DURIPANS OF THE OWYHEE PLATEAU REGION OF IDAHO - GENESIS OF OPAL AND SEPIOLITE

Mineralogical and micromorphic techniques were used to investigate secondary minerals that occur in duripans (Durargids and Durixeralfs) on the Owyhee Plateau region of southwestern Idaho. Opaline silica and sepiolite were the principal secondary minerals found. Opaline silica exhibited three morphogenetic forms. Clearopal, consisting of opal-A with included calcite and unknown crystallites, occurred throughout the duripan, principally as pseudomorphic replacements after calcite pendants. Dirty-opal, a gray, semitransparent and nearly x-ray amorphous material, was found in highly altered cores of complex pedogenic concretions. Some opaline silica occurred in a web-like arrangement intimately mixed with calcite and was located in the matrix among loess agglomerates, in the laminar cap and in loess pedotubules. The opal in this microgranular calcite-silica was opal-A with included quartz and appeared to form, in part, by silica supplied in situ from alteration of primary aluminosilicate minerals. Transmission electron microscopic examination of colloidal silica revealed the presence of two types of coatings. Gray-gel was visually homogeneous and semielectron opaque whereas mottled-gel was less electron opaque and contained many, minute, more electron opaque spheroids. The reorganization of these gels seems to control the formation of opaline silica spheres. In addition, most opaline silica spheres exhibited bubble-like substructures heretofore unrecognized. The presence of poorly crystalline sepiolite was indicated by x-ray diffraction. Imaged with the transmission electron microscope, the sepiolite exhibited a hierarchical structure. At the highest scale of magnification, laths with a cross-sectional area of 13 by 23 nm were evident. The laths were organized in bundles. The most visually distinctive bundles were 950 by 1200 nm in length and lacked gel-like coatings. Other bundles were less distinct, more curvilinear, and exhibited less parallel orientation or mesh-like arrangements. These laths and fibers were from 250 to 375 nm in length, coated wtih structurally amorphous material resembling the grey-gel, and surficially sprinkled with opaline silica spheres and spherical units of mottled-gel. In the duripans studied, sepiolite was largely limited to pedogenic concretions in which it occurred as acicular crystals that radiated outward from concretionary laminae of opaline silica. This finding and the association of opal spheres with sepiolite laths suggest that some sepiolite forms via the addition of Mg to opal.