Tissint (Morocco) is the fifth martian meteorite collected after it was witnessed falling to Earth. Our integrated mineralogical, petrological, and geochemical study shows that it is a depleted picritic shergottite similar to EETA79001A. Highly magnesian olivine and abundant glass containing martian atmosphere are present in Tissint. Refractory trace element, sulfur, and fluorine data for the matrix and glass veins in the meteorite indicate the presence of a martian surface component. Thus, the influence of in situ martian weathering can be unambiguously distinguished from terrestrial contamination in this meteorite. Martian weathering features in Tissint are compatible with the results of spacecraft observations of Mars. Tissint has a cosmic-ray exposure age of 0.7 ± 0.3 million years, consistent with those of many other shergottites, notably EETA79001, suggesting that they were ejected from Mars during the same event.

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Compounds of the meta-autunite group containing monovalent cations (Li, Na, K, Rb, Cs, Ag, Tl) have been synthesized by diffusion in gels or by hydrothermal methods, and their crystal structures determined. Single-crystal X-ray-diffraction intensity data were collected at room temperature using MoK␣ radiation and a CCD-based area detector. These compounds contain the autunite-type sheet with composition [(UO 2)(XO 4)]-, X = P or As, which involves the sharing of equatorial vertices of uranyl square bipyramids with tetrahedra. The interlayer region contains cations and H 2 O groups, and the sheets are linked by hydrogen bonding and through bonds from the interlayer cations to oxygen atoms of the sheets. The structural roles of the interlayer cations in determining the symmetries and hydration states observed are discussed. The smallest monovalent cation, Li, occurs in tetrahedral coordination between fourfold squares of hydrogen-bonded H 2 O groups. Despite a wide range in ionic radius, Na, K, Rb, Ag and Tl randomly substitute for H 2 O groups in the interlayer, in the same fashion as their ammonium and oxonium analogues. The large Cs cation adopts independent crystallographic sites in the interlayer. The size difference between Cs and the other monovalent cations probably prevents their direct substitution, and may limit the extent of solid solution. With the exception of the Rb and Cs compounds, chemically corresponding uranyl phosphates and uranyl arsenates are isostructural. The structural similarity of Rb[(UO 2)(AsO 4)](H 2 O) 3 with metazeunerite, Cu[(UO 2)(AsO 4)] 2 (H 2 O) 8 , may indicate a mechanism of solid solution for monovalent and divalent interlayer cations in the meta-autunite group. Crystallographic data: Li[(UO 2)(PO 4)](H 2 O) 4 : tetragonal, P4/n, a 6.9555(2), c 9.1389(3) Å, R1 = 1.2%; Na[(UO 2)(PO 4)](H 2 O) 3 : tetragonal, P4/ncc, a 6.9616(2), c 17.2677(9) Å, R1 = 2.3%; Na[(UO 2)(AsO 4)](H 2 O) 3 : tetragonal, P4/ncc, a 7.1504(3), c 17.325(1) Å, R1 = 1.7%; K[(UO 2)(AsO 4)](H 2 O) 3 : tetragonal, P4/ncc, a 7.1669(17), c 17.867(6) Å, R1 = 3.4%; Rb[(UO 2)(PO 4)](H 2 O) 3 : tetragonal, P4/ncc, a 7.0106(2), c 17.9772(8) Å, R1 = 2.6%; Rb[(UO 2)(AsO 4)](H 2 O) 3 : tetragonal, P4/n, a 7.1904(3), c 17.643(1) Å, R1 = 1.9%; Ag[(UO 2)(PO 4)](H 2 O) 3 : tetragonal, P4/ ncc, a 6.9332(1), c 16.9313(6) Å, R1 = 1.6%; Ag[(UO 2)(AsO 4)](H 2 O) 3 : tetragonal, P4/ncc, a 7.0901(2), c 17.0453 (8) Å, R1 = 2.1%; Tl[(UO 2)(PO 4)](H 2 O) 3 : tetragonal, P4/ncc, a 7.019(3), c 17.98(1) Å, R1 = 3.2%; Tl[(UO 2)(AsO 4)](H 2 O) 3 : tetragonal, P4/ ncc, a 7.1905(8), c 17.970(3) Å, R1 = 3.4%; Cs 2 [(UO 2)(PO 4)] 2 (H 2 O) 5 : monoclinic, P2 1 /n, a 9.8716(7), b 9.9550(7), c 17.6465(13) Å, ␤ = 90.402(2)°, R1 = 2.8%; Cs(H 3 O)[(UO 2)(AsO 4)] 2 (H 2 O) 5 : monoclinic,