Ardennite

Formula:  (Mn,Ca)₄(Al,Mn)₅Mg(SiO₄)₂(Si₃O₁₀)(As,V)O₄(OH)₆

Origin of name:  Locality; Salmchâteau, in the Ardennes, Belgium

Crystal system:  Orthorhombic

Crystal class:  2/m 2/m 2/m

Twinning:

Unit cell: a = 8.7126 Å, b = 18.5124 Å, c = 5.8108 Å

Colour:  Yellow to yellowish brown, black

Diaphaneity: Nearly opaque, translucent in thin splinters

Luster:  Subadamantine

Habit:  Individual crystals are rare. Crystals prismatic, acicular; crystalline aggregates and small rosettes of tapered acicular crystal

Hardness:  6 to 7

Specific gravity:  3.69-3.75

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

Tenacity:

Streak: Pale brownish yellow

Synonyms/varieties: Dewalquite

Comments: Found in pegmatites and quartz veins in schists; in highly oxidised Mn, Al-rich metasediments