DEMARTINITE, A NEW POLYMORPH OF K2SiF6 FROM LA FOSSA CRATER, VULCANO, AEOLIAN ISLANDS, ITALY

Abstract

Demartinite, ideally K 2 SiF 6 , is a new hexagonal polymorph of potassium ß uorosilicate, 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 knasibÞ te, a recently approved tetraß uoroborate-hexaß uorosilicate of Na and K. The mineral is hexagonal (6mm), space group P6 3 mc, with a 5.6461(8) and c 9.2322(18) Å, V 254.88(7) Å 3. The crystals are sharp hexagonal pyramids {112}, terminated by the {001} pedion. The crystal structure has been reÞ ned to a Þ nal R index of 0.018; its SiF 6 2groups have the same geometry as in the other natural ß uorosilicates 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 SiF 6 2anion 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 (K 2.00 Na 0.02) ⌺2.02 Si 0.99 F 5.99 , based on nine atoms per formula unit. The calculated density is 2.87 g cm-3 , the measured density obtained by ß otation 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 [d obs 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 signiÞ cant 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).

K2B12F12: A rare A2X structure for an ionic compound at ambient conditions

“…It is the packing of ions in the stoichiometrically simple A 2 X compound K 2 B 12 F 12 that is unusual and therefore of interest. Several drawings of the structure of K 2 B 12 F 12 , using the same relative orientation, are shown in Figs [6], K 2 GeF 6 [7], and K 2 MnF 6 [14], all of which have a Cs 2 S-like structure. However, unlike those structures, which have half of the K + ions filling every interstitial O h hole and half filling every other T d hole, half of the K + ions in K 2 B 12 F 12 fill every O h hole and half fill every D 3h trigonal hole within the close-packed layers, holes that are midway between two T d holes.…”

“…Similarly, K 2 SiF 6 and K 2 GeF 6 each exhibit two polymorphs. For K 2 SiF 6 , the situation is similar to K 2 MnF 6 : one polymorph has the antifluorite structure [5] and the other, which is a naturally occurring mineral recently found in the La Fossa crater on Volcano Island near Sicily, has a Cs 2 S-type structure [6]. However, one polymorph of K 2 GeF 6 has a Cs 2 S-type structure [7] and the other has an unusual hexagonal structure consisting of superimposed close-packed layers of anions with K + ions significantly off-center in half of the trigonal-prismatic holes [8].…”

“…For example, there are at least two polymorphs of K 2 MnF 6 . One has the antifluorite structure [2] (in this paper we equate the packing arrangement of EF n mÀ anions to the packing arrangement of their central atoms, not their fluorine atoms) and one has a Cs 2 S-type structure [3] (approximate HCP packing of anions with K + ions near the centers of all Mn 6 O h holes and near the centers of half of the Mn 4 T d holes) [4]. Similarly, K 2 SiF 6 and K 2 GeF 6 each exhibit two polymorphs.…”

Li2PbF6 and SrPbF6 Revisited

“…[11,12] The crystal structures of the alkali metal hexafluoridosilicates(IV) and hexafluoridogermanates(IV) are also known for Li-Cs. [13][14][15][16][17][18][19][20][21][22][23][24] The compounds M I 2 SnF 6 (M I = Li-Cs) are structurally characterized as well. [13,[25][26][27] For the stannates(IV), the crystal structures of the compounds M II SnF 6 (M II = Mg, Ca, determined the decomposition temperature of Li 2 PbF 6 .…”

Fumarolic Minerals: An Overview of Active European Volcanoes

“…Rare. Demartinite, Vu[62] polymorph of hieratite, probably metastable. In colourless hexagonal pyramidal sub-mm crystals.…”