Riebeckite Na2(Fe,Mg)5Si8O22(OH)2 Monoclinic

Description: Riebeckite is a sodium-rich member of the amphibole family. It typically forms dark blue elongated to fibrous crystals in highly sodic granites, syenites, iron formations and schists.

JACKSON COUNTY: Minor riebeckite occurs in the iron formation and along later joints at the Jackson County Iron Mine, SE sec. 15 T.21N. R.3W. near Black River Falls. Some riebeckite forms cores rimmed by later cummingtonite. (Kimball and Spears, 1984).

MARATHON COUNTY: Riebeckite is relatively common in the syenites of the Stettin Pluton, west of Wausau. It is found associated with aegirine, hedenbergite, zircon, fluorite, K feldspar and other rare minerals.
- Weidman (1907a) reports riebeckite in a syenite pegmatites in the NE NW sec. 15 and NW sec. 22 T.29N. R.6E and in nepheline syenite in the NE sec. 29 T.29N. R.7W.
- Sood, Myers and Berlin (1980) report riebeckite in syenites exposed in numerous outcrops in sec. 14 as well as in the NW sec. 23 T.29N. R.6E.
- Myers, et. al. (1984) note riebeckite in the syenitized wall rock where metavolcanic rocks are in contact with the Stettin pluton. Such a wall zone is exposed in road cuts along County Hwy U 0.5 miles east of the Little Rib River (SW SW sec. 18 T.29N. R.7E).

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