Suanite in two boron-bearing magnesian skarns from Romania: data on a longtime ignored mineral species

摘要

This paper provides new mineralogical information on suanite from two boron-bearing magnesian skarn deposits from Romania, namely those from Baita Bihor and Pietroasa. In both occurrences, the mineral occurs in the innermost boron-bearing zone of well-defined metasomatic columns or skarn bodies and is extensively replaced by szaibelyite. The skarn zones are developed at the contact of igneous bodies of Upper Cretaceous age with sequences of Triassic dolostones. Associated minerals include calcite, dolomite, magnesite, kotoite, ludwigite, fluoborite, fluorite, chondrodite, magnetite, brucite, chrysotile, lizardite, clinochlore, lepidocrocite and goethite. Suanite occurs as submillimeter-sized rough prismatic relics, altered by szaibelyite and grouped in sheaf-like aggregates, generally surrounded by dolomite. The physical and crystallographic constants of a representative sample from Pietroasa (181) are: D_x = 2.905 g/cm~3, alpha = 1.601 (1), beta (calc.) = 1.649 (1), gamma= 1.674(1), 2V_(alpha) = 70 deg, a = 12.337(6) A,b = 3.132(3) A, c = 9.157(5) A and beta = 104.15 (3) deg. The same constants measured for a sample from Baita Bihor (1397) are: D~x = 2.919g/cm~3, alpha = 1.600(2), beta (calc.) = 1.647(1), gamma= 1.672(1), 2V_(alpha) = 70 deg, a = 12.204(9) A, b = 3.134(4) A, c = 9.219(7) A and/3 = 104.23 (4) deg. No evidence for a deviation from the monoclinic symmetry was obtained after refinement of the cell parameters. The Fe~(2+)_(-), Mn- and Ca-substitutions for Mg are very limited. The compositional limits for formulas calculated on the basis of five oxygen atoms per formula unit (apfu) are given by 0.007 to 0.015 apfu Fe, 0.005 to 0.010 apfu Mn and 0.000 to 0.008 apfu Ca at Pietroasa, and by 0.005 to 0.012 apfu Fe, 0.007 to 0.010 apfu Mn and 0.001 to 0.023 apfu Ca at Baifa Bihor. It is reasonable to assume that in both occurrences suanite crystallized at temperatures higher than 400 deg C, at pressures of 0.6-3 kbar and at high activities of CO_2.