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Formula: Y(MoO4)(OH)
Molybdate of yttrium
Crystal system: Monoclinic
Specific gravity: 4.90 calculated for the empirical formula
Hardness: 3 to 4
Streak: White
Colour: White
Magnetism: Not magnetic
Environments
Marsaalamite-(Y) is a new mineral, approved in 2024 and to date (October 2025) reported only from the type
locality.
Localities
At the type locality, Um Safi, Red Sea Governorate, Egypt, marsaalamite-(Y)molybdate mineral; it and
tancaite-(Ce) are the only minerals known (in 2024) to contain both
molybdenum and a
rare earth element as defining constituents. Marsaalamite-(Y) was
encountered at Um Safi as inclusions and intergrowths with a fluorine-rich
zinnwaldite
(polylithionite -
siderophyllite series) in a
greisen alteration assemblage within an fluorine-rich
granite. Although its environment is very rich in fluorine,
marsaalamite-(Y) is proposed to have formed after available fluorine was depleted by earlier phases including
zinnwaldite, fluorite and
fluorine-bearing
rare earth element minerals — explaining why marsaalamite-(Y)
formed rather than a phase equivalent to its fluorine analog
(AM 110.5.827).
Marsaalamite occurs as micaceous aggregates, with sizes varying from 0.1 to 1 mm in the highly evolved
granite. Associated minerals include
arsenopyrite, baryte,
bastnäsite-(Ce),
cassiterite,
chernovite-(Y),
columbite-(Fe),
fluocerite-(Ce), fluorite,
limonite, löllingite,
molybdenite, monazite-(Ce),
pyrite, quartz,
rutile, thorite,
wolframite group minerals,
wulfenite and xenotime-(Y)
Extreme fractionation of the parental halogen-rich, granitic magma
triggered the greisenisation of the
granite. Marsaalamite-(Y) occurred simultaneously with or
immediately after the crystallisation of fluorine-rich zinnwaldite.
Therefore, the crystallisation of marsaalamite-(Y) was most likely to have been controlled by fluid-induced
processes rather than magmatic conditions
(MM 89.3.443-453).
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