Fontarnauite, (Na,K)2(Sr,Ca)(SO4)[B5O8(OH)](H2O)2, A New Sulfate-Borate Mineral From Doğanlar (Emet), Kütahya Province, Western Anatolia, Turkey

Abstract

Fontarnauite was discovered in cores recovered from the Kutahya-Emet 2 and 188 (named here as Doganlar) boreholes drilled in the Emet borate basin near the village of Doganlar, Kutahya Province, Western Anatolia, Turkey. The Emet (or Emet- Hisarcik) basin is one of the Neogene basins in western Turkey bearing a borate-rich unit intercalated with Miocene sediments. Fontarnauite is most commonly associated with probertite, glauberite, and celestine and occurs as isolated colorless to light-brown prismatic crystals or as clusters of crystals less than 5 mm long. Fontarnauite is brittle, with a Mohs hardness of 2½–3, and perfect {010} cleavage. D calc = 2.533 g/cm3. The new mineral is optically biaxial (−), α 1.517(2), β 1.539(2), γ 1.543(2) (590 nm); 2 V meas = 46(1)o; 2 V calc = 46o; X ^ a 95.0° (β obtuse); Y // b , Z ^ c 81.9° (β acute). Dispersion is r > v , medium to weak. The chemical composition (electron microprobe; B and H from the crystal-structure refinement) is as follows: SO3 17.75, B2O3 38.66, CaO 2.26, SrO 18.98, Na2O 12.65, K2O 1.70, H2O 10.01, total 102.01 wt.%. The empirical formula (based on 15 O atoms per formula unit) is (Na1.84K0.16)Σ2.00(Sr0.82Ca0.18)Σ1.00S1.00B5H5O15; the endmember formula is Na2Sr(SO4)[B5O8(OH)](H2O)2 based on the crystal-structure refinement. Single-crystal X-ray studies gave the space group P 21/c, a 6.458(2), b 22.299(7), c 8.571(2) A, β 103.047(13)o, V 1202.5(1.0) A3, Z = 4. Structure refinement ( R 1 = 2.9%) revealed that two BO4 tetrahedra and three BO3 triangles share vertices to form B5O10(OH) units that link to other B5O10(OH) units along [100] and [001] to give a [B5O8(OH)] sheet parallel to (010). Within the central cavities of opposing sheets are the H2O groups, SO4 tetrahedra, and Na (1) sites; the Sr and Na (2) sites occupy the interstices of a given sheet. The region of the structure where opposing cusps of neighboring sheets approach each other is dominated by weaker H-bonding associated with the OH and H2O groups, in accord with the observed perfect {010} cleavage. The strongest lines in the powder X-ray diffraction pattern, obtained after profile fitting using the Le Bail method, are as follows [ d in A ( I ) ( hkl )]: 11.1498 (100)(020), 3.3948 (8)(061), 3.3389 (20)(042), 3.1993, 3.1990 (10)(160, ![Formula][1] 42), 3.0458(10)(052), 3.0250(7)(220), 2.7500 (10)(![Formula][2] 22,142), 2.3999 (8)(260), 2.2300, 2.2284(7)(0 10 0, 222), 1.9241, 1.9237(7)(311,![Formula][3] 24). The holotype is deposited in the mineralogy collection of the Royal Ontario Museum, 100 Queen's Park, Toronto, Ontario M5S 2C6, Canada, accession number [M56745][4]. [1]: /embed/mml-math-1.gif [2]: /embed/mml-math-2.gif [3]: /embed/mml-math-3.gif [4]: /lookup/external-ref?link_type=GEN&access_num=M56745&atom=%2Fcanmin%2Fearly%2F2016%2F02%2F23%2Fcanmin.1400088.atom