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had been replaced by more temperate conditions by unit 3 time.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.
Parham, W.E., Clay Mineralogy and Geology of Minnesota's Kaolin Clays. Special Publications Series 10. 142 p.
142 p. Second printing, 1976.
Parham, Walter E..
SP-10 Clay Mineralogy and Geology of Minnesota's Kaolin Clays.
Minnesota Geological Survey.
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