Cubic QZ? melanophlogite
3 messages in this thread
> You found it in "old" books; I hope this does not mean your formula has been
> debunked with no specific one to take its place –
As I said in other thread, my book =1980; reference =1972. BUT — I found the
original work in our Technical Information Center (i.e. library): 'A
Contribution to the Crystal Chemistry of Melanophlogite', Lubor Zak, in
*American Mineralogist* V57pp779-796 (1972). Written about an occurrence (the
second known!) at Chvaletice, Bohemia (Czechoslovakia). Considering the
exceedingly in-depth study represented in this paper, I do not think the
formula has been/will be debunked: the diffraction was single-crystal of
several orientations as well as powder; and the paper includes a SEM photos and
several micrographs. A plate shows drusy crystals, cubes to 0.5cm, described
as colorless, dull, and covered by a light brown limonite coating. Additional
tests included microprobe and several instruments I had not heard of.
Incidentally, the mineral is not fluorescent.
The mineral is shown to have a "clathrate" structure (it was new to me also,
means 'cage-like'), and the contents of the cages called 'guest molecules' are
variable — in particular, zoned (visible in the micrographs). At the close of
all the testing, the formula is given –to two decimal places!– 46SiO2 * C2.17
H17.25 O5.42 S0.09 per 1/4-unit-cell. (see final paragraph).
So you see I did not type a wrong number. The explanation for the odd
stoichiometry seems to be that the guest molecules are "without chemical bounds
to the silica framework." That is, not really part of the crystalline
structure. Different molecules are present in different cages; the formula
represents a summation. The guest molecules can be "C + H +/- S +/- O, H + O,
C + O, or S +/- O +/- H", and they vary through the zoning; different rates of
occupancy are even hypothesized.
However, this formula is particularly stated to be "for the Chvaletice
melanophlogite". Differences between this occurrence and the first (from
Sicily) are heavily emphasized. Compare to Rice (1951) referencing Dana (1932)
on the Silician variety: "SiO2 with 5% to 7% SO3, perhaps SiS2." So the
formula may be different at each occurrence. Also, I recall you were
originally confounded by the presence of organics in a mineral (now we can say,
'surrounded by a mineral'); so I include this: "The Chvaletice
melanophlogite…crystallized in the last stage of metamorphic hydrothermal
vein formation… The source of carbon was country rock sediments: graphite
schists and fine-grained rhodochrosite with graphite."
Further info for hard-cores: the "tetragonal distortion of the cubic 13A
cell…is most probably due to the guest molecules." The tetragonal supercell
(2:2:1 x 13A) reverts to a 13A cubic on decomposition of the organic molecules
by heat. The unit-cell formula, with 46 Si-atoms, represents the cubic
sub-cell, not the 4-times-as-big supercell.
Elizabeth,
Clarthates aren't all that unusual. Zeolites do it all the time, usually the
included fractions are bulky ions like rare earths. However, in the case of
the Zeolites, rather than a neutral molecule, you have a positive ion
displacing one (or more) of the small hydrogen ions.
Physical clarthates occur all across chemistry, so I don't suppose it's very
surprising that they occur in nature, too. Some examples are the complexes
formed from urea and hydrocarbons, and at low temperature, from water ice and
gaseous argon. For a while, this particular clarthate had people going, they
thought they had found a compound of argon and H2O.
— Norm
Hi, Elizabeth–
I have heard of clathrates in discussions of ices on other worlds, where water
or dry ice may hold things like methane and whatnot. I did not expect one to
bite me on my home turf.
I guess I will just have to learn to like that squirrelly melanophlogite
formula. Just don't ask me to balance the valences! It still seems to me
there would have to be neutral hydrogen (H2) molecules, which I would expect to
diffuse out, as they can from solid metals.
–Doug