Vote for your favorite mineral in #MinCup26! - Sphalerite vs. Anorthite
Both zinc ore Sphalerite and calcium plagioclase feldspar Anorthite have perfect cleavage that will split cleanly if you hit them with a hammer, but only one can claim a larger split of your votes!
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Sillimanite from
Cotopaxi Mine (Cotopaxi lode; Gumaer Mine; Fannie lode; Fannie Mill), Mine Gulch, Cotopaxi, Cotopaxi Mining District, Fremont County, Colorado, USA


Locality type:Mine
Classification
Species:Sillimanite
Formula:Al2(SiO4)O
Confirmation
Validity:Believed Valid
Data
Mineral Data:Click here to view Sillimanite data
Locality Data:Click here to view Cotopaxi Mine (Cotopaxi lode; Gumaer Mine; Fannie lode; Fannie Mill), Mine Gulch, Cotopaxi, Cotopaxi Mining District, Fremont County, Colorado, USA
Data Identifiers
Mindat Occurrence Record ID:34977
Long-form Identifier:1:3:34977:8
GUID (UUID V4):0
Nearest other occurrences of Sillimanite
0.2km (0.2 miles) ⓘMine Gulch, Cotopaxi, Cotopaxi Mining District, Fremont County, Colorado, USA
14.1km (8.7 miles) ⓘDevil's Hole Mine, Fremont County, Colorado, USA
14.4km (9.0 miles) ⓘFive Points Gulch, Fremont County, Colorado, USA
24.9km (15.5 miles) ⓘCopper Girl (Copper Girl 1906; Patented Claims: Valley View), Copper Gulch Area, Grape Creek Mining District (Greenhorn Mining District), Fremont County, Colorado, USA
26.6km (16.6 miles) ⓘGreen Mountain Mine (Copper Boy Mine), Grape Creek Mining District (Greenhorn Mining District), Fremont County, Colorado, USA
30.9km (19.2 miles) ⓘEl Plomo Mine, Grape Creek Mining District (Greenhorn Mining District), Fremont County, Colorado, USA
34.4km (21.4 miles) ⓘDawson Gold project, Fremont County, Colorado, USA
35.9km (22.3 miles) ⓘCalumet Mine, Turret Mining District, Chaffee County, Colorado, USA
38.1km (23.7 miles) ⓘWhopper Mine and Prospect, Micanite Mining District (Currant Creek Mining District), Colorado, USA
38.1km (23.7 miles) ⓘSedalia Mine (Sedalia Copper Mine), Sedalia Mining District, Chaffee County, Colorado, USA
References
Am Min 50:1179-1212
Eckel, Edwin Butt; Cobban, Robert R.; Mosburg, Shirley K.; Foord, Eugene E. (1997) Minerals of Colorado. Fulcrum Publishing.
The Canadian Mineralogist Vol. 43, pp. 601-622 (2005)
Spry, Paul G.; McFadden, Scott; Teale, Graham S.; Alers, Brian; Shallow, John M.; Glenn, Justin M. (2022) Nodular sillimanite rocks as field indicators to metamorphosed massive sulfide deposits. Ore Geology Reviews, 141. 104632 doi:10.1016/j.oregeorev.2021.104632
 
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