Chemical and growth zoning in trapiche tourmaline from Zambia — a re-evaluation

Abstract

Trapiche tourmalines from Zambia show the hexagonal prism a {112 - 0}, the positive pyramids r {101 - 1} and o {022 - 1} and the negative pyramid -r {011 - 1 } as dominant external crystal forms. The trapiche pattern is formed by the positioning of liquid and solid inclusions which are trapped at growth boundaries between pyramidal and prismatic growth sectors. Growth-induced elongate voids are also formed perpendicular to the dominant growth faces, which are inclined at 27°, at 46° or at 90° to the pedion {0001} of the tourmaline hosts. All the trapiche tourmalines are chemically zoned with isomorphic substitutions of Ca by Na and Mg by Al, which is characteristic for tourmalines of the uvite-dravite solid solution series. Content of Na increases and of Ca decreases from core to rim of all tourmalines. However, at the boundaries between different growth sectors (forming the geometric trapiche pattern of the samples), tourmaline compositions with higher Na and lower Ca contents than measured in adjacent pyramidal and prismatic growth sectors were found. The formation of this complex structural and chemical pattern is consistent with the formation of the tourmaline crystals in a two step process, with the skeletal growth of a sodium-rich dravite in a fi rst step and the subsequent, layer-by-layer growth of a second tourmaline generation. The growth of the second generation started with calcium-rich fl uor-uvite and was followed step-by-step by a more sodium-rich dravite. The green coloration of the samples is caused by minor contents of vanadium and chromium.