Grandreefite

Grandreefite
General
Category	Sulfate minerals
Formula (repeating unit)	Pb2SO4F2
Strunz classification	7.BD.45
Crystal system	Monoclinic Prismatic class
Space group	Monoclinic H-M symbol: (2/m) Space group: A2/a
Unit cell	a = 8.667, b = 4.4419 c= 14.242 [Å]; β = 107.448° Z = 4; V = 523.15 Å^3
Identification
Color	Colorless
Twinning	Polysynthetic twins visible under cross polarized light
Cleavage	None
Fracture	Conchoidal
Tenacity	Brittle
Luster	Subadamantine
Streak	White
Specific gravity	6.96
Optical properties	Biaxial(+)
Refractive index	nα=1.872, nβ=1.873, nγ=1.895
2V angle	23°
Other characteristics	Decomposes in water
References	[1][2][3]

Grandreefite is a rare secondary lead sulfate-fluoride mineral with a general chemical formula, Pb₂SO₄F₂. It is named for the location in which it was discovered in 1989, the Grand Reef Mine in Graham County, Arizona.

Grandreefite is monoclinic, falling in the 2/m symmetry group. Crystallographically this means, grandreefite has three axes of unequal length, with two angles at 90° to one another and a third, obtuse angle. The 2/m symmetry group indicates a two-fold axis with a perpendicular mirror plane. Grandreefite was originally classified as an orthorhombic mineral but was determined monoclinic instead upon further analysis. Pseudograndreefite, one of three other newly discovered minerals in 1989 at the Grand Reef Mine site, has orthorhombic crystallography. Optically, grandreefite is an anisotropic mineral, meaning the velocity of light varies depending on the direction through which it is passing through the mineral. Its calculated relief, i.e. its relative difference in index of refraction compared to the surrounding medium's index of refraction, is 1.96–1.98; and its birefringence is .025.

Grandreefite, while not a valuable mineral in terms of commercial use, is useful in the sense that it is a good indicator of the very specific environment in which it forms and therefore the same conditions required to produce minerals similar to it in structure, composition, etc. For example, its structure is very similar to that of lanthanide oxide sulfates and therefore may be used to help better describe them. The Grand Reef Mine is located near a large granitic intrusion. Grandreefite is usually found in the mine in isolated veins containing hydrothermal copper, silver, and lead minerals hosted in oxidized breccias.

References

1. "Grandreefite". mindat.org. Retrieved 15 September 2010.
2. "Grandreefite". webmineral.com. Retrieved 3 December 2010.
3. "Grandreefite" (PDF). Handbook of Mineralogy. Retrieved 3 December 2010.

• Kampf, A.R. (1991) Grandreefite, Pb₂SO₄F₂;Crystal Structure and relationship to the lanthanide oxide sulfates, Ln₂O₂SO₄. American Mineralogist, 76, 278-282.
• Kampf, A.R., Dunn P.J., and Foord E.E. (1989) Grandreefite, pseudograndreefite, laurelite, and aravaipaite: Four new minerals from the Grand Reef Mine, Graham County, Arizona. American Mineralogist, 74, 927-933.