Abstract
The formulae commonly cited for cordierite usually ignore the occupants of the structural channels that run parallel to the c-axis. Recently, however, it has been shown1 that two of the more common channel occupants, Na+ and molecular H2O, significantly modify cordierite's refractive indices (α, β, γ), optic angle (2Vx), unit cell dimensions, and the distortion index (Δ) defined by Miyashiro2. Indeed, for 11 natural cordierites, excluding indialites, Δ correlates almost entirely with channel Na+ and/or H2O and with Ra, the average radius of the ions occupying the octahedral sites, but not (as long believed) with Al/Si ordering. After heating at 800 °C for 6 h, Δ increased for the four crystals remeasured, and 2Vx increased for the nine optically (−) crystals but decreased to below 90° for the two optically (+) crystals. Selkregg and Bloss1 attributed these effects to loss of channel H2O and/or Na+ but did not explain why the two (+) crystals became (−) after heating. We report here that loss of molecular CO2, a frequently reported channel occupant3–5, was the reason. We observe that for Mg-cordierite, 2Vx is ∼87° if its channels are vacant, decreases to 45° or less as channel H2O increases and increases to 114° with increasing channel CO2 (but no H2O). Channel CO2 is apparently the long-sought-for cause of optically (+) cordierites.
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References
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Armbruster, T., Bloss, F. Channel CO2 in cordierites. Nature 286, 140–141 (1980). https://doi.org/10.1038/286140a0
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DOI: https://doi.org/10.1038/286140a0
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