Open AccessThe optical properties of astrophyllite from N arssarssuk, South Greenland
Author(s) -
Ole V. Petersen
Publication year - 1982
Publication title -
bulletin of the geological society of denmark
Language(s) - English
Resource type - Journals
eISSN - 2245-7070
pISSN - 0011-6297
DOI - 10.37570/bgsd-1981-30-06
Subject(s) - cleavage (geology) , statement (logic) , orientation (vector space) , geology , mica , geometry , physics , mathematics , philosophy , paleontology , linguistics , fracture (geology)
The present investigation proves that astrophyllite from Narssarssuk, like astrophyllite from elsewhere, is biaxial positive and renders it probable that B0ggild's statement in his original description, that astrophyllite from Narssarssuk is biaxial negative, is due to a combination of misreading and miscalculation. Astrophyllite from Narssarssuk has 2Va = 98° ± 1° - 91° ± 1°; a .L to the mica-like cleavage (001) and y = the needleaxis (a), i.e. in optical orientation identical with astrophyllite from El Paso County, Colorado.