Abstract
ABSTRACT Chemical analyses, weight-loss studies, differential thermal analyses, and X-ray diffraction studies show that massive opal in the Kimball Member of the Ogallala Formation, like opal described by Florke, is composed of hydrated low-cristobalite that is disordered by differing amounts of low-tridymite, which forms an integral part of the opal structure. The development of tridymite structure in the opal, as proposed by Florke, is due to incorporation of cations (e.g., Na+, K+, Ca+2, and Al+3 among others) into the low-cristobalite structure. It is proposed that the Kimball Member, which is everywhere rich in feldspar and quartz, was leached during formation of the overlying pisolitic caliche, freeing SiO2 and metal cations which we e redeposited as opal having a disordered low-cristobalite structure. Difference in hardness of opal samples is seemingly inversely related to difference in water content; the more hydrated samples have lesser hardness.
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