Alstonite

Alstonite, also known as bromlite, is a member of the aragonite group of carbonate minerals. It consists of an isomorphous mixture of calcium and barium carbonates in various proportions, CaBa(CO3)2, it is triclinic in crystal structure and differs structurally from barytocalcite, which has a monoclinic structure. Simple crystals are not known. The crystals are invariably complex twins, and have the form of doubly terminated pseudo-hexagonal pyramids, like those of witherite but more acute; the faces are horizontally striated and are divided down their center by a twin suture.
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General Information:
 Chemical Formula:
BaCa(CO3)2
 Composition:
(Molecular Weight = 297.42 gm)
Barium 46.17 % Ba
Calcium 13.48 % Ca
Carbon 8.08 % C 
Oxygen 32.28 % O 
 Empirical Formula:
BaCa(CO3)2
 Environment:
Low-temperature hydrothermal deposits. See other trimorphs paralstonite and barytocalcite.
 Locality:
Brownley Hill lead mine, Alston, Cumberland and Fallowfield lead mine hear Hexham, Northcumberland, England.
 Name Origin:
Named after the locality.
Physical Properties:
 Cleavage:
[110] Imperfect
 Color:
colorless, gray, white, or pink.
 Density:
3.69 - 3.71, Average = 3.7
 Diaphaniety:
Transparent to translucent
 Habits:
Euhedral Crystals - Occurs as well-formed crystals showing good external form., Pseudo Hexagonal - Crystals show a hexagonal outline.,
 Hardness:
4-4.5 - Between Fluorite and Apatite
 Luminescence:
Fluorescent, Long UV=weak yellow.
 Luster:
Vitreous (Glassy)
 Streak:
white
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