Fluorellestadite is a rare nesosilicate of

calcium Calcium is a chemical element with the symbol Ca and atomic number 20. As an alkaline earth metal, calcium is a reactive metal that forms a dark oxide-nitride layer when exposed to air. Its physical and chemical properties are most similar to ...

, with sulfate and

fluorine Fluorine is a chemical element with the symbol F and atomic number 9. It is the lightest halogen and exists at standard conditions as a highly toxic, pale yellow diatomic gas. As the most electronegative reactive element, it is extremely reacti ...

, with the chemical formula Ca₁₀(SiO₄)₃(SO₄)₃F₂. It is a member of the

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common e ...

group, and forms a series with hydroxylellestadite.

Etymology

The mineral was originally named wilkeite by Eakle and Rogers in 1914, in honor of R. M. Wilke, a mineral collector and dealer. In 1922, a sample of “wilkeite” was analysed and found to be sufficiently different from the material reported by Eakle and Rogers to consider it a new species.Duncan McConnell (1937
The substitution of SiO4 – and SO4 – groups for PO4 -groups in the apatite structure; ellestadite, the end member
American Mineralogist 22: 977–986 The name “ellestadite” was proposed, in honor of Reuben B Ellestad (1900–1993), an American analytic chemist from the Laboratory for Rock Analysis,

University of Minnesota The University of Minnesota, formally the University of Minnesota, Twin Cities, (UMN Twin Cities, the U of M, or Minnesota) is a public university, public Land-grant university, land-grant research university in the Minneapolis–Saint Paul, Tw ...

, US. In 1982 Rouse and Dunn showed that the Si:S ratio was close to 1:1, giving the formula Ca₁₀(SiO₄)₃(SO₄)₃X₂, where X represents

(F),

hydroxyl In chemistry, a hydroxy or hydroxyl group is a functional group with the chemical formula and composed of one oxygen atom covalently bonded to one hydrogen atom. In organic chemistry, alcohols and carboxylic acids contain one or more hydroxy ...

(OH) or

chlorine Chlorine is a chemical element with the Symbol (chemistry), symbol Cl and atomic number 17. The second-lightest of the halogens, it appears between fluorine and bromine in the periodic table and its properties are mostly intermediate betwee ...

(Cl), and they named minerals in this group the ellestadite group.Roland C. Rouse and Pete J. Dunn (1982
A contribution to the crystal chemistry of ellestadite and the silicate sulfate apatites
American Mineraleralogist 67: 90–96 The end members of the group were named hydroxylellestadite (X = OH), fluorellestadite (X = F) and chlorellestadite (X = Cl); ideal end-member chlorellestadite is assumed not to exist in nature, although it has been synthesized. Wilkeite was discredited as a unique species, as it is not an

end member An endmember (also end-member or end member) in mineralogy is a mineral that is at the extreme end of a mineral series in terms of purity of its chemical composition. Minerals often can be described as solid solutions with varying compositions of ...

of any solid solution series, but an intermediate member. The name fluorellestadite was changed to ellestadite-(F) in 2008Burke (2008) The Mineralogical Record 39: 131 and changed back to fluorellestadite in 2010.

Structure

The ellestadites are nesosilicates, which are minerals with isolated SiO₄

tetrahedra In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the o ...

. They are members of the

group, but whereas

phosphorus Phosphorus is a chemical element with the symbol P and atomic number 15. Elemental phosphorus exists in two major forms, white phosphorus and red phosphorus, but because it is highly reactive, phosphorus is never found as a free element on Ear ...

is one of the chief constituents of apatite, in ellestadite it is almost completely replaced by

sulfur Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...

and

silicon Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic tab ...

, without appreciably altering the structure. The crystal class is

hexagonal In geometry, a hexagon (from Greek , , meaning "six", and , , meaning "corner, angle") is a six-sided polygon. The total of the internal angles of any simple (non-self-intersecting) hexagon is 720°. Regular hexagon A '' regular hexagon'' has ...

6/m, space group P6₃/m. The tetrahedral groups are arranged to create the 6₃

screw axis A screw axis (helical axis or twist axis) is a line that is simultaneously the axis of rotation and the line along which translation of a body occurs. Chasles' theorem shows that each Euclidean displacement in three-dimensional space has a screw ...

, and the

atoms are located in channels parallel to this direction. Some sources give unit cell parameters for one formula unit per unit cell (Z = 1), but some scientists consider the formula to be half the value accepted by the

International Mineralogical Association Founded in 1958, the International Mineralogical Association (IMA) is an international group of 40 national societies. The goal is to promote the science of mineralogy and to standardize the nomenclature of the 5000 plus known mineral species. Th ...

(IMA), i.e. Ca₅((Si,S)O₄))₃F, with two formula units per unit cell (Z = 2). Cell parameters for natural, as opposed to synthetic, material are a = 9.41 to 9.53 Å, and c = 6.90 to 6.94 Å. Rouse and Dunn postulated a hypothetical pure end-member with a = 9.543 Å and c = 6.917 Å. Synthetic material has a = 9.53 to 9.561 Å, and c = 6.91 to 6.920 Å.

Appearance

Fluorellestadite occurs as acicular or hexagonal

prismatic An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are ''not'' prisms. The most familiar type of optical ...

, poorly terminated crystals, and as fine-grained aggregates.John J. Jambor and Jacek Puziewicz (1989
New Mineral Names
American Mineralogist 74: 500, abstract of Chesnokov, Bazhenova and Bushmakin (1987) Zapiski Vses. Mineralog, Obshch 116:743 (in Russian) Crystals are transparent and aggregates are translucent. Material from Crestmore, California, is light rose-red or yellow in color, and typically occurs in a matrix of blue

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

. Material from Russia is pale bluish-green or colorless. The streak is white with a weak bluish tint, and the luster is sub-resinous on broken surfaces, but very brilliant on prism faces.

Physical properties

Fluorellestadite shows imperfect cleavage perpendicular to the long crystal axis. The mineral is very brittle, and breaks with a conchoidal

fracture Fracture is the separation of an object or material into two or more pieces under the action of stress. The fracture of a solid usually occurs due to the development of certain displacement discontinuity surfaces within the solid. If a displa ...

. Its

hardness In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion. In general, different materials differ in their hardness; for example hard ...

is , between that of fluorite and

, and its

specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water (molecule), wa ...

is 3.03 to 3.07, similar to that of fluorite. It is easily soluble in dilute hydrochloric and

nitric acid Nitric acid is the inorganic compound with the formula . It is a highly corrosive mineral acid. The compound is colorless, but older samples tend to be yellow cast due to decomposition into oxides of nitrogen. Most commercially available nitri ...

s and is not

radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is consid ...

. When intensely heated, ellestadite (wilkeite) becomes colorless and then assumes a pale bluish green color on cooling. The mineral is uniaxial (-), with refractive indices n_ω = 1.638 to 1.655 and n_ε = 1.632 to 1.650. It is sometimes

fluorescent Fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation. It is a form of luminescence. In most cases, the emitted light has a longer wavelength, and therefore a lower photon energy, tha ...

, white to blue-white or yellow-white in short-wave

ultraviolet Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nanometer, nm (with a corresponding frequency around 30 Hertz, PHz) to 400 nm (750 Hertz, THz), shorter than that of visible light, but longer than ...

light, and medium white-yellow-brown or weak white in long-wave light.

Occurrence and associations

The type locality is Coal Mine No. 44, Kopeisk, Chelyabinsk coal basin,

Chelyabinsk Oblast Chelyabinsk Oblast (russian: Челя́бинская о́бласть, ''Chelyabinskaya oblast'') is a federal subject (an oblast) of Russia in the Ural Mountains region, on the border of Europe and Asia. Its administrative center is the city ...

, Southern Urals, Russia, and

type material In biology, a type is a particular specimen (or in some cases a group of specimens) of an organism to which the scientific name of that organism is formally attached. In other words, a type is an example that serves to anchor or centralizes the ...

is held at the

Fersman Mineralogical Museum ) , native_name = , native_name_lang = , logo = , logo_upright = , logo_alt = , logo_caption = , image = Fersman Mineralogical Museum.JPG , image_upright = , a ...

, Academy of Sciences, Moscow, Russia. Ellestadite is a skarn mineral. It occurs associated with diopside, wollastonite, idocrase, monticellite,

okenite Okenite (CaSi2O5·2H2O) is a silicate mineral that is usually associated with zeolites. It most commonly is found as small white "cotton ball" formations within basalt geodes. These formations are clusters of straight, radiating, fibrous crystals ...

, vesuvianite,

and others at Crestmore, Riverside County, California, US. At Crestmore a contact zone exists between crystalline

limestone Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...

and granodiorite. The area was quarried for limestone in the early 1900s, revealing varied associations of

metamorphic minerals Metamorphic rocks arise from the transformation of existing rock (geology), rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of ...

, including ellestadite (named as wilkeite) with

garnet Garnets () are a group of silicate minerals that have been used since the Bronze Age as gemstones and abrasives. All species of garnets possess similar physical properties and crystal forms, but differ in chemical composition. The different s ...

, vesuvianite and diopside, in blue

. At the type locality it was formed in burned fragments of petrified wood in coal dumps, associated with

lime Lime commonly refers to: * Lime (fruit), a green citrus fruit * Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide * Lime (color), a color between yellow and green Lime may also refer to: Botany ...

, periclase,

magnesioferrite Magnesioferrite is a magnesium iron oxide mineral, a member of the magnetite series of spinels. Magnesioferrite crystallizes as black metallic octahedral crystals. It is named after its chemical composition of magnesium and ferric iron. The densit ...

hematite Hematite (), also spelled as haematite, is a common iron oxide compound with the formula, Fe2O3 and is widely found in rocks and soils. Hematite crystals belong to the rhombohedral lattice system which is designated the alpha polymorph of . ...

, srebrodolskite and anhydrite. Ellestadite (wilkeite) is often altered to

References

External links

JMol
{{Commons category, position=left Calcium minerals Nesosilicates Crestmore Heights, California Hexagonal minerals Minerals in space group 176