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The name is in honour of the first three astronauts who traveled to Earth's Moon, Neil ARMstrong, Edwin ALdrin and
Michael COLlins, the Apollo 11 astronauts who collected the type samples in 1969
(Mindat).
Formula: (Mg,Fe2+)Ti2O5
Valence: (Mg,Fe2+)Ti4+2O5
Oxide of magnesium, ferrous iron and
titanium,
pseudobrookite group, forms a series with
pseudobrookite
Crystal System: Orthorhombic
Specific gravity: 4.94 measured, 4.64 calculated
Hardness: 5
Colour: Grey
Environments
Igneous environments
Derived from the mantle
Lunar environments
Armalcolite typically is formed at low pressures and high temperatures; in
titanium-rich basalts and
microbreccias of lunar samples; in salic (containing large amounts of
silica and alumina) volcanics; rarely in granite
pegmatites; in
ultramafic rocks,
lamproites,
kimberlites; in terrestrial impact craters; in inclusions in a
carbonaceous chondrite
(HOM).
Localities
There are two co-type localities, Disko Island, Qeqertalik, Greenland and Tranquillity Base (Apollo 11 landing site),
Mare Tranquillitatis, The Moon.
At Disko Island, Qeqertalik, Greenland, armalcolite is associated with
ilmenite, Fe–Ti oxides, iron and
graphite
(HOM).
Mexico. Aluminous armalcolite has been found in two
sillimanite-bearing xenoliths that were recently exhumed from the
lower crust of central Mexico. The occurrence of armalcolite is unusual in crustal
paragneiss because most terrestrial armalcolite occurs in
volcanic rocks that are derived from partial melting in the Earth’s mantle. Textures suggest that armalcolite
is a late product formed by the reaction:
rutile + ilmenitess
= armalcolitess
TiO2 + Fe2+TiO3 = (Mg,Fe2+)Ti2O5
during rapid transport of the xenoliths to the surface. Indications are that armalcolite is stable in the crust
at 900°C to 1200°C
(AM 80.810–822).
At Kimberley, Sol Plaatje Local Municipality, Frances Baard District Municipality, Northern Cape, South Africa,
armalcolite is associated with rutile and
ilmenite
(HOM).
At Smoky Butte (Smoky Butte Lamproite), Garfield County, Montana, USA, armalcolite is associated with
phlogopite, analcime and
diopside
(HOM).
Armalcolite from Smoky Butte - Image
At Tranquillity Base (Apollo 11 landing site), Mare Tranquillitatis, The Moon, armalcolite occurs as subhedral
to anhedral grains to 300 µm across in titanium-rich
basalt and microbreccias
of lunar samples, formed at low pressures and high temperatures
(Mindat).
Armstrong, Aldrin and Collins - Image
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