Hydrothermal synthesis and mineralogy of the alkali amphiboles

Abstract

The mineralogy of the alkali amphiboles is reviewed., Various suggestions are put forward to rationalise the nomenclature of these minerals. Several alkali amphibole compositions have been successfully synthesized for the first tine using hydrothermal synthesis techniques. The compositions of these minerals correspond to richterite-K, eckermannite-K and sundiusite-K". In addition five amphiboles of intermediate composition have been synthesized. Two of these minerals have compositions between richterite and tremolite indicating only partial 'A' site occupation. Attempts to synthesize- amphiboles of miyashiroite-K composition have failed to yield amphibole and the condensed product is a phyllosilicate. A survey of all the available alkali amphibole analyses has revealed only two possible miscibility gaps in the alkali amphiboles, one in the area of the miyashiroite composition and the other in the ferroeckerraannite field. Many of the analyses in the literature are of a suspect quality and various suggestions have been made to decide on the quality of an analysis. A method of representing alkali amphiboles of intermediate composition has been found by using the substitutions in the Y sites which are balanced by substitutions in the 2 and X sites. This method has revealed that there is complete solid solution between riebeckite and arfvedsonite, magnesioarfvedsonite and magnesioriebeckite, and richterite and eckermannite. A suggested nomenclature fbr these compositions has also been proposed. Alkali amphiboles from the celebrated nepheline syenite ofXIlimaussaq, S.W. Greenland have been analysed and these analyses reveal a compositional trend from close to katophorite to arfvedsonite. Electron microprobe analyses of alkali amphiboles from the saturated rocks of Tugtutoq, S.W. Greenland show a trend in compositions from ferrorichterite to riebeckite-arfvedsonite. XArfvedsonites from the late-stage lujavrites of Ilimaussaq contain high concentrations of potassium. This element has been shown to be exchanged between amphibole and hepheline in these rocks. Electron microprobe studies have shown the chemical variations within continuously and discontinuously zoned alkali amphiboles and in addition a study of co-existing glaucophane and hornblende has demonstrated that glaucophane develops around the hornblende as the rock retrogresses from the eclogite facies to the glaucophane schist facies. X-ray diffraction studies of synthetic and natural alkali amphiboles demonstrate that the aosin β parameter is controlled by the occupants of the 'A' site and the octahedral sites. The bo axis depends upon the M(4.) and M(2) occupancy; and the β angle shows a strong positive correlation with the M(4) occupant and to a lesser extent the magnesium and potassium contents. The C(_o) axes of alkali amphiboles are longest in iron-rich compositions. Infra-red spectroscopic studies indicate that it is possible to "fingerprint" the various alkali amphibole sub-groups. The presence of K(^+) or Na(^+) in the 'A' sites of synthetic alkali amphiboles raises the stretching frequency of adjacent OH groups by about 60 cm(^-1). In natural amphiboles, however, the increase in frequency is only 20-40 cm(^-1), due probably to the effects of substitutions such as F(^-) for 0H(^-) and the presence of trivalent cations in M(l) and M(3).