Rare accessory mineral in aluminous boron-rich rocks, thermally and high-grade regionally metamorphosed under low pressure; in pegmatites, aplites, gneisses; in xenoliths.
IMA Status:
Valid Species (Pre-IMA) 1902
Locality:
Andrahomana, southwest of Fort Dauphin, southern Madagascar. Link to MinDat.org Location Data.
Name Origin:
Named for Alfred Grandidier (1836-1912), French naturalist and explorer, an authority on Madagascar.
Brittle - Uneven - Very brittle fracture producing uneven fragments.
Habit:
Tabular - Form dimensions are thin in one direction.
Hardness:
7.5 - Garnet
Luster:
Vitreous (Glassy)
Streak:
white
Optical Properties of Grandidierite
Gladstone-Dale:
CI meas= -0.019 (Excellent) - where the CI = (1-KPDmeas/KC) CI calc= -0.008 (Superior) - where the CI = (1-KPDcalc/KC)
KPDcalc= 0.2079,KPDmeas= 0.2102,KC= 0.2063 Ncalc = 1.61 - 1.62
Optical Data:
Biaxial (-), a=1.602, b=1.636, g=1.639, bire=0.0370, 2V(Calc)=32, 2V(Meas)=24-32. Dispersion strong, r < v.
Pleochroism (x):
dark bluish green.
Pleochroism (y):
colorless.
Pleochroism (z):
dark green.
Calculated Properties of Grandidierite
Electron Density:
Bulk Density (Electron Density)=2.96 gm/cc note: Specific Gravity of Grandidierite =3.01 gm/cc.
Fermion Index:
Fermion Index = 0.01 Boson Index = 0.99
Photoelectric:
PEGrandidierite = 2.74 barns/electron U=PEGrandidierite x rElectron Density= 8.10 barns/cc.
Radioactivity:
GRapi = 0 (Gamma Ray American Petroleum Institute Units) Grandidierite is Not Radioactive
Grandidierite Classification
Dana Class:
54.01.01.01(54)Nesosilicate Borosilicates and Some Beryllosilicates
Chemical Formula: 
