The structure hierarchy hypothesis states that structures may be ordered hierarchically according to the polymerization of coordination polyhedra of higher bond valence. Ramsdellite occurs in isolated individualsĪggregates of colourless, glassy octahedralįinally, an interesting Y-dominant gadolinite-datolite solid solution in spray of
These are matlockite and ramsdellite,Īnd tschermigite found at Punta Chistafòrt. Si tratta di I) matlockite e ramsdellite, rinvenute alla ex miniera di Lausetto, rispettivamente in microcristalli tabulari allungati, perlacei, di colore bianchiccio e in gruppi o individui isolati di cristalli pseudocubici e ottaedrici appiattiti, neri, metallici, sovente striati, su tappeti di finissimi e fragili microcristalli del medesimo ossido, di colore bruno-grigiastro e II) di tschermigite, rinvenuta a Punta Chistafòrt, in aggregati di cristalli trasparenti ottaedrici, vitrei, da incolori a bianchi, unitamente al secondo ritrovamento provinciale di pickeringite in ciuffi di soffici microcristalli bianchi. Recenti indagini SEM-EDS e microRaman condotte su campioni rinvenuti da uno di noi (SF) alla miniera abbandonata di Lausetto (San Lorenzo, Valdieri) e a Punta Chistafòrt (Entracque), hanno permesso di identificare tre specie minerali nuove per la provincia di Cuneo e la regione Piemonte. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart Moreover, “bakerite” is discredited as mineral species because it does not meet the conditions of the dominant-constituent rule. The drugmanite subgroup is defined by the dominance of divalent cations (usually Pb2þ) in the A site, vacancy in the Q site and the occupation of the M site.
The herderite subgroup is defined by the dominant divalent cation (usually Ca2þ) in the A site and Be2þ in the Q site, while the M site is vacant. The herderite group is divided into two subgroups. Accordingly, the Q site is dominantly occupied by B3þ in the datolite subgroup and by Be2þ in the gadolinite subgroup. The A site is dominantly occupied by divalent cations in the datolite subgroup and by trivalent cations in the gadolinite subgroup. The gadolinite group is divided into the gadolinite and datolite subgroups. The gadolinite supergroup is divided into two groups defined by prevailing charge occupancy at the T site Si4þ in gadolinite group and P5þ or As5þ in herderite group. The classification of the gadolinite supergroup is based on the occupancy of A, M, Q, T and’ sites and application of the dominant-valency and dominant-constituent rules. Each site is occupied by: A Ca, REE (Y and lanthanoids), actinoids, Pb, Mn2þ, Bi M Fe, □ (vacancy), Mg, Mn, Zn, Cu, Al Q B, Be, Li T Si, P, As, B, Be, S and’ O, OH, F. The newly defined gadolinite supergroup approved by the IMA CNMNC (vote 16-A) includes mineral species that have the general chemical formula A2MQ2T2O8’2 and belong to silicates, phosphates and arsenates. The pegmatites at Ehrenfriedersdorf, Germany have produced samples at the boundary between herderite and hydroxyl-herderite, if not possibly herderite as well. Herderite is now documented from Mogok, Myanmar Yichung, China Brazil and probably Erongo, Namibia. Most herderite-series minerals are the product of late-stage processes in granite pegmatite evolution, typically of an exsolved fluid upon crystallization, and significant F-enrichment in these fluids appears to result in the domination of herderite over hydroxyl-herderite. Herderite from Mogok is associated with complex rare-element beryl-type granite pegmatite.
The crystal structures of herderite, CaBePO4, a = 9.7446(4), b = 7.6769(3), c = 4.7633(2) angstrom, beta = 90.667(1)degrees, V = 356.31(4) angstrom, P2(1)/a, and Z = 4, from the Mogok Stone Tract, Myanmar, and hydroxyl-herderite, CaBePO4, a = 9.7615(4), b = 7.6680(3), c = 4.7853(2) angstrom, beta = 90.184(1)degrees, V = 358.18(4) angstrom(3), P2(1)/a, and Z = 4, from the Sauberg mine, Morgenrother Zug, Ehrenfriedersdorf, Germany, have been refined to R-1 indices of 1.7%, in both cases, using MoK alpha single-crystal X-ray diffraction data.
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