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.