Abstract

Synthetic chevkinite and perrierite, Ln3+4M2+ (Ti4+M2+)2(Ti4+O4)2·(Si2O7)2;M2+ = Fe2+, Co, Ni and Mg; Ln3+ = La, Ce, Pr, Nd and Sm, demonstrate polymorphism (1 st order transition) within certain chemical compositions. Mg2Pr4Ti3Si4O22, and Ni2Pr4Ti3Si4O22 crystallized from the gels as perrierites (low temperature form) at temperatures of approximately 990°C in air, transform to chevkinites (high-form) by elevating temperatures above 1070°C. The unit-cell dimensions for these two sets of perrierite-chevkinite polymorphs were obtained from computer refined X-ray powder diffractometer data.

Nearly all of the chevkinites and perrierites are primary phases. Single crystal growth was effected directly from melt either by slow cooling or pulling. The crystals of all the probable end members have been obtained up to maximum size 2mm.

Single crystal X-ray studies using the precession method indicate space group P21/a for the synthetic Co-Pr and Mg-Nd chevkinite, and Mg-La perrierite. The space group originally reported for the natural specimens is C2/m.

Wet chemical analyses of the synthetic crystals for Mg-La perrierite and Mg-Nd chevkinite confirmed the existing structural formula for both minerals.

Infrared spectra of chevkinite and perrierite are very similar except very small changes in absorption frequencies accompanied with the polymorphic transitions.

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