Magnesio-arfvedsonite

Formula:  NaNa₂(Mg,Fe)₂Si₈O₂₂(OH)₂

Origin of name:  From magnesium and arfvedsonite

Crystal system:  Monoclinic

Crystal class:  2/m

Twinning: 

Unit cell: a = 9.796, b = 17.932, c = 5.289, β = 104.99°

Colour:  Greenish black, black

Diaphaneity:  Nearly opaque

Luster:  Vitreous

Habit:  Crystals short to long prismatic or tabular; prismatic aggregates

Hardness:  5 to 6

Specific gravity:  3.17

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

Tenacity: 

Streak:  Dark bluish grey

Synonyms/varieties:

Comments:  Series with arfvedsonite