Abstract

The compositional variation from core to edge of a (001) section of liddicoatite (diameter~20 cm) from the Anjanabonoina granitic pegmatite in Madagascar shows pronounced oscillatory zones. Over the bulk crystal, the compositional variation corresponds to the two substitutions, XNa + YM* → XCa + YLi and YM* + YM* → YLi + YAl (M* = Fe + Mg + Mn), and the oscillatory zoning is superimposed on a smooth monotonic variation in the principal constituents. Both Fe and Mg show prominent oscillatory behavior superimposed on background values of~0 apfu, and Mn and (Mg + Fe) show antithetic behavior with regard to compositional variation. The pattern of compositional zoning is completely different in the {021} and {110} sectors of the crystal. In the pyramidal sector, individual zones range in width from <1 to~8 mm, and each is marked by a sharp, dark discontinuity that fades in color intensity until the start of the next zone. The abundance of Mn decreases monotonically from 0.60 to <0.05 apfu from the center to approximately halfway toward the edge of the crystal, and remains close to zero out to the edge. Four colors of zones are observed: purple, light green, dark green, and greenish black, owing to an overall decrease and increase in Mn and Fe contents, respectively. In the pyramidal sector, each zone starts with a sharp increase in Fe and Mg, which then monotonically decrease to zero across the zone, and this pattern is repeated across the series of zones. In the prism sector, Fe and Mg are zero at the beginning of each zone and gradually increase to reach maximum values at the end of each zone, followed by a sharp drop to~0 apfu. The complexity of the compositional behavior reported here requires future development of a more complex model for the zoning process of liddicoatite at Anjanabonoina.

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