CompuServe Messages

Rod — Cubic system silicon dioxide is high cristobalite. The only *crystal* form referenced by Hurlbut & Klein or Mason & Berry is octahedral ("small"). I believe the cleavage is {111} or parallel to the octahedron's faces. The other habit, mentioned by both, is small "spheroids" (either rounded octahedral crystals or ???). One of the sources also mentions that, at ambient temperatures, the octahedral crystals are milky-translucent. The octahedral crystal shape is retained if high cristobalite inverts to tetragonal system low cristobalite. I believe cristobalite is also common between crystals in some extrusive rocks, and I know it occurs in some sedimentary rocks (cement?) but don't know the mechanism. Also don't know whether these occurrences are low or high cristobalite… but I suspect low. It is evident that I favor Doug's third suggestion — a misstatement "cubic quartz" rather than "cubic SiO2". His first two suggestions are also possible — I have not heard of cubic pseudomorphs of quartz — but straight-forward mis-identification of minerals is common. (Clear fluorite occurs to me, and properly cubic too, not just close! Get out your Mohs' kit.) But even if cristobalite, if the crystal habit is *cubes* — that is apparently unique. So be sure to let us know what info you receive on this mystery mineral occurrence! — E. 1: 2: Why would they exempt this from the FIA?reedom of Information Act? 3: /post