SP-10 Clay Mineralogy and Geology of Minnesota's Kaolin Clays

Abstract

Humid tropical weathering during the latter part of the Mesozoic Era, probably during early Late Cretaceous time, produced a thick kaolinitic residuum (unit 1) over much of Minnesota, mainly from Precambrian metamorphic and igneous rocks. The weathered zone is now covered by younger Cretaceous sedimentary rocks and Pleistocene glacial deposits except locally along the Minnesota River Valley in southwestern Minnesota and between St. Cloud and Little Falls in the central part of the state. As much as 100 feet of residuum is exposed along a 45-mile long section of the Minnesota River Valley between Granite Falls and Fort Ridgley State Park; marginal to the valley, the residuum is overlain by 40 or more feet of clays and shales. and glacial deposits. The clay minerals of the residuum-unit 1 of this report-that were formed from weathering of felsic rock types are composed primarily of kaolinite. In the least weathered parts of the profile, the kaolinite has an irregular platy form. Tubular halloysite is present in minor amounts, especially in the lower part of the weathering profile. Mafic rock types weathered first to montmorillonite and under progressively more intense weathering to kaolinite. Two Upper Cretaceous units of kaolinitic sedimentary rocks (units 2 and 3) overlie the residuum. The lowermost of these (unit 2), which was derived from erosion of the weathered residuum and which also underwent tropical weathering, has a maximum observed thickness of 45 feet, and is composed of varying proportions of kaolinite and quartz, with trace amounts of halloysite. A three- to five-foot, generally iron-rich, kaolinitic, pisolitic clay that contains small amounts of gibbsite and boehmite lies at the top of unit 2. Sedimentary rocks of unit 3 disconformably overlie unit 2, and consist of gray to black, organic-rich clays and shales, thin beds of lignite. and at least one thin bed of bentonite. Kaolinite is abundant in the basal part of unit 3 but gives way upward progressively to montmorillonite and illite, suggesting that the humid tropical climate under which units 1 and 2 had formed had been replaced by more temperate conditions by unit 3 time. Some of the kaolinitic clays of units 1 and 2 are potentially important as raw material for paper coating and filler. However, the presence of minor amounts of halloysite in some of these kaolin clays might adversely affect the flow properties of clay-water suspensions during paper-coating operations by increasing the suspensions' viscosity; some kaolinite having an irregular particle form may also produce a similar effect. It may be difficult to improve the whiteness and brightness of some of the clays of unit 1 if their natural color is pale green; the clays of unit 2, on the other hand, generally respond better to chemical bleaching. In addition, some of the kaolin clays present in all three stratigraphic units may be satisfactory for use in the ceramics and refractories industries. Ball clays of unit 3, which are very plastic and burn white, could be mixed with less plastic kaolin clays of units 1 and 2 for the production of a variety of refractory products. Most of the kaolin clays of the three units when fired become tan, pink. or white and could be used in common types of light-colored ceramic products.