Sacrofanite

Sacrofanite
General
Category	Cancrinite group, tectosilicate
Formula; (repeating unit)	(Na,Ca)9(Si,Al)12O24(SO4,CO3,OH,Cl)4·n(H2O)
Strunz classification	9.FB.05
Crystal system	Hexagonal
Crystal class	Ditrigonal dipyramidal (6m2) ; H-M symbol: (6m2)
Space group	P62c
Unit cell	a = 12.86, c = 72.24 [Å] ; V = 10,346.52 Å3; Z = 14
Identification
Color	Colorless
Cleavage	Perfect on {0001} {0110}
Mohs scale hardness	5 1⁄2 - 6
Luster	Pearly to vitreous
Streak	white
Diaphaneity	Transparent
Specific gravity	2.423
Optical properties	Uniaxial negative
Refractive index	nω = 1.505, nε = 1.486 (both ± 0.001)
Birefringence	0.019
References	[1][2][3]

Sacrofanite is a rare silicate mineral that has the general formula of (Na,Ca)₉(Si,Al)₁₂O₂₄(SO₄,CO₃,OH,Cl)₄·n(H₂O).[4] It was approved as a mineral by the International Mineralogical Association in 1980.[4] Its name comes from the Sacrofano Caldera in the Monti Sabatini from which it was discovered in Latium, Italy.[1]

Structure and optical properties

It has a hexagonal crystal system, meaning crystallographically it has three horizontal axes of equal length that make an angle of 120° with their positive ends, with a long vertical axis running perpendicular to them. The mineral is anisotropic, meaning the velocity of light through a mineral will vary depending on the direction that the light is going. It has a moderate relief ranging from 1.486 to 1.505.[2] It has a birefringence of 0.019, with birefringence being the decomposition of a light ray into two different rays when light hits a mineral.[2] Sacrofanite is uniaxial negative, meaning it has one optic axis and that the index of refraction for the ordinary ray is greater than the index of refraction for the extraordinary ray.[5]

Occurrence

Sacrofanite is only found in the Sacrofano Caldera in Italy, yet it represents one of many new minerals found in Italian volcanoes. It can be found on display in The Mineralogical Museum at the University of Rome.[1]

It occurs as a crystal coating within a vug in a rock ejected from the volcano. Other minerals occurring in the rock include sanidine, andradite, fassaite, leucite and hauyne.[3]

References

Cabri, L., Fleischer, M., and Pabst, A. (1981) New Mineral Names. American Mineralogist, 66, 1100.
Ralph, J. & Chau, I. "Sacrofanite mineral information and data". Retrieved 20 September 2010.
Handbook of Mineralogy
Nickel, E., and Nichols, M. (1980) IMA/CNMNC List of Mineral Names. Official IMA-CNMNC List of Mineral Names, 247.
Nelson, Stephen, 25 October 2010, "Uniaxial Minerals, Uniaxial Indicatrix, Optic Sign, and Ray Path." http://www.tulane.edu/~sanelson/eens211/uniaxial_minerals.htm. Accessed 1 December 2010.

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[Cabri-1] Cabri, L., Fleischer, M., and Pabst, A. (1981) New Mineral Names. American Mineralogist, 66, 1100.

[mindat-2] Ralph, J. & Chau, I. "Sacrofanite mineral information and data". Retrieved 20 September 2010.

[HBM-3] Handbook of Mineralogy

[IMA/CNMNC-4] Nickel, E., and Nichols, M. (1980) IMA/CNMNC List of Mineral Names. Official IMA-CNMNC List of Mineral Names, 247.

[5] Nelson, Stephen, 25 October 2010, "Uniaxial Minerals, Uniaxial Indicatrix, Optic Sign, and Ray Path." http://www.tulane.edu/~sanelson/eens211/uniaxial_minerals.htm. Accessed 1 December 2010.