Acta Musei Moraviae

Enstatite from type locality Ruda nad Moravou (Czech Republic)

We have undertaken a study of the enstatite and its mineral association from the historical and newly found specimens from its type locality at the Žďár hill (547 m a.s.l.) near Ruda nad Moravou, 10 km W of Šumperk, northern Moravia (Czech Republic). Enstatite occurs as euhedral to subhedral elongated prismatic crystals up to 3.5 cm in length with a characteristic pearly luster on cleavage surfaces in fine-grained greenish grey rock. Enstatite crystals are colorless, yellowish, very light gray-green, green or brownish green; it is translucent, in small cleavage fragments even transparent. Enstatite is orthorhombic, the space group Pbca, with the unit-cell parameters refined from X-ray powder diffraction data: a 18.2435(18), b 8.8296(8), c 5.1840(7) Å and V 835.06(12) ų. The chemical composition of all studied enstatite samples corresponds to end-member MgSiO₃ with Fe contents only in the range 0.03–0.07 apfu and traces of Mn, Ca, Ni, Al and Cr. Mineral associations of historical and new specimens are partly different. Historical specimens are characterized by the complete absence of primary and newly formed magnetite, abundant presence of large enstatite crystals, predominance of clinochlore over serpentine in the fine-grained rock matrix and absence of Ni and Cr in newly formed silicates. In principle, two respective associations were observed here – the first with rare forsterite relics and no tremolite, the second with abundant tremolite and without forsterite relics. Newly found specimens with enstatite crystals show abundant presence of primary Cr-rich magnetite and newly formed almost pure magnetite, abundant forsterite relics, predominance of serpentine over clinochlore, absence of diopside and tremolite and regular minor contents of Ni and Cr in newly formed silicates. The character of the observed mineral associations is probably caused by differences in the petrographic composition of the protolith of serpentinized rocks.