Crystal-chemical relations and classification problems of tourmalines belonging to the oxy-schorl–oxy-dravite–bosiite–povondraite series

Two oxy-tourmalines showing intermediate compositions between the oxy-schorl and oxy-dravite series (sample from Sludyanka crystalline complex, Baikal, Russia) and the oxy-dravite and bosiite series (sample from the Valletta mine, Piedmont, Italy) were structurally and chemically characterized by single-crystal X-ray diffraction, electron microprobe analysis, optical absorption and Synchrotron Mössbauer source spectroscopy. A comparative analysis of the results, along with data from literature on additional samples corresponding to the species oxy-schorl, oxy-dravite, bosiite and povondraite, show that the main structural variations within the Fe-bearing, Na-dominant oxy-tourmalines are related to variations in the content of Fe, which is incorporated into the structure via substitution mechanisms that involves Al. The results also show the occurrence of a complete substitution series from oxy-schorl via oxy-dravite and bosiite to povondraite, but no (or a limited) direct substitution series between oxy-schorl and bosiite (or povondraite). The substitution of Al by larger cations causes changes in 〈Y–Oi and 〈Z–Oi distances as well as in the unit-cell parameters. The effect of 〈Y–Oi on the a-parameter is evident in the oxy-schorl–oxy-dravite series, but it is of marginal significance in the oxy-dravite– bosiite–povondraite series. On the other hand, a good correlation exists between 〈Z–Oi and the a-parameter only in the oxy-dravite– bosiite–povondraite series. The c-parameter is strongly correlated with 〈Z–Oi along the entire substitution series. Classification problems related to the naming of Fe-bearing, Na-dominant oxy-tourmalines are discussed. Application of the nomenclature rules in force resulted in ambiguous naming of the studied samples. This makes, for example, direct comparisons of physical and chemical properties of tourmalines originating from different deposits difficult

Ambrinoite, (K,NH4)2(As,Sb)8S13·H2O, a new mineral from Upper Susa Valley, Piedmont, Italy: The first natural (K,NH4)-hydrated sulfosalt

Ambrinoite, ideally (K,NH4)2(As,Sb)8S13⋅H2O, occurs as a rare sulfosalt species in the Triassic evaporitic formation of Gessi (gypsum) outcropping near the hamlet of Signols (Oulx, Susa Valley, Torino, Piedmont, Italy). The new species is associated with sulfur and orpiment; in the same occurrence galkhaite, stibnite, and enargite were also identified. Ambrinoite occurs as aggregates of tabular crystals up to 1 mm in length. The color is red, with an orange-red streak; the luster is vitreous to resinous. The mineral is transparent; its microhardness VHN(10 g) = 30 kg/mm2, corresponding to a Mohs hardness of about 2. Electron microprobe analysis gives the empirical formula [K1.43(NH4)0.42Na0.02Tl0.01]Σ=1.88 (As5.82Sb2.18)Σ=8.00S13.22⋅1.2H2O, close to stoichiometric [K1.5(NH4)0.5]Σ=2(As6Sb2)Σ=8S13⋅H2O; the calculated density is 3.276 g/cm3. Micro-Raman spectroscopy confirmed the presence of water and ammonium cation. Ambrinoite is triclinic, space group P1, with a = 9.704(1), b = 11.579(1), c = 12.102(2) Å, α = 112.82(1), β = 103.44(1), γ = 90.49(1)°, V = 1211.6(3) Å3, Z = 2. The strongest X-ray powder diffraction lines [d in Å (I) (hkl)] are: 10.78 (100) (001), 5.79 (55) (021), 4.23 (35) (102), 5.31 (34) (102), 5.39 (32) (002). Its crystal structure has been solved by X-ray single-crystal diffraction on the basis of 2667 unique reflections, with a final R = 0.035. It is formed by two kinds of modules: slabs (110)PbS of modified PbS archetype (type A slabs) and openwork slabs with channels accomodating (K,NH4)+ cations and H2O molecules (type B slabs). Its structure can be described as an order-disorder (OD) structure, built up by two different kinds of layers. Taking into account only the short (As,Sb)-S bonds, (As,Sb)S3 triangular pyramids form double chains similar to those described in other natural and synthetic compounds, among which its homeotype gillulyite, as well as gerstleyite. Ambrinoite belongs to the hutchinsonite merotypic family. It is probably the product of late-stage hydrothermal fluid circulation. The name of this new mineral species (IMA 2009-071) honors Pierluigi Ambrino (b. 1947), the mineral collector who kindly provided us with the studied specimens

Ambrinoite, (K,NH4)2(As,Sb)8S13·H2O, a new mineral from Upper Susa Valley, Piedmont, Italy: The first natural (K,NH4)-hydrated sulfosalt

Ambrinoite, ideally (K,NH4)2(As,Sb)8S13⋅H2O, occurs as a rare sulfosalt species in the Triassic evaporitic formation of Gessi (gypsum) outcropping near the hamlet of Signols (Oulx, Susa Valley, Torino, Piedmont, Italy). The new species is associated with sulfur and orpiment; in the same occurrence galkhaite, stibnite, and enargite were also identified. Ambrinoite occurs as aggregates of tabular crystals up to 1 mm in length. The color is red, with an orange-red streak; the luster is vitreous to resinous. The mineral is transparent; its microhardness VHN(10 g) = 30 kg/mm2, corresponding to a Mohs hardness of about 2. Electron microprobe analysis gives the empirical formula [K1.43(NH4)0.42Na0.02Tl0.01]Σ=1.88 (As5.82Sb2.18)Σ=8.00S13.22⋅1.2H2O, close to stoichiometric [K1.5(NH4)0.5]Σ=2(As6Sb2)Σ=8S13⋅H2O; the calculated density is 3.276 g/cm3. Micro-Raman spectroscopy confirmed the presence of water and ammonium cation. Ambrinoite is triclinic, space group P1, with a = 9.704(1), b = 11.579(1), c = 12.102(2) Å, α = 112.82(1), β = 103.44(1), γ = 90.49(1)°, V = 1211.6(3) Å3, Z = 2. The strongest X-ray powder diffraction lines [d in Å (I) (hkl)] are: 10.78 (100) (001), 5.79 (55) (021), 4.23 (35) (102), 5.31 (34) (102), 5.39 (32) (002). Its crystal structure has been solved by X-ray single-crystal diffraction on the basis of 2667 unique reflections, with a final R = 0.035. It is formed by two kinds of modules: slabs (110)PbS of modified PbS archetype (type A slabs) and openwork slabs with channels accomodating (K,NH4)+ cations and H2O molecules (type B slabs). Its structure can be described as an order-disorder (OD) structure, built up by two different kinds of layers. Taking into account only the short (As,Sb)-S bonds, (As,Sb)S3 triangular pyramids form double chains similar to those described in other natural and synthetic compounds, among which its homeotype gillulyite, as well as gerstleyite. Ambrinoite belongs to the hutchinsonite merotypic family. It is probably the product of late-stage hydrothermal fluid circulation. The name of this new mineral species (IMA 2009-071) honors Pierluigi Ambrino (b. 1947), the mineral collector who kindly provided us with the studied specimens