Arsenotučekite from serpentinite and listvenite in the Median Tectonic Line shear zone, eastern Ehime Prefecture, Japan

Abstract

Arsenotučekite (Ni₁₈Sb₃AsS₁₆), a rare mineral of the hauchecornite group, is newly identified from carbonated serpentinite and schistose listvenite in the Median Tectonic Line (MTL) shear zone in eastern Ehime Prefecture, Japan. This is the second occurrence after the type locality and the first report from outcrop samples. At the Urayama River, arsenotučekite occurs in carbonated serpentinite and in schistose listvenite, and at the Ichiba River it occurs in schistose listvenite. Arsenotučekite is associated with millerite in antigorite, magnesite, and dolomite in carbonated serpentinite, and in quartz and dolomite in schistose listvenite. The chemical compositions are (Ni_16.08Fe_1.61Co_0.23)(Sb_3.47As_0.80)S_15.81 (carbonated serpentinite, Urayama River), (Ni_16.20Fe_1.14Co_0.57)(Sb_3.50As_0.60)S_15.98 (schistose listvenite, Urayama River), and (Ni_15.84Fe_1.73Co_0.28)(Sb_3.55As_0.59)S_16.02 (schistose listvenite, Ichiba River). The unit-cell parameters of arsenotučekite in carbonated serpentinite from the Urayama River are a = 10.184(4), c = 10.758(6) Å, and V = 1115.8(8) ų. The a-axis length is markedly larger than that reported from the type locality. The mineral paragenesis indicates that arsenotučekite formed in relation to the carbonation of serpentinite and pelitic schist by CO₂-rich slab-derived fluids.