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Burns P C , Cooper M A , Hawthorne F C , The Canadian Mineralogist , 32 (1994) p.397-403, Jahn-Teller-distorted MnO6 octahedra in fredrikssonite, the fourth polymorph, of Mg2Mn(BO3)O2
Physical Properties of Fredrikssonite
Cleavage:
{???} Poor, {???} Indistinct
Color:
Reddish brown.
Density:
3.84
Diaphaneity:
Translucent
Habit:
Prismatic - Crystals Shaped like Slender Prisms (e.g. tourmaline).
Hardness:
6 - Orthoclase
Luster:
Vitreous (Glassy)
Streak:
yellowish brown
Optical Properties of Fredrikssonite
Gladstone-Dale:
CI meas= 0.061 (Fair) - where the CI = (1-KPDmeas/KC) CI calc= 0.051 (Good) - where the CI = (1-KPDcalc/KC)
KPDcalc= 0.2342,KPDmeas= 0.2318,KC= 0.2467 Ncalc = 1.94 - 1.95