Rockbridgeite is an

anhydrous A substance is anhydrous if it contains no water. Many processes in chemistry can be impeded by the presence of water; therefore, it is important that water-free reagents and techniques are used. In practice, however, it is very difficult to achie ...

phosphate In chemistry, a phosphate is an anion, salt, functional group or ester derived from a phosphoric acid. It most commonly means orthophosphate, a derivative of orthophosphoric acid . The phosphate or orthophosphate ion is derived from phospho ...

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ( ...

in the "Rockbridgeite" supergroup with the

chemical formula In chemistry, a chemical formula is a way of presenting information about the chemical proportions of atoms that constitute a particular chemical compound or molecule, using chemical element symbols, numbers, and sometimes also other symbols, ...

Fe²⁺Fe³⁺₄(PO₄)₃(OH)₅. It was discovered at the since-shut-down Midvale Mine in

Rockbridge County Rockbridge County is a county in the Shenandoah Valley on the western edge of the Commonwealth of Virginia. As of the 2020 census, the population was 22,650. Its county seat is the city of Lexington. Rockbridge County completely surrounds the ...

Virginia Virginia, officially the Commonwealth of Virginia, is a state in the Mid-Atlantic and Southeastern regions of the United States, between the Atlantic Coast and the Appalachian Mountains. The geography and climate of the Commonwealth ar ...

United States The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...

. The researcher who first identified it, Clifford Frondel, named it in 1949 for its region of discovery, Rockbridge County.

Structure

The stable form of rockbridgeite is

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic lattices result from stretching a cubic lattice along two of its orthogonal pairs by two different factors, resulting in a rectangular prism with a r ...

2/m2/m2/m,

space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...

Bbmm. Face-sharing trimers in the structure share octahedron corners, forming chains of octahedra extending in the direction of the b-axis. Adjacent chains formed from two trimers and (PO₄) groups extending along the direction of the a-axis, resulting in sheets parallel to (001). Each chain of octahedra is connected by a layer of tetrahedra extending along the direction of the a-axis.

Unit cell

The

unit cell In geometry, biology, mineralogy and solid state physics, a unit cell is a repeating unit formed by the vectors spanning the points of a lattice. Despite its suggestive name, the unit cell (unlike a unit vector, for example) does not necessaril ...

parameters are a = 14 Å, b = 17 Å, and c = 5.2 Å, with 4 formula units per unit cell (Z = 4).
More accurate values are given by various sources as follows. *a = 13.783 Å, b = 16.805 Å, c = 5.172 Å, β = 100.313(1)°

Optical properties

Rockbridgeite varies from green, to black, to brownish green, to reddish brown. It is subtranslucent, with a greenish gray streak and vitreous to dull luster. The

refractive indices In optics, the refractive index (or refraction index) of an optical medium is a dimensionless number that gives the indication of the light bending ability of that medium. The refractive index determines how much the path of light is bent, or ...

are N_x = 1.875, N_y = 1.880, N_z = 1.897.

Physical properties

Rockbridgeite shows perfect cleavage on , distinct cleavage on , and distinct cleavage on . Rockbridgeite is a brittle mineral, with an uneven

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 ...

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 ...

4.5 and

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 ...

3.40. It is soluble in

hydrochloric acid Hydrochloric acid, also known as muriatic acid, is an aqueous solution of hydrogen chloride. It is a colorless solution with a distinctive pungent smell. It is classified as a strong acid Acid strength is the tendency of an acid, symbol ...

HCl, but not in

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 ...

HNO₃ or

sulfuric acid Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...

H₂SO₄.

Environment

Rockbridgeite primarily forms in

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main The three types of rocks, rock types, the others being Sedimentary rock, sedimentary and metamorphic rock, metamorphic. Igneous rock ...

s when the primary iron and manganese phosphates in

granite Granite () is a coarse-grained (phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies undergro ...

pegmatite A pegmatite is an igneous rock showing a very coarse texture, with large interlocking crystals usually greater in size than and sometimes greater than . Most pegmatites are composed of quartz, feldspar, and mica, having a similar silicic com ...

s undergo

mineral alteration Mineral alteration refers to the various natural processes that alter a mineral's chemical composition or crystallography. Mineral alteration is essentially governed by the laws of thermodynamics related to energy conservation, relevant to environ ...

through

redox reactions Redox (reduction–oxidation, , ) is a type of chemical reaction in which the oxidation states of substrate change. Oxidation is the loss of electrons or an increase in the oxidation state, while reduction is the gain of electrons or a d ...

, resulting in these phosphates becoming oxidized and causing rockbridgeite to form. More rarely, rockbridgeite can also form in

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s in association with

limonite Limonite () is an iron ore consisting of a mixture of hydrated iron(III) oxide-hydroxides in varying composition. The generic formula is frequently written as FeO(OH)·H2O, although this is not entirely accurate as the ratio of oxide to hydroxid ...

from

iron ore Iron ores are rocks and minerals from which metallic iron can be economically extracted. The ores are usually rich in iron oxides and vary in color from dark grey, bright yellow, or deep purple to rusty red. The iron is usually found in the fo ...

deposits. Associated minerals include

triphylite Triphylite is a lithium iron(II) phosphate mineral with the chemical formula LiFePO4.IMA-CNMNC List of Miner ...

, hureaulite, barbosalite, roscherite, and

Localities

The type locality is the since-shut-down Midvale Mine in

. Other localities for rockbridgeite include sites in Brazil, Portugal, Bavaria, Saxony, South Australia, Namibia, Zimbabwe, and Morocco. Type material is conserved for reference in

Harvard University Harvard University is a private Ivy League research university in Cambridge, Massachusetts. Founded in 1636 as Harvard College and named for its first benefactor, the Puritan clergyman John Harvard, it is the oldest institution of higher le ...

Cambridge Cambridge ( ) is a university city and the county town in Cambridgeshire, England. It is located on the River Cam approximately north of London. As of the 2021 United Kingdom census, the population of Cambridge was 145,700. Cambridge bec ...

Massachusetts Massachusetts (Massachusett language, Massachusett: ''Muhsachuweesut assachusett writing systems, məhswatʃəwiːsət'' English: , ), officially the Commonwealth of Massachusetts, is the most populous U.S. state, state in the New England ...

References

{{Reflist

External links

JMol: http://rruff.geo.arizona.edu/AMS/viewJmol.php?amcsd=0000198 Phosphate minerals Orthorhombic minerals Minerals in space group 63