The weathering of titanium-bearing silicates in syenites from Saline County, Arkansas

Abstract

Samples collected from syenites and associated lateritic weathering profiles in the bauxite mining region of central Arkansas allow the study of redistribution of titanium and associated elements from titanium-bearing silicates when subjected to intense weathering. An average syenite in Saline County, Arkansas contains by volume 66% K-feldspar, 4% albite, 21% feldspathoids (nepheline, sodalite, and cancrinite), 2% hornblende, 2% pyroxenes, and lesser amounts of sphene, biotite, perovskite and opaque minerals. Titanium in the unweathered syenite is largely concentrated in the accessory silicate minerals. Nearly 65% of the total TiO2 in the unweathered syenite is present in the structure of sphene, with an additional 12% found in hornblende, biotite, and pyroxenes. Petrographic and x-ray diffraction analysis of bulk samples form weathering profiles indicate that alteration during the initial stage of weathering is not uniform, but rather is influenced by fractures and inhomogeneities in the rock. Many of the minerals examined alter to halloysite in the earlies stages of weathering. Kaolinite is the dominant mineral in rocks which have undergone intermediate weathering. Gibbsite dominates in the heavily weathered bauxitic clays farthest from the weathering surface. Late stage siderite veins occur throughout the profile. The concentration of major elements in the weathering profile is directly related to the alteration of rock forming silicate minerals. Ca, Na, and K were leached from the syenites immediately upon weathering, Al was concentrated with time by the formation of clays and oxides, and Fe was redistributed within the profile and may form siderite and pyrite where reducing conditions exist. Scanning electron microscopy examination of partially weathered titanium-bearing silicates indicates that anatase is the principal Ti alteration product. The morphology of anatase varies with distance from the weathering surface and is thought to be related to crystal growth. Close to the weathering surface, the Ti-oxide forms as cryptocrystalline or amorphous spherical particles 1μm in diameter. With increased weathering, the secondary Ti-oxide changes to ellipsoidal grains of anatase 3-5 μm across...