#Cubic Quartz
7 messages in this thread
Quartz – we all know all about quartz. Right??? Quartz if crystallized is
probably commonly found as prismatic, horizontially striated crystals
in matrix, occasionally doubley terminated. Quartz is a member of the
hexagonal crystal system and therefore when looking down on the crystal from
above, one sees the six sided hexagonal structure. How about cubic quartz?????
More later as I obtain further information from a serious field collector of
the Santa Monica Mountains, California.
Rod
Hi, Rod–
(a) Cubes of quartz: pseudomorphs of quartz replacing some cubic mineral?
(b) An article on calcite crystal forms in a semi-recent rockhound mag issue
showed how some calcite forms could come awfully close to cubic in shape, so
only a goniometer could tell it was not a true cube. Calcite is of the same
rhombohedral crystal system as quartz.
(c) While silicon dioxide might crystallize in the cubic system, it would not
be quartz – a mineral is characterized by both a specific chemical composition
and a specific crystal structure. Are we talking about a polymorph of quartz?
(d) None of the above
Save some cubes for me!
–Doug
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
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
I can't get too technical (I'm an arctic geomorphologist, not a geologist) but
I have a sample here of quartz (it's been tested!) with perfect cleavage in two
directions. This apparent cleavage is caused by a specific set of
pressure-heat conditions. I'm afraid I know little about this, but could the
heat-pressure produce cubes?
L.McC.
Hi, Larissa–
Welcome to the forum/section!
Thanks for the alert; quartz is so bashful about exhibiting its cleavage, that
it is not often noticed, mentioned, or thought of. But I have read that quartz
has a rhombohedral cleavage, rather like that of calcite. I am no
crystallographer myself, but the hexagonal/rhombohedral system symmetry of
quartz (and calcite) makes a cubic cleavage seem just as improbable as a cubic
crystal, for the same reasons.
–Doug
Many thanks. I had noticed that tendancy and I appreciate the explaination!
L.McC.