Formula: Al2Si2O5(OH)4 Monoclinic
Halloysite is a clay mineral formed by weathering or hydrothermal alteration of other aluminum-rich minerals. It occurs intermixed with kaolin and other clay minerals. Its identification requires x-ray analysis. Many “clay” occurrences within the state are likely partly halloysite, hence the occurrences listed under other clay minerals (dickite, kaolin, montmorillonite) are also likely halloysite occurrences. The occurrences listed below are points were halloysite has been verified.
DODGE COUNTY: Halloysite occurs as white spherical porcellaneous shells in the oolites of the Neda iron formation, as exposed in old mines in the east 1/2 of NW sec. 1 T.11N. R.16E. (Hawley and Beavan, 1934).
IOWA COUNTY: Halloysite is a microscopic component of duricrusts formed on outcrops of St. Peter Sandstone in NW sec. 15 T.4N. R.2E., near the Pecatonia river southwest of Mineral Point. (Habermann, 1976).
JACKSON COUNTY: Found as crusts and interstitial material in the Mt. Simon Sandstone with wavellite on the west side of East Snow Creek Road, SW NW sec. 14 T.22N. R.4W. (Cordua, 1993 field notes).
SAUK COUNTY: Halloysite is a microscopic component of duricrusts in sec. 29 T.12N. R.5E. (Habermann, 1978