Contribution to the Crystal Chemistry of Beryl

Abstract

Chemical analyses and crystallographic and some optical data have been obtained for 28 samples of beryl from Bahia State, Brazil. The larger range of variability in the chemical composition is sho~m by Mg, Fe and Li. Sodium is the more diffuse alkali element. Potassium is always very limited. Calcium appears in noticeable amounts only in three samples. The Ti, Cr, Rb, and Cs elements were also tested. The samples studied here can be defined as sodium-potassium beryls with low alkali content. Unit cell parameters show the following ranges: a=9.210-9.245 and c=9.190-9.220 A. From a statistical analysis of these data it may be seen that: an increase of Fe and (Fe d-Mn d-Mg) percentage has a positive correlation with a, but no influence on c, which in turn has a close positive correlation with Li and is negatively correlated with Be. Less negative correlations also exist between the pairs {Be, Li}, {a, A1}, {A1, Fe} and {A1, (Fe ~-Mn-[-Mg)}. A positive correlation also exists between sodium and the parameter a.