Crystal field and covalency of octahedral chromium in natural ^[8](Mg_1−x Ca _x ) ^[6]₃ (Al_0.67Cr_0.33)₂Si₃O₁₂ garnets from upper mantle rocks

Abstract

In the course of a thorough study of the influences of the second coordination sphere on the crystal field parameters of the 3d ^N-ions and the character of 3d ^N–O bonds in oxygen based minerals, 19 natural Cr³⁺-bearing (Mg,Ca)-garnets from upper mantle rocks were analysed and studied by electronic absorption spectroscopy, EAS. The garnets had compositions with populations of the ^[8] X-sites by 0.881 ± 0.053 (Ca + Mg) and changing Ca-fractions in the range 0.020 ≤ w _Ca[8] ≤ 0.745, while the ^[6] Y-site fraction was constant with x ³⁺_{Cr [6]} = 0.335 ± 0.023. The garnets had colours from deeply violet-red for low Ca-contents (up to x _Ca = 0.28), grey with 0.28 ≤ x _Ca ≤ 0.4 and green with 0.4 ≤ x _Ca. The crystal field parameter of octahedral Cr³⁺ 10Dq decreases strongly on increasing Ca-fraction from 17,850 cm⁻¹ at x _Ca[8] = 0.020 to 16,580 cm⁻¹ at x _Ca[8] = 0.745. The data could be fit with two model which do statistically not differ: (1) two linear functions with a discontinuity close to x _Ca[8] ≈ 0.3,

$$ \begin{aligned}{} {\text{range }}0.0 & \le x_{{{\text{Ca}}[8]}} \le 0.3\quad 10{\text{Dq}} = ( - 2638.3x_{{{\text{Ca}}[8]}} ) + 17846.1\quad (r = 0.993) \\ {\text{range }}0.3 & \le x_{{{\text{Ca}}[8]}} \le 0.75\quad 10{\text{Dq}} = ( - 1417.8x_{{{\text{Ca}}[8]}} ) + 17616.5\quad (r = 0.997) \\ \end{aligned} ,$$

(2) one continuous second order function,

$$ {\text{range }}0.0 \le x_{{{\text{Ca}}[8]}} \le 0.75\quad 10{\text{Dq}} = 1242.1x^{{\text{2}}}_{{{\text{Ca}}[8]}} - 2576.5x_{{{\text{Ca}}[8]}} + 17,836\quad (r = 0.996) $$

The behaviour of the crystal field parameter 10Dq and band widths on changing Ca-contents favour the first model, which is interpreted tentatively by different influences of Ca in the structure above and below x _Ca[8] ≈ 0.3. The covalency of the Cr–O bond as reflected in the behaviour of the nephelauxetic ratio $\beta {\text{ = (}}B_{{{\text{crystal}}}} {\text{/}}B_{{{\text{free ion}}}} {\text{)}}_{{{\text{Cr}}^{{{\text{3+}}}} }} ,$ decreases on increasing Ca-contents.