Original paper
Crystallo-chemical evolution of melilite due to cationic substitution
Kimata, Mitsuyoshi
Neues Jahrbuch für Mineralogie - Abhandlungen Band 159 Heft 2 (1988), p. 181 - 197
59 references
published: Jul 4, 1988
DOI: 10.1127/njma/159/1988/181
ArtNo. ESP154015902005, Price: 29.00 €
Abstract
Extension of the isomorphous substitution in mineral, as an example of melilite, has been investigated by the syntheses and substitution of isovalent cations with different ionic radii. By unsuitable replacement of one kind of cation in a melilite structure by the other isovalent cation, whenever the corresponding melilite cannot be crystallized directly after overstepping the stability field of the melilite structure, there inevitably occurs the phase isostructural with the mineral coexisting with melilite in nature. This extension bears a systematic interpretation by several crystallo-chemical factors: stoichiometry (cation/anion ratio), ionic radius, packing index, coordination number and partition coefficient. This leads to consequences that stoichiometry of the non-melilite phase is in accordance with either of 5/7, 3/4 = (5 + 1)/ (7 + 1) or 2/3 = (5 - 1)/ (7 - 1) in reference to 5/7 of melilite. Prescience is made of the crystal structure generating in the non-melilite crystallization field outside the isomorph boundary. Moreover the range of packing index for each of the melilite series suggests that gehlenite is stable under higher pressure and not åkermanite. The approach by the cationic substitution developed in this study has it’s application in constraining the crystallization process of the magma and more practically in identifying the unknown phases obtained in syntheses.
Keywords
Melilite • cationic substitution • evolution • crystal chemistry