Arfvedsonite

Formula:  NaNa₂(Fe, Mg)₄FeSi₈O₂₂ (OH)₂

Origin of name:  For John A. Arfvedson (1792-1841), Swedish chemist

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning: Simple or lamellar parallel to {100}

Unit cell: a = 10.007 Å, b = 18.077 Å, c = 5.332 Å, b = 104.101°

Colour:  Greenish black

Diaphaneity: Translucent to nearly opaque

Luster:  Vitreous

Habit:  Crystals short to long prismatic, often tabular; prismatic aggregates and radiating fibrous clusters

Hardness:  5 to 6

Specific gravity:  3.37 to 3.50

Cleavage:  2; {110} perfect, {010} parting

Tenacity:

Streak:  Dark bluish grey-green

Synonyms/varieties:

Comments:  Series with magnesio-arfvedsonite and with riebeckite. Member of amphibole group. Occurrence common in alkalic granites and other plutonic rocks and pegmatites