In Oregon the plagioclase of basaltic rocks weathered to kaolinite or to halloysite; ferromagnesian minerals and the basaltic glass of the groundmass and of tuffaceous fragments weathered to nontronite. Then, by loss of silica, the clay minerals changed to gibbsite and to high-iron bauxite. Gibbsite and a trace of boehmite are the chief aluminous minerals in the bauxite which contains from 47 to 25 percent Al 2 O 3 . Titaniferous magnetite, ilmenite, maghemite, goethite, hematite, limonite, and nontronite are the chief iron minerals in the bauxite which contains from 49 to 18 percent Fe 2 O 3 . Development of pisolitic structure in the upper part of the bauxite obscured early structures. Bauxite, which is formed from tuffaceous fragments or from detrital materials containing some quartz and other minerals foreign to basaltic rocks, lacks relict shapes of minerals that are preserved in the bauxite formed from basaltic lavas. A transitional layer of clay, composed of kaolinite, halloysite, and nontronite, and locally 100 feet thick, separates the bauxite at the top of the deposit from the underlying parent basaltic rocks of Miocene age and indicates that bauxite in Oregon formed by a two-stage process of weathering.

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