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,

1
42), 3.0458(10)(052), 3.0250(7)(220), 2.7500 (10)(
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22,142), 2.3999 (8)(260), 2.2300, 2.2284(7)(0 10 0,222), 1.9241, 1.9237(7)(311,
2
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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