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University of Missouri Department of Geological Sciences

Rock type:      Iron Ore
Location:        Pilot Knob, MO

This rock is made of layers that alternate in grain size and composition from coarser-grained and

quartz-rich to finer-grained and hematite-rich. Hematite (Fe2O3) is an iron oxide that can appear

red or grey. The grain size changes are most obvious on a rough face, while the color changes

are most obvious on a smooth cut face. Preserved hexagonal mudcracks suggest that the rock

formed from sediment on the floor of a lake that periodically dried out. Ideas for the formation

of the hematite in this rock include (i) infiltration of sandstone by iron-rich fluids, replacing other

minerals with hematite, and (ii) deposition in a volcanic lake, where the hematite precipitated

directly from hot volcanic fluids discharged into the lake at a hot spring. The original rock is

about 1.47 billion years old. Quarrying at Pilot Knob began in 1835 and ended in the 1920’s.

A subsurface deposit of magnetite (Fe3O4) was mined from 1965 until the 1980’s.

Photo by Alan Whittington, MU Geology

MU Geology Department
http://geology.missouri.edu/