DISTRIBUTION OF APATITE AND OTHER MINERAL INCLUSIONS IN A RHYOLITIC PUMICE ASH AND BEACH SANDS FROM NEW ZEALAND: AN ELECTRON‐MICROPROBE STUDY

Abstract

Summary Apatite, magnetite, ilmenite, and magnetite‐ulvöspinel inclusions were identified in certain primary minerals in a rhyolitic pumice ash and beach sands from New Zealand using an electron microprobe. In addition to being present in apatite inclusions, P was uniformly distributed throughout the matrix of plagioclase feldspars. Included apatite was particularly common in hypersthene and also occurred within Fe and Fe‐Ti rich phases in hypersthene. Quantitative data for Ca and P and the presence of Cl and F indicate the similarity in composition between the apatite inclusions and a chlorofluorapatite from Kola (U.S.S.R.). The coexistence of various mineral inclusions in hypersthene reflects magma composition, oxygen fugacity, and temperature conditions at the time of crystallization. The presence of included apatite and lattice‐P within certain primary minerals adequately explains the low results reported previously for total P in these samples, obtained by HClO 4 digestion as compared to Na 2 O 3 fusion. Apatite and lattice‐P present within magnetite, ilmenite, and magnetite‐ulvöspinel inclusions are expected to be pedogenically inert in weakly and moderately weathered soils.