Formula: CaZrSi6O15 . 2.5H2O
Origin of name: For Neil Alden Armstrong (1930-), American astronaut, first man to walk on the Moon
Crystal system: Monoclinic
Crystal class: 2/m, m or 2
Twinning: polysynthetic by rotation about [100]
Unit cell: a = 14.04 Å, b = 14.16 Å, c = 7.81 Å, b = 109°33'
Colour: Dark to pale brown
Diaphaneity: Semi-transparant
Habit: Crystals up to 2 cm and in aggregates
Hardness: about 4.5
Specific gravity: 2.56 to 2.59
Cleavage: 2; {001} perfect, {100}good
Streak: Brownish white
Synonyms/varieties:
Comments: Formed in schlieren of alkalic granite pegmatite, at the contact of arfvedsonite granite with xenoliths of felsic volcanic rocks.
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