Chromium-rich kyanite, magnesiostaurolite and corundum in ultrahigh-pressure eclogites (examples from Pohorje Mountains, Slovenia and Tromsø Nappe, Norway)
Janák, Marian; Uher, Pavel; Ravna, Erling Krogh; Kullerud, Kåre; Vrabec, Mirijam
European Journal of Mineralogy Volume 27 Number 3 (2015), p. 377 - 392
published: Jun 1, 2015
ArtNo. ESP147052703010, Price: 29.00 €
Chromium-rich kyanite, magnesiostaurolite and corundum (ruby) were found in ultrahigh-pressure (UHP) eclogites associated with garnet peridotite and serpentinite from Pohorje Mountains (Eastern Alps) and the Tromsø Nappe (Northern Scandinavian Caledonides). The concentrations of Cr2O3 in kyanite and magnesiostaurolite from the Pohorje Mts. (17.0 and 15.0 wt%, respectively) are the highest Cr values ever reported for these minerals, and the Cr content of corundum from this area ( 12.2 wt% Cr2O3) also belong to the highest reported from natural samples. The Cr-rich kyanite of turquoise-blue colour contains inclusions of chromite (Cr* = 0.74–0.78) and Cr-rich magnesiostaurolite (6 to 15 wt% Cr2O3; XMg = 0.7–0.8). Kyanite is commonly surrounded by a corona composed of the assemblage Cr-bearing sapphirine (up to 5.5 wt% Cr2O3) + corundum + Al-rich spinel + plagioclase. Blue-green, Cr-rich magnesiostaurolite (5.4–8.5 wt% Cr2O3; XMg = 0.76–0.82) forming ≤500 μm large crystals is associated with Cr-rich kyanite, Cr-rich spinel and Cr-rich corundum which are surrounded by pargasitic amphibole and zoisite. Chromium enrichment in kyanite, magnesiostaurolite and corundum was controlled by substitution for Al in the octahedral sites (Cr3+Al–1). Magnesiostaurolite compositional variations indicate at least two additional substitutions: Fe2+Mg–1 and M 2+Si(Al,Cr)–2, where M 2+ are the divalent cations Mg, Fe, Mn, Ni and Zn. The highest Si content and near absence of tetrahedrally coordinated Al was observed in the most Cr-rich magnesiostaurolite. Chromian kyanite, magnesiostaurolite and ruby formed by reactions involving primary chromite derived from a magmatic protolith characterized by high Cr, Mg, Al and low Si, during prograde metamorphism and formation of eclogite at HP-UHP conditions.