Simmonsite is a halide mineral, being a tertiary light metal fluoride, with formula Na₂ Li Al F₆. It was first discovered in nature in Mineral County in the Gillis Range of

Nevada Nevada ( ; ) is a landlocked state in the Western United States. It borders Oregon to the northwest, Idaho to the northeast, California to the west, Arizona to the southeast, and Utah to the east. Nevada is the seventh-most extensive, th ...

, U.S.A. The mineral is found intergrown with

cryolite Cryolite ( Na3 Al F6, sodium hexafluoroaluminate) is a rare mineral identified with the once-large deposit at Ivittuut on the west coast of Greenland, mined commercially until 1987. It is used in the reduction ("smelting") of aluminium, in pest ...

, cryolithionite and trace elpasolite. The mineral has a

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

structure of P2 or P2/m. The ideal chemical formula for simmonsite is Na₂ Li Al F₆. The mineral has a no visible cleavage, Mohs hardness of 2.53, a pale white color with a white streak and feels somewhat greasy. Simmonsite was named for the Professor of Mineralogy and Petrology at the

University of New Orleans The University of New Orleans (UNO) is a Public university, public research university in New Orleans, Louisiana, United States. First opened in 1958 as Louisiana State University in New Orleans, it is the largest public university and one of t ...

, William B. Simmons.

Occurrence

Simmonsite, cryolite and cryolithionite form together as part of the alumino-fluoride aassemblage in a late stage

breccia Breccia ( , ; ) is a rock composed of large angular broken fragments of minerals or Rock (geology), rocks cementation (geology), cemented together by a fine-grained matrix (geology), matrix. The word has its origins in the Italian language ...

pipe structure that cross the Zapot amazonite-topaz-zinnwaldite pegmatite located in the described location above in Nevada. This assemblage occurs with around a third of each of theses phases. The three phases form an intergrowth of anhedral to subhedral grains that can be micrometers to approximately two or three hundred micrometers in size. The main anhydrous alumino-fluorides results in size almost 20 cm or more across and weight more than 1 kg. Another assemblage of alumino-fluorides exist, some that contain water or hydroxyl, pachnolite, weberite, thomsenollite, prosopite, ralstonite, as well as a second generation of cryolithionite.

Physical properties

Simmonsite contains no observable euhedral crystals but it does have complex polysynthetic twinning. Its color is pale buff to cream in the hadn specimens accompanied by a white streak and has a greasy appearance. It exhibits a hardness of 2.5-3 on the

Mohs hardness scale The Mohs scale ( ) of mineral hardness is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by the Germ ...

. There is no apparent cleavage, a subconchoidal fracture, no parting, and is not brittle. The measured density is 3.05(2) g/cm³.

Optical properties

The optical properties of this mineral were discovered by using the standard optical spindle stage procedure in 589 nm light. It was determined to have

birefringence Birefringence, also called double refraction, is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are described as birefrin ...

with a Berek compensator.

Chemical properties

Simmonsite is a fairly simple mineral chemically as the only elements present in major or minor amounts are Na, Al, Li, and F. The empirical chemical formula for simmonsite is Na₂ Li Al F₆. The four primary alumino-fluorides behave distinctly different under an electron beam with elpasolite and cryolithionite showing no beam damage with time, even though simmonsite did have some and cryolite having the greatest extent of damage.

Chemical composition

X-ray crystallography

Natural simmonsite is monoclinic with P2 or P2/m. The unit cell dimensions are a=7.5006(6) Å, b= 7.474(1) Å, c= 7.503(1) Å, β = 90.847(9)°, V= 420.6(1) Å3, Z= 4. These dimensions are almost identical to those of babingtonite. Simmonsite has had a long history of it being difficult to understand as there have been previous studies trying to understand the

crystallography Crystallography is the branch of science devoted to the study of molecular and crystalline structure and properties. The word ''crystallography'' is derived from the Ancient Greek word (; "clear ice, rock-crystal"), and (; "to write"). In J ...

. Errors were discovered in previous work by Holm and Holm(1970) that suggested it was monoclinic, B-centered, a=7.54 Å, b= 7.52 Å, c= 7.53 Å, β = 90.81°.

References

{{Authority control Natural materials Monoclinic minerals Minerals in space group 3 Minerals in space group 10