JOESMITHITE, A PLUMBOUS AMPHIBOLE REVISITED AND COMMENTS ON BOND VALENCES

Joesmithite is a complex clinoamphibole which contains at least 14 components in its formula. Its ideal formula would be PbCa2Mg3Fe23+(Si6Be2O22)(OH)2. It is monoclinic holosymmetric, an ordered derivative of the C2/m clinoamphiboles, a = 9.915(2), b = 17.951(4), c = 5.243(1)angstrom, beta = 105.95(2)degrees, Z = 2, space group P2/a, D(obs) = 3.83(1), D(cal) = 3.91 g cm-3. R = 0.056 for 2299 independent F0. The cells of joesmithite and pargasitic amphibole were compared according to differences, DELTA, in their structure cells. The same procedure was applied to the margarosanite, PbCa2Si3O9 and walstromite, BaCa2Si3O9 pairs. In both pairs, the anion positions differed by 0.2 angstrom at most, but the Pb/A, Pb/Ba differences ranged from 0.4 to 0.6 angstrom. The lone pair Pb2+ cation responds according to the bond valence sums of its coordinating anions compared with its closed core counterpart. It appears that when the bond valence sums for the coordinating anions are different, Pb2+ moves toward the most underbonded anion. If the anion bond valence sums are equal for all vertices of the coordinating polyhedron (as in wulfenite, Pb2+Mo6+O4), then Pb2+ does not shift.