Abstract
Marsaalamite-(Y), ideally Y(MoO4)OH, is a new molybdate mineral discovered in
the greisenized Um Safi F-rich granite located in the Marsa Alam District, Central
Eastern Desert of 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 color, and has an earthy lustre and white
streak. It is brittle (34 Mohs) and has basal cleavages {001}. The calculated density is
4.90 g.cm–3 based on the empirical formula and unit-cell parameters refined from X-ray
powder 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
2
formula is (Y0.67Er0.10Dy0.08Yb0.08Ho0.02Lu0.02Tm0.02Ca0.01)Σ1.00 (Mo0.95S0.03As0.01P0.01)Σ1.00
O4.00[(OH)0.88F0.11Cl0.01]Σ1.00; the ideal end member formula is Y(MoO4)(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(MoO4)F. It is monoclinic, space group
P21/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) Å3, and Z = 4. Extreme fractionation of the parental
halogen-rich, A-type granitic magma triggered the greisenization of the granite. The
marsaalamite-(Y) occurred simultaneously with or immediately after the crystallization
of F-rich zinnwaldite based on the textural relationship. Therefore, the crystallization of
marsaalamite-(Y) was most likely controlled by fluid-induced processes rather than
magmatic conditions. The new mineral was approved by the Commission on New
Minerals, Nomenclature and Classification of the International Mineralogical Association
(CNMNC-IMA) (IMA 2024-050) and named after the Marsa Alam District, Al-Bahr Al- Ahmer Governorate, Egypt
