CompuServe Messages

> 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.