Morphology, crystallography and origin of needle fiber calcite in quaternary pedogenic calcretes of South Australia

Abstract

In several calcrete profiles from South Australia, needle-fibre calcite is concentrated in the channels and voids of nodules, hardpans, carbonate silt and platy calcrete. The morphology, spatial arrangements and crystallography of the needle-fibre calcite were determined using scanning and transmission electron microscopy. Needle-fibre calcite is composed of single crystals of calcite containing less than 0 5 wt % Mg. Two distinct morphological classes are evident: (1) small single rods or micro-rods, 1 �m in length and 0.1 �m or less in diameter and (2) larger needle-fibres 2-120 �m in length and 0.5-6 �m in diameter which display a variety of habits including multiple rods and serrated forms. The basic unit for the second group is that of a double rod which may be modified by cementation to other rods, and by epitaxial growth or dissolution. Spatial arrangements of needle-fibre calcite are determined by their association with organic matter. The large fibres form within mycelial strands. Lysis of the strand by rod-shaped bacteria releases the needle-fibres for redistribution in the profile. The micro-rods are calcified rod-shaped bacteria.