Abstract
A detailed analysis of x-ray powder diffraction graphs of chromium chlorite yields evidence for chromium substitution between 0 per cent and 8 per cent Cr2O3 in both the octahedral and tetrahedral sites of the talc and brucite layers. “Comorphism” is suggested for this substitution of one element in two different structural positions.
X-ray powder diffraction measurements based on x-ray spectrographic analyses provide accurate determinations of total chromium content. Certain x-ray “d” spacings and intensity values correlate with Cr2O3 content, while others indicate the extent of octahedral or tetrahedral chromium substitution.
Indices of refraction increase almost linearly with chromium content, the octahedral and tetrahedral substitution curves having slightly different slopes which converge at approximately 2 per cent Cr2O3. A similar convergence is noted for x-ray powder and differential thermal data.
Below 2 per cent, there is no correlation of properties and chromium content, because the net effects are overcome by Fe substitutions for Mg. Thermal data indicates that the chromium chlorite structure may have either greater or lesser stability than the Fe-Mg chlorites. Thermal stability is inversely proportional to chromium content.
Chromium chlorites with more than 2 per cent Cr2O3 may be classified as kotschubeite (dominantly tetrahedral chromium) and kammererite (dominantly octahedral chromium). When the Cr2O3 content is less, a terminology analogous to the corundophilite-prochlorite-clinochlore-penninite grouping for the Fe-Mg chlorites may be used. The term “chromium chlorite” is a useful field term applicable to pink or rose-violet chlorite.
