Channel CO₂ in cordierites

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 (2V_x), 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 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 2V_x 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 occupant^3–5, was the reason. We observe that for Mg-cordierite, 2V_x 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.