DISTINCTIVE FEATURES OF THE HEALING OF A CRACK IN A CRYSTAL UNDER CONDITIONS OF DECLINING TEMPERATURE

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Abstract
The concentrations of aqueous solution from which quartz, feldspar, topaz, tourmaline and other minerals crystallize during the hydrothermal stage, is important to an understanding of pegmatite formation. Two opposing theories have been advanced on solution concentration: 1) Pegmatitic minerals separate from highly concentrated solution, without marked change in concentration, by gradual cooling. 2) Pegmatitic minerals separate as a result of flow and of renovation of the exhausted contents of solutions; this need is based on low solubility of silica in pure water. Study of liquid inclusions in minerals reveals that separation of hydrothermal minerals is known to occur from aqueous solutions heavily saturated with alkaline-halide salts, carbonates, borates, and free carbonic acid. From observations made on secondary liquid inclusions in topaz samples from pegmatites, it was found that one-third of the inclusion-cavity volume was occupied by the gas bubble and the remainder of the volume, largely, by small crystals of several solid phases. Inclusion contents could not be completely homogenized, because they exploded at 400° - 450° C; homogenization required temperatures above 500° C. Flattened inclusions, formed by healing of cracks parallel to the (001) cleavage plane, were examined. At room temperature, the included bubble was disk like; on heating to temperatures of 180° - 200° C, the bubble contracted revealing beneath it a circular crescent-shaped depression, probably an indication that crystal substance had been deposited on inclusion-cavity walls. Accordingly, crystal substance precipitated out after entrapment of solution in the cavity because redeposition occurred around the bubble; this could have been possible only after the solution had been completely enclosed and subsequently cooled. To determine the original amount of substance entrapped, measurements were made of inclusion-cavity volume, thickness of substance deposited within the cavity (found to be about 2 percent of inclusion-cavity height), and depth of crescent-shaped depression. From these data it was found that up to 2 percent volume of the topaz substance remained in dissolved state at relatively low temperatures. Deposition traces were found on walls of sufficiently flat inclusions where a large part of the wall surface was blocked by a flattened bubble; presence of these conditions was required in order to detect the deposited layer and measure its height.

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