Crystal structure and optical spectroscopy (300 to 2200 nm) of CaCrSi₄O₁₀

Howard L. Belsky¹, George R. Rossman², Charles T. Prewitt¹, Tibor Gasparik³

¹ Department of Earth and Space Sciences, State University of New York,
Stony Brook, New York 11794, U.S.A.

² Division of Geological and Planetary Sciences, 170-25, California Institute of Technology,
Pasadena, California 91125, U.S.A.

³ Department of Geophysical Sciences, The University of Chicago,
Chicago, Illinois 60637, U.S.A.

ABSTRACT

Crystals of CaCr²⁺Si₄O₁₀ were synthesized at 1 atm and 1400-1350°C in a closed system with f_O2 <10^-16 bar.  CaCrSi₄O₁₀ crystallizes in the space group P4/ncc with a = 7.378(5)Å and c = 15.119(2)Å. The structure was refined to R = 0.026 (R_w = 0.031) based on 546 reflections. CaCrSi₄O₁₀ is a structural analog of gillespite and other  ABSi₄O₁₀ compounds, and although not itself a mineral, has been shown to exhibit structural and comporession properties nearly identical to those of gillespite. The most interesting aspects of the structure are the square-planar coordination ofCr ²⁺, and the Si-O1-Si bond angle of 176.8°.  Apparently, Cr²⁺ is stable in the gillespite structure because it is readily accepted into the square-planar site, in contract to other Cr²⁺ compounds that are distorted from type structures as a result of the Jahn-Teller effect in Cr²⁺.  The optical absorption spectrum of CaCrSi₄O₁₀ has bands at 511 and 670 nm polarized perpendicular to c and at 453 nm parallel to c, with all absorption occuring only through vibronic coupling. Diamond cell experiments indicte that CaCrSi₄O₁₀ remains tetragonal to at least 50 kbar, with compression behavior identical to that of gillespite.