Department of Earth Sciences, Faculty of Science, Ibaraki University
Abstract
Opaque-mineral nodules in Y-691 consist mainly of kamacite, perryite, schreibersite, troilite, daubreelite, niningerite, oldhamite, sphalerite, djerfisherite, caswellsilverite, graphite, roedderite, a silica mineral, albite, enstatite, and forsterite. The Ni and Si contents of kamacite are 2-3.5wt% and about 2wt%, respectively, being controlled by exsolution of perryite and schreibersite. Troilite shows a wide range of the Cr contents from 2.4 to 0.05wt%, and is classified into two types, massive and porous. The massive troilite is high in the Cr contents more than 0.6wt% and contains sometimes thin daubreelite lamellae, although the porous troilite is low in the Cr contents less than 1.2wt% and nearly free from daubreelite lamellae. The porous texture seems to be due to tiny inclusions of FeCl_2 in the troilite. Niningerite in Y-691 is the lowest in Mn and Fe contents in comparison to those in other enstatite chondrites. Djerfisherite in Y-691 is richer in Cu, Ni and Na in comparison to those in Qingzhen. Roedderite in Y-691 is similar in chemical composition to that in Indarch. Enstatite in Y-691 nodules shows a wide range of MgO/(MgO+FeO) molar ratio from 0.91 to 0.99,and sometimes forsterite occurs with a silica mineral in a nodule, suggesting that the silicates in the opaque-mineral nodules were not in equilibrium. Opaque-mineral nodules were classified into two main types based on the texture; concentric and massive, which are further divided into subtypes based on the modal volume of the main-minerals. The constituent minerals in these nodules seem to have polygenesis, and the genesis of nodules is discussed in detail. Any thermal metamorphic effects after accretion was not petrographycally detected in Y-691 nodules
