Robertsite

Formula: Ca₂Mn₃(PO₄)₃O₂.3H₂O

Origin of name: for W. Roberts, American mineralogist

Crystal system: monoclinic

Crystal class: 2/m

Twinning: about axis perpendicular to {100}

Unit cell: a = 17.36 Å, b = 19.43 Å, c = 11.30 Å, b = 96.0°

Colour: reddish brown, deep blood red to almost black

Diaphaneity: 

Luster: adamantine

Habit: crystals barrel-shaped or pseudohexagonal plates, to 0.5 mm, botryoidal aggregates, twinning common

Hardness: 3.5

Specific gravity: 3.17

Cleavage: 

Tenacity: 

Streak: brown

Synonyms/varieties:

Comments: dimorphous with pararobertsite