Fontarnauite was discovered in cores recovered from the Kütahya-Emet 2 and 188 (named here as Doğanlar) boreholes drilled in the Emet borate basin near the village of Doğanlar, Kütahya Province, Western Anatolia, Turkey. The Emet (or Emet-Hisarcık) 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. Dcalc = 2.533 g/cm3. The new mineral is optically biaxial (−), α 1.517(2), β 1.539(2), γ 1.543(2) (590 nm); 2Vmeas = 46(1)°; 2Vcalc = 46°; 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)∑2.00(Sr0.82Ca0.18)∑1.00S1.00B5H5O15; 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 P21/c, a 6.458(2), b 22.299(7), c 8.571(2) Å, β 103.047(13)°, V 1202.5(1.0) Å 3, Z = 4. Structure refinement (R1 = 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 Å (I) (hkl)]: 11.1498 (100)(020), 3.3948 (8)(061), 3.3389 (20)(042), 3.1993, 3.1990 (10)(160,

42), 3.0458(10)(052), 3.0250(7)(220), 2.7500 (10)(
22,142), 2.3999 (8)(260), 2.2300, 2.2284(7)(0 10 0,222), 1.9241, 1.9237(7)(311,
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.

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