Mineralogy and paragenesis of amblygonite-montebrasite with special reference to the Tanco (Chemalloy) pegmatite, Bernic Lake, Manitoba

Abstract

This thesis describes the results of a study of about 219 specimens of the amblygonite - montebrasite series, LiAlPO4F - LiAlPO4(OH), with 157 specimens from the Tanco pegmatite and the remainder from other world localities. The investigation had two more or less separate objectives, one being a study of the variations of certain physical properties with variable F/OH ratios within the series, and the other being a study of the chemical variation in amblygonite - montebrasites from different zones within the Tanco pegmatite. The variations of physical properties with F/OH ratio were studied on 15 members of the amblygonite - montebrasite series varying from 1.4 to 11.8% F. Specific gravity increases with F-content, following closely a straight line connecting the end-members values of 2.98 for montebrasite and 3.11 for amblygonite. The refractive index v~ increases from 1.586 for amblygonite to 1.644 for montebrasite. Differential thermal analysis showed a regular variation with F-content in the positions of the principal endotherms on the DTA curves. The products after heating to 1000o C have been identified as Li4P2O7, Li3PO4 and three forms of AlPO4, the mutual portions of which vary with the F-contents of the original phase. Unit cell dimensions showed non-linear variations from F-rich to F-poor members. A chart for the indirect estimation of the F-contents in amblygonite - montebrasites was constructed using a group of six powder reflections whose 20o CuK~ angles vary most conspicuously; the accuracy is regarded as +-0.5% F. Of these physical properties, X-ray powder diffractograms are considered to yield the most accurate estimate of F-contents. For the second part of the study, 157 specimens of the amblygonite - montebrasites in the Tanco pagmatite were X-rayed and their F-contents estimated, twelve were analyzed for F and 35 were analyzed for Na2O, K2O and CaO. All of the last 35 specimens were found to contain negligible amounts of the three oxides... In all zones, the primary amblygonite - montebrasite is frequently replaced by a polysynthetically twinned, F-poor montebrasite (1.8 - 4.0% F), the loss in F-content from the parent mineral being as much as 4.0%