Alstonite

Formula:  BaCa(CO₃)₂

Origin of name:  After locality; Alston, Cumbria, England

Crystal system:  Triclinic

Crystal class:  1 or -1

Twinning:  on {110}or {130}

Unit cell:   a = 17.36 Ǻ, b = 14.40 Ǻ, c = 6.12 Ǻ, a = 90.4°, b = 90.1°, g = 120.1°

Colour:  Colourless, white, grayish, pink

Diaphaneity: Transparent to translucent

Luster:  Vitreous

Habit:  Crystals commonly twinned; as pseudo-hexagonal prisms, pseudo-hexahagonal dipyramids.

Hardness:  4 to 4.5

Specific gravity:  3.707

Cleavage:  1; {110} imperfect

Tenacity:  Uneven

Streak:  White

Synonyms/varieties:  Bromlite

Comments:  Trimorphous with barytocalcite and paralstonite