Marsaalamite-(Y), Y(MoO₄)OH, a new molybdate mineral from the Um Safi area, Marsa Alam District, Central Eastern Desert, Egypt

Abstract Marsaalamite-(Y), ideally Y(MoO 4 )OH, is a new molybdate mineral discovered in the greisenised Um Safi F-rich granite located in the Marsa Alam District, Central Eastern Desert, Egypt. It typically occurs as inclusions in or intergrowths with F-rich zinnwaldite. It forms micaceous aggregates, with sizes varying from 0.1 to 1 mm. Marsaalamite-(Y) is non-magnetic, white in colour, and has an earthy lustre and white streak. It is brittle (3–4 Mohs) and has basal cleavages {010}. The calculated density is 4.90 g.cm –3 based on the empirical formula and unit-cell parameters refined from powder X-ray diffraction data. Marsaalamite-(Y) is associated with arsenopyrite, baryte, bastnäsite-(Ce), cassiterite, chernovite-(Y), columbite-(Fe), fluocerite-(Ce), fluorite, iron oxy-hydroxides, löllingite, molybdenite, monazite-(Ce), pyrite, quartz, rutile, thorite, wolframite, wulfenite, xenotime-(Y) and several unidentified phases. The empirical formula is (Y 0.67 Er 0.10 Dy 0.08 Yb 0.08 Ho 0.02 Lu 0.02 Tm 0.02 Ca 0.01 ) Σ1.00 (Mo 0.95 S 0.03 As 0.01 P 0.01 ) Σ1.00 O 4.00 [(OH) 0.88 F 0.11 Cl 0.01 ] Σ1.00 ; the ideal end-member formula is Y(MoO 4 )(OH). The presence of a hydroxyl group has been confirmed by Raman and infrared spectroscopy, and its concentration has been calculated from the stoichiometry. Marsaalamite-(Y) is the natural (OH)-dominant analogue of synthetic Y(MoO 4 )F. It is monoclinic, space group P 2 1 /c , with unit-cell parameters a = 5.1863(7) Å, b = 12.3203(11) Å, c = 6.6953(7) Å, β = 114.173(8)°, V = 390.30(8) Å 3 , and Z = 4. Extreme fractionation of the parental halogen-rich, A-type granitic magma triggered the greisenisation of the granite. Marsaalamite-(Y) occurred simultaneously with or immediately after the crystallisation of F-rich zinnwaldite based on the textural relationship. Therefore, the crystallisation of marsaalamite-(Y) was most likely to have been controlled by fluid-induced processes rather than magmatic conditions. The new mineral has been approved by the Commission on New Minerals, Nomenclature and Classification of the International Mineralogical Association (IMA 2024-050) and named after the Marsa Alam District, Al-Bahr Al-Ahmer Governorate, Egypt.