monoclinic Tectosilicates
Moonstone
KAlSi3O8
Crystal Structure
#! Species: Moonstone (Orthoclase Feldspar)
#! System: Monoclinic (2/m)
#! Habit: Prismatic with adularescence phenomenon
monoclinic[2/m]:{010}@1.0 + {110}@0.8 + {001}@0.7 + {100}@0.5 | phenomenon[adularescence]
monoclinic
2/m
{010} {110} {001} {100}
Quick Facts
Hardness
6.0
Specific Gravity
2.56
Refractive Index
1.518
Optical Character
Biaxial -
Physical Properties
Crystal Systemmonoclinic
Hardness (Mohs)6.0
Specific Gravity2.56
CleavagePerfect in two directions at ~90 degrees
FractureUneven to conchoidal
LustreVitreous to pearly
Optical Properties
Refractive Index1.518
Birefringence0.005-0.008
Optical CharacterBiaxial -
Dispersion0.012
PleochroismNone to weak
PhenomenonAdularescence
FluorescenceWeak bluish-white in some specimens
Colours
Colorless with blue sheenWhite with white sheenGreyPeach with white sheenRainbow (multicoloured flashes)
Localities
Sri LankaIndiaMyanmarTanzaniaMadagascarAustralia
Common Inclusions
Centipede inclusionsStress cracks from cleavageExsolution lamellae (cause of phenomenon)
Known Treatments
Generally not treatedSurface coating (rare, to enhance sheen)
Crystal Forms
prismpinacoid
Related Minerals
Other minerals in the monoclinic system
