above. Al-O-Al bonds, which tend to be unfavorable in aluminosilicates, are more numerous in series (b) scapolite; it seems likely that the highly charged [Ca4 ·CO3]6+ clusters stabilize such bonds.

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