Schroeckingerite crystals have been found at Moab, Utah, and San Isidro, Argentina. They are orthorhombic pseudohexagonal, flattened on {001} with additional forms {010}, {110}, {014}, {012}, {011}, {021}, {041}, {118}, {114}, {112}, {111}. Unit cell dimensions are; a0 = 9.69 Å, b0 = 16.83, c0 = 14.26 (a0:b0:c0 = 0.5758:l:0.8473). Space group Cmmm. Z = 4. Optically biaxial (—), X = c, Y = b. Argentina: nX = 1.492, nY = 1.543, nZ= 1.544. 2V = 10°. G=2.550. Utah: nX = 1.490, nY = 1.537, nZ = 1.538. 2V = 18°. G = 2.544. Calculated G = 2.547. Dehydrated over H2SO4, schroeckingerite goes to NaCa3 UO2(CO3)3SO4F · 4H2O and becomes hexagonal with unit cell dimensions: a0 = 9.72 Å, c0 = 11.03 (a:b = l:1.1348). Z = 2. Optically uniaxial (—). nO = 1.581, nE = 1.532. Measured G = 2.86, calculated G = 2.874.

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