Features of Amethyst Crystals Synthesized in the K2O—CO2— SiO2—H2O System

Abstract

Synthetic amethysts may be made hydrothermally in the system K2O—CO2—SiO2—H2O in the presence of iron [1, 2]; when the (R) and (r) pyramids of such crystals are exposed to ionizing radiation,* they acquire an amethyst color, and the strength of the color in the (R) pyramid is usually much greater than that in the (r) pyramid. An analogous color distribution is characteristic of natural amethyst crystals. We have defined closely the formation conditions for amethyst crystals in this system, and have compared some physical properties of synthetic and natural amethyst. The amethyst color purity is better when the synthesis temperature is low; the characteristic color is related to the presence of iron, but aluminum is also taken up as a structural impurity during the synthesis, and this is [3, 4] related to hole centers producing smoky color. The absorption spectra for the two types of center are superimposed in the irradiated crystal, which naturally results in a poorer amethyst color. It has been shown [5] that the trapping factor for the structural impurity is directly related to the growth temperature, whereas the uptake factor for the iron is largely independent of temperature in the range covered. Therefore, one reduces the crystallization temperature to reduce the proportion of impurity and thus to improve the amethyst color. The same result can be obtained by more careful removal of traces of aluminum; then a pure and strongly colored amethyst material may be grown at higher temperatures.