Formula: Na3(Fe2+4Fe)Si8O22(OH)2 Monoclinic
Description: Arfvedsonite is a member of the amphibole group usually found in low silica but highly alkaline igneous rocks such as nepheline syenites. It may be mistaken for hornblende, but is distinguished by its occurrence, darker streak and optical properties.
MARATHON COUNTY: Arfvedsonite has long been known from the syenites of the Wausau region (Weidman, 1904a). More recent work has verified its presence in numerous outcrops in the Stettin pluton, where it occurs as elongated crystals up to 25 cm. long and radiating crystal groups and as rims surrounding augite Arfvedsonite and riebeckite frequently coat fracture surfaces in the pegmatites and syenites. (Falster et al., 2000). Some specific localities of arfvedsonite-bearing rocks are: NE Sec. 31 T.29N. R.7E (Weidman, 1907b), throughout Sec. 14 T.29N. R.6E. and in NW Sec. 23 T.29N. R.6E. (Sood, Myers and Berlin, 1980).