Prosopite
Current inventory:  1 gem
 

  
Prosopite is named from the Greek word prosopon meaning mask in allusion to the deceptive character of the mineral (as pseudomorphs).

Discovered in 1853;   IMA status:  Valid (pre-IMA; Grandfathered)

Chemistry

Chemical Formula:

CaAl₂F₆(OH)₂

Calcium Aluminum Flouride Hydroxide

Molecular Weight:

242.05 gm

Composition:

Calcium

16.56 %

Ca

23.17 %

CaO

Aluminum

22.29 %

Al

42.12 %

Al₂O₃

Hydrogen

0.83 %

H

7.44 %

H₂O

Oxygen

13.22 %

O

Fluorine

47.09 %

F

47.09 %

F

— %

F

-19.83 %

-O = F₂

100.00 %

100.00 %

= TOTAL OXIDE

Classification

Mineral Classification:

Halides

Strunz 8th Ed. ID:

3/C.03-10

Nickel-Strunz 10th Ed. ID:

3.CD.10

3 : HALIDES
C : Complex halides
D : Ino-aluminofluorides

Related to:

None

Varieties:

None

Synonyms:

None

Crystal Data

Crystallography:

Monoclinic - Prismatic

Crystal Habit:

Crystals, prismatic || [001], flattened on {010}, with {021}, {111}, and {221} also noted, to 6 mm; commonly granular or powdery.

Twinning:

None

Physical Properties

Cleavage:

Perfect on {111}

Fracture:

Irregular/Uneven to Conchoidal

Tenacity:

Brittle

Moh's Hardness:

4.5

Density:

2.880 - 2.894 (g/cm³)

Luminescence:

May be fluorescent; sky blue to bluish white under SW UV and LW UV

Radioactivity:

Not Radioactive

Optical Properties

Color:

Colorless, Grayish to White, rarely Turquoise-Blue; Colorless in transmitted light.

Transparency:

Transparent when fresh; becomes translucent on exposure.

Luster:

Vitreous, weak

Refractive Index:

1.500 - 1.510  Biaxial ( + )

Birefringence:

0.009

Dispersion:

None

Pleochroism:

None

Occurances

Geological Setting:

Disseminated in fluorine-rich granites, greisens, and granite pegmatites; more rarely in base-metal deposits.

Common Associations:

Fluorite, Cryolite, Thomsenolite, Pachnolite, Gearksutite, Ralstonite, Weberite, Jarlite, Siderite, Hematite, Cassiterite, Quartz.

Common Impurities:

Cu (Santa Rosa Mine, Noche Buena, Zacatecas, Mexico)

Type Locality:

Tin mine, Altenberg, Erzgebirge, Saxony, Germany

Year Discovered:

1853

View mineral photos:

Prosopite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Prosopite is a rare halide mineral that is commonly found as granular or powdery masses or as minute colorless crystals to about 6 mm. Prosopite is usually a nondescript mineral of no gem significance except for the beautiful turquoise to sky blue material found at the Santa Rosa Mine, Noche Buena, Zacatecas, Mexico. This source has produced small quantities of massive material of turquoise to sky blue that can be made into beautiful cabochons (see picture above). This is the only location to produce such beautiful blue material large enough for gem use. Blue Prosopite from Zacatecas, Mexico may appear very similar to light colored Turquoise from various sources. Distinction from Turquoise is mainly based on the lower refractive index (1.50 vs. 1.62 for Turquoise). This blue Prosopite gets its color from approximately 1.4% Copper content.

Prosopite is found at the following localities: In Germany, from Altenberg, Saxony. At Horní Slavkov (Schlaggenwald), Czech Republic. From Perga, Volyn, Ukraine. In Russia, from Miass, Ilmen Mountains, Southern Ural Mountains; on the Yenisei Ridge, Siberia; and in the Zharchikhinsk molybdenum deposit, on the west side of Lake Baikal, eastern Siberia. In the Ivigtut cryolite deposit, southwestern Greenland. From Mt. Bischoff, Tasmania, Australia. In the USA, at St. Peters Dome, near Pikes Peak, El Paso County, and in the Goldie carbonatite, McClure Mountain-Iron Mountain, Fremont County, Colorado; in the Dugway district, Tooele County, Utah; at the Zapot pegmatite, 25 km northeast of Hawthorne, Fitting district, Mineral County, Nevada; from the Grand Reef mine, Aravaipa district, Graham County, and 16 km north of Bouse, La Paz County, Arizona; in the Morefield pegmatite mine, Amelia, Amelia County, Virginia. In Mexico, at the Santa Rosa mine, Mazapil, Zacatecas, Mexico. From Tsumeb, Namibia.
 

  
Prosopite gems for sale: