Channel CO2 in cordierites

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 shown 1 that two of the more common channel occupants, Na + and molecular H 2 O, significantly modify cordierite's refractive indices ( α , β , γ ), optic angle (2 V x ), unit cell dimensions, and the distortion index (Δ) defined by Miyashiro 2 . Indeed, for 11 natural cordierites, excluding indialites, Δ correlates almost entirely with channel Na + and/or H 2 O and with R a , 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 2 V x increased for the nine optically (−) crystals but decreased to below 90° for the two optically (+) crystals. Selkregg and Bloss 1 attributed these effects to loss of channel H 2 O and/or Na + but did not explain why the two (+) crystals became (−) after heating. We report here that loss of molecular CO 2 , a frequently reported channel occupant 3–5 , was the reason. We observe that for Mg-cordierite, 2 V x is ∼87° if its channels are vacant, decreases to 45° or less as channel H 2 O increases and increases to 114° with increasing channel CO 2 (but no H 2 O). Channel CO 2 is apparently the long-sought-for cause of optically (+) cordierites.