Crystallographic studies of some Norwegian aventurinised feldspars by optical, X-ray, and electron optical methods

Four aventurinised oligoclases from SE Norway have non-feldspathic inclusions of which the majority are based on hematite-like structures. Detailed examination of these inclusions, however, shows considerable variability in diffraction data suggesting that they do not have a regular ?- Fe2O3 structural arrangement; this is interpreted in terms of unusual and sometimes irregular dispositions of Fe3+ (and possibly other cations) in the approximately hexagonal close-packed oxygen framework. There are minor amounts of other inclusions, but the only certain identification is that of alkali feldspar due to anti-perthitic exsolution. Data have been obtained on the mutual orientation of the feldspar and hematite-Iike inclusions, which, whilst demonstrating some accord with previous observations, indicate !hat such relations are variable and complex. Monotactic control exerted by 'close-packed' oxygen directions is examined, but the evidence is inconclusive.