MICROSTRUCTURE OF CALCITE SAND CRYSTAL AND IMPLICATION ON ITS CRYSTALLIZATION PROCESS

A single calcite sand crystal of 8 cm in length from Washabaugh County, South Dakota is examined under microscopes to investigate the crystallization process of the poikilitic calcite. The crystal shows prominent {211} and {100} faces. The sand is composed of median grain quartz, feldspars and other minerals. The sand and the crystal were reported to be eolian in origin (Wanless, 1922)

A thin section was made on a cut oriented perpendicular to the c-axis of the calcite crystal. Features reported in many similar cases such as high porosity of original sand deposit (>30%) and rounded sand of eolian origin are noticed. In addition, there are at least four more significant features observed. 1) Floating texture of sand grain on poikilitic calcite. This implies the compaction is minimum and the fluid pressure was higher than normal groundwater flow. 2) The contact between calcite cement and mineral grain is clear. This suggests there were no chemical reaction took place between the alkaline solution and any mineral species it contacted; 3) Not a single calcite cleavage is shown; 4) Abundant silt-size mineral grains uniformly scattered in calcite cement. SEM observation further indicates: 1) The bond between calcite and sand grain is weak; 2) The surface of sand grains is usually pitted and has no surface deposit.

The mechanism on the crystallization of sand crystal was suggested to go through process such as diffusion or flow. However, none of the suggested process can explain the observed features satisfactorily. For example, in order to keep silt particles float in the pore, either a very high speed, almost instant, crystallization of calcite throughout the whole crystal must take place, or a constant pore fluid flow in the order of 1 cm/s, which is tens of times faster than normal pore water velocity in a typical dune sand deposit must be maintained.