CompuServe Messages

Hi, Elizabeth and Rod– Finally remembering melanophlogite, another SiO2 mineral "containing organic compounds", I looked it up in the Encyclopedia of Minerals (2nd ed.) and found its crystals are simple cubes. The Encyclopedia of Minerals says cristobalite (SiO2) crystals are "usually pseudo-octahedral, rarely pseudocubic … Also massive; stalactitic; spherulitic; as crusts and botyroidal aggregates". I assume the "pseudo" prefixes refer to the fact that high (beta) cristobalite is of the cubic system, generating true octahedra and rarely cubes, but since that is not stable below 1470 C or even metastable below 200 C (as implied by the following test), these forms are low cristobalite pseudomorphs after high cristobalite. Hurlbut & Klein says when cristobalite (low = alpha, presumably) is heated to 200 C it inverts to high = beta cristobalite, becoming clear, and on cooling reverts to a milky appearance. This may be a worthwhile test if there is too little for density measurements (which should easily distinguish cristobalite from quartz). Hurlbut & Klein says that high cristobalite "transforms reconstructively" (whatever that means) to tridymite below 1470 C at 1 bar; I am unclear as to how it "inverts" to low cristobalite if that is the case, unless it is a different transformation at higher pressures, or when high cristobalite has remained metastable at lower temperatures, as the book says it does. My understanding of opalescence is that opal has "spheres" of cristobalite embedded in the hydrous silica matrix; when these are layer-sorted by size one gets the play of colors that make it precious opal. I suspect this would not work unless the cristobalites were very close to spherical. I seem to remember reading of a quartz pseudomorph after fluorite somewhere; that would likely be cubes. If I had an electronic copy of all my rockhound magazine back issues, I could hunt up this reference easily; as it is, this would be a daunting task. Rod, if you can tell us what sort of environment this "cubic quartz" was found in, and what minerals are associated, it may help rule out some of the possibilities. –Doug