Scandian actinolite from Jordanów Śląski, Lower Silesia, Poland: Compositional evolution, crystal structure, and genetic implications

Abstract

Abstract Scandian actinolite evolving to scandio-winchite (up to 5.45 wt% Sc2O3) has been found in chlorite-dominant xenoliths incorporated into marginal portion of a granitic pegmatite. The pegmatite intruded a blackwall schist zone developed around rodingite-type rocks exposed in a serpentinite quarry at Jordanów Śląski near Sobótka, ~30 km south of Wrocław, Lower Silesia, Poland. The amphiboles form irregular overgrowths around cascandite and represent a complex solid-solution series among actinolite and scandio-winchite end-members, with a trace contribution of “scandio-magnesio-hornblende.” Structural studies of a scandian actinolite crystal with composition A[☐0.995(2)K0.005(2)]Σ1B[Na0.24(5)Ca1.73(4)]Σ1.98(1)C[Mg3.74(7)Fe0.90(3)2+Mn0.04(1)Sc0.26(3)Al0.05(1)]Σ4.99(1)T[Si7.98(2)Al0.02(2)]Σ8.00O22(OH)2 revealed monoclinic C2/m structure with unit-cell parameters a = 9.8517(3), b = 18.0881(6), c = 5.28501(18) Å, β = 104.809(4)°, in which scandium is located solely at the CM2 site. Scandian amphiboles are uncommon in geological environments, and invite comments on the origin of the observed Sc enrichment in the amphibole structure. Textural appearance of the chlorite-cascandite-amphibole clusters suggests that the formation of the amphiboles is related to the evolution of the country rocks followed by partial alteration of blackwall schist xenoliths by pegmatite-forming melt.