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 Floerke, 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 Floerke, is due to incorporation of cations (e.g., Na (super +) , K (super +) , Ca (super +2) , and Al (super +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 SiO 2 and metal cations which were 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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