Szilagyiite, a new uranyl carbonate-selenite mineral related to schröckingerite from the Pickett Corral mine, Montrose County, Colorado, USA

Szilagyiite (IMA 2024-063), NaCa 3 (UO 2 )(CO 3 ) 3 (SeO 3 )F(H 2 O) 6 , is a new uranyl-carbonate-selenite mineral from the Pickett Corral mine, Montrose County, Colorado, USA.The new mineral occurs on sandstone and asphaltite matrix in close association with ferroselite, andersonite, schrckingerite, magselite, and an unidentified Na-Ca-uranyl carbonate--selenite-sulfate.Szilagyiite is trigonal, space group R3c (#161), with unit cell parameters a = 9.6542(9), c = 33.465(5), V = 2701.2(6) 3 and Z = 6.Crystals occur as dense yellow-green rosettes up to 1 mm wide and individual tablets up to ~200 m.Szilagyiite crystals are predominantly tabular on {001} and exhibit {001}, {00-1}, {102}, and {0-1-2} forms, with frequent twinning by inversion and perfect {001} cleavage.It has a pale yellow-green streak and fluoresces dimly green under longwave UV and 405 nm illumination, but has no apparent fluorescence under SWUV.Crystals are transparent with vitreous luster and exhibit a brittle, uneven fracture, with a Mohs hardness between 2-3.The calculated density based on the empirical formula is 3.17 g/cm 3 , and 3.16(2) g/cm 3 as measured by flotation in a mixture of diiodomethane and toluene.The mineral is optically uniaxial (-), with = 1.628( 2), = 1.538(2) measured in white light.It is pleochroic: O yellow, E colorless; O > E. The empirical formula is Na 0.76 Ca 3.11 (UO 2 )(CO 3 ) 3 (Se 1.16 O 3 )F 0.82 O 20.18 H 13.19 based on 21 O + F, U = 1, with C = 3 apfu based on the structure and H set to achieve charge balance.The eight strongest powder X-ray diffraction lines are [d obs (I)(hkl)]: 5.916(100)(104), 4.836(58)(110), 3.744(77)(018), 3.125(33)(211,122), 2.960(60)(214), 2.795(62)(300), 1.828(40)(410) and 1.744(35)(238,146).The structure of szilagyiite (R 1 = 0.0314 for 2335 reflections with I > 2I) is based on infinite sheets built from 3 major components: distorted cubane-like [(SeO 3 ) Ca 3 (F,OH)(H 2 O) 3 ] units, NaO 4 (H 2 O) 3 monocapped trigonal antiprisms, and hexagonal bipyramidal uranyl tricarbonate cluster units, [UO 2 (CO 3 ) 3 ].The sheets are cross-linked by a thin layer of hydrogen bonds formed between interlayer H 2 O bound to Na, with F/OH, and O in the sheets.