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Dipartimento di Chimica Strutturale e Stereochimica Inorganica, Università degli Studi di Milano, via Venezian 21, I-20133 Milano, Italy
E-mail address:: carlo.gramaccioli{at}unimi.it
Demartinite, ideally K2SiF6, is a new hexagonal polymorph of potassium fluorosilicate, found in a fumarole at the rim of La Fossa crater, Vulcano Island, Aeolian Archipelago, Sicily, Italy. The mineral occurs as colorless hexagonal pyramidal crystals up to 0.3 mm across on a fragment of altered pyroclastic breccia, together with hieratite, avogadrite and knasibfite, a recently approved tetrafluoroborate-hexafluorosilicate of Na and K. The mineral is hexagonal (6mm), space group P63mc, with a 5.6461(8) and c 9.2322(18) Å, V 254.88(7) Å3 . The crystals are sharp hexagonal pyramids {112}, terminated by the {00
} pedion. The crystal structure has been refined to a final R index of 0.018; its SiF62– groups have the same geometry as in the other natural fluorosilicates known. There are two independent twelve-coordinated K sites, displaying distorted cuboctahedral and hexagonal cuboctahedral coordination, respectively. The K–F distances are in the range 2.800(2) – 3.087(2) Å, with an average of 2.885 Å. The Si–F distances in the octahedral SiF62– anion range from 1.681(2) to 1.689(2) Å, with an average of 1.685 Å. The chemical analysis obtained by EDS microprobe gave (wt%) K 35.1, Si 12.4, F 51.0, Na 0.2, total 98.7, corresponding to the empirical formula (K2.00Na0.02)
2.02Si0.99F5.99, based on nine atoms per formula unit. The calculated density is 2.87 g cm–3, the measured density obtained by flotation in heavy liquids is 2.85 g cm–3. The indices of refraction are: 
1.350 ± 0.005; 
1.340 ± 0.005 (589 nm). Demartinite is uniaxial (–) and nonpleochroic (colorless). The strongest six lines in the X-ray powder-diffraction pattern [dobs in Å(I) hkl] are: 2.301(100)(004), 4.62(75)(002), 2.155(54)(022), 4.32(43)(011), 4.90(25)(010) and 2.358(22)(021). The mineral is named in honor of Prof. Francesco Demartin (b. 1953), who has made significant contributions to the chemistry of metallic clusters and to the crystal structure of Alpine rare-earth minerals and uranium minerals. Both the mineral and the mineral name have been approved by the Commission on New Minerals and Mineral Names of the IMA (No. 2006–034).
Keywords: demartinite, new mineral species, hieratite, crystal structure, Vulcano Island, Aeolian Islands, Italy.
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