WGNHS
Wisconsin Geological and Natural History Survey

Aegirine

Posted on
Green aegirine in pink K feldspar, near Stettin, Marathon County, Wisconsin. Field of view is about 6 cm across. (Image courtesy W. S. Cordua.)

Formula: NaFeSi2O6 Monoclinic

Description:Aegirine (also called acmite) is a sodium-rich member of the pyroxene family. Aegirine grades into other pyroxenes such as augite or hedenburgite as a result of solid solution series. Aegirine is most common in sodium-rich igneous rocks such as syenites, where it is often associated with sodium-rich amphiboles such as riebeckite and arfvedsonite. More rarely, small amount of acmite may be found in rocks formed by the metamorphism of sodium-rich rocks or iron formations.MARATHON COUNTY: The region west and south of Wausau has two major acmite-bearing syenite bodies—the Stettin pluton, a circular pluton largely found in T29N R6E and R.7E, and the Wausau pluton, a large arcuate body largely in T 27N-T29.N and R.6E to R. 7E (Sood, Meyers and Berlin, 1980). Both contain aegirine (Patton et al., 1989).—Falster (1987) notes stubby black crystals of aegirine as large as 3 cm in mariolitic cavities within the Wausau pluton as exposed in other granite quarries in sec. 19 and 20 T28 R7E.

—Weidman (1904) notes aegirine in massive, gneissic and pegmatitic nepheline syenite in the SE1/4 SE1/4 Sec. 22. This locality is described in great detail in Sood, Myers and Berlin (1980). The latter authors note the aegirine is associated with large perthitic K-feldspar xls., soda-amphiboles, euhedral zircon xls., titanite, apatite, sodalite and thorogummite (?).

—Weidman (1904) notes additional aegirine-bearing nepheline syenite in Sec. 26 and 27 T29N R6E, west of Wausau.

—Weidman (1907a) describes a syenite pegmatite with aegirine associated with K feldspar, quartz, crocidolite, riebeckite, pyrochlore, fluorite, lithia mica and zircon at the SW 1/4 Nw 1/4 Sec. 22 T.29N R6E.

—Weidman (1907b) records the following additional aegirine localities: in a syenite pegmatite with zircon and riebeckite in NE 1/4 Nw1/4 Sec. 15 T29N R6E; in nepheline syenite exposed both in NW1/.4 Sec. 28, NE 1/4 Sec. 29 (with hedenburgite, crocidolite, fluorite and riebeckite) and the NE 1/4 sec. 31 T29N R7E; in nepheline syenite in NW 1/4 Sec. 15, NW 1/4 Sec. 24 and the NE 1/4 Sec. 27 T.29N R6E; and in nepheline syenite in sec. 2 T28N R8E.

—Weidman (1907) also reports aegirine from the Summit Mine in T 29N R6E at the intersection of County Routes O and U.

—Aegirine is an accessory mineral in the quartz syenites found in roadcuts and an old quarry at NW 1/4 SE 1/4 Sec. 27 T.29N R.7E. just west of Wausau (Sood, Myers and Berlin, 1980).

—Excellent aegirine sprays occur with K feldspar in outcrops in the east 1/2 sec. 21 T.29N. R.6E. (Brown, personal communication)

—Aegirine is an accessory mineral in the syenites exposed at the Old Technical Institute near the Power Dam in Wausau in the NE 1/4 NE 1/4 Sec. 35 T.29N R.7E. (Sood, Myers and Berlin, 1980). They also find aegirine in pyroxene syenite of Stettin pluton rimmed by arfvedsonite in the SE and SW Sec 14 and NW Sec.23 T29N R6E.

—Aegirine occurs with zircon and microcline along fence rows SW NW Sec 23 T29N R6E (Vickers, 1956).