Braggite is a

sulfide mineral The sulfide minerals are a class of minerals containing sulfide (S2−) or disulfide () as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenide mineral, selenides, the tell ...

platinum Platinum is a chemical element; it has Symbol (chemistry), symbol Pt and atomic number 78. It is a density, dense, malleable, ductility, ductile, highly unreactive, precious metal, precious, silverish-white transition metal. Its name origina ...

palladium Palladium is a chemical element; it has symbol Pd and atomic number 46. It is a rare and lustrous silvery-white metal discovered in 1802 by the English chemist William Hyde Wollaston. He named it after the asteroid Pallas (formally 2 Pallas), ...

and

nickel Nickel is a chemical element; it has symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel is a hard and ductile transition metal. Pure nickel is chemically reactive, but large pieces are slo ...

with chemical formula: (Pt, Pd, Ni)S. It is a dense (

specific gravity Relative density, also called specific gravity, is a dimensionless quantity defined as the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for solids and liquids is nea ...

of 10), steel grey, opaque mineral which crystallizes in the

tetragonal In crystallography, the tetragonal crystal system is one of the 7 crystal systems. Tetragonal crystal lattices result from stretching a cubic lattice along one of its lattice vectors, so that the Cube (geometry), cube becomes a rectangular Pri ...

crystal system In crystallography, a crystal system is a set of point groups (a group of geometric symmetries with at least one fixed point). A lattice system is a set of Bravais lattices (an infinite array of discrete points). Space groups (symmetry groups ...

. It is the central member in the platinum group end-members cooperite and vysotskite. It was first described in 1932 for an occurrence in the

Bushveld Igneous Complex The Bushveld Igneous Complex (BIC) is the largest Layered intrusion, layered igneous intrusion within the Earth's Crust (geology), crust. It has been tilted and Erosion, eroded forming the outcrops around what appears to be the edge of a great Ba ...

South Africa South Africa, officially the Republic of South Africa (RSA), is the Southern Africa, southernmost country in Africa. Its Provinces of South Africa, nine provinces are bounded to the south by of coastline that stretches along the Atlantic O ...

. Its name came from

William Henry Bragg Sir William Henry Bragg (2 July 1862 – 12 March 1942) was an English physicist and X-ray crystallographer who uniquelyThis is still a unique accomplishment, because no other parent-child combination has yet shared a Nobel Prize (in any fiel ...

(1862–1942) and his son,

William Lawrence Bragg Sir William Lawrence Bragg (31 March 1890 – 1 July 1971) was an Australian-born British physicist who shared the 1915 Nobel Prize in Physics with his father William Henry Bragg "for their services in the analysis of crystal structure by ...

(1890–1971). It was the first mineral that was discovered with the assistance of

X rays An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

. It occurs as

magmatic Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma (sometimes colloquially but incorrectly referred to as ''lava'') is found beneath the surface of the Earth, and evidence of magmatism has also ...

segregations in layered igneous intrusions such as Bushveld, the

Stillwater igneous complex The Stillwater igneous complex is a large layered mafic intrusion (LMI) located in southern Montana in Stillwater, Sweet Grass and Park Counties. The complex is exposed across 30 miles (48 km) of the north flank of the Beartooth Mount ...

, the Lac des Îles igneous complex, the island of

Rùm Rùm (), a Scottish Gaelic language, Scottish Gaelic name often Anglicisation, anglicised to Rum ( ), is one of the Small Isles of the Inner Hebrides, off the west coast of Scotland, in the district of Lochaber. For much of the 20th century the ...

intrusive, the

Great Dyke The Great Dyke or Dike is a linear geological feature that trends nearly north-south through the centre of Zimbabwe passing just to the west of the capital, Harare. It consists of a band of short, narrow ridges and hills spanning for approximate ...

and many others. It is one of the most common

platinum group The platinum-group metals (PGMs) are six noble, precious metallic elements clustered together in the periodic table. These elements are all transition metals in the d-block (groups 8, 9, and 10, periods 5 and 6). The six platinum-group ...

minerals.

Composition

The braggite composition series is between the platinum rich cooperite and palladium rich vysotskite end members in solid solution and is thus considered of primary economic importance as an

ore Ore is natural rock or sediment that contains one or more valuable minerals, typically including metals, concentrated above background levels, and that is economically viable to mine and process. The grade of ore refers to the concentration ...

for both of these precious metals. Braggite, as well as vysotskite, was named prior to knowledge of phase relations in the Pt-Pd-S System and prior to the extensive

microprobe A microprobe is an instrument that applies a stable and well-focused beam of charged particles (electrons or ions) to a sample. Types When the primary beam consists of accelerated electrons, the probe is termed an electron microprobe, when the pr ...

analyses now available. Using electron probe analyses the average proportions for metal in the

sulfide Sulfide (also sulphide in British English) is an inorganic anion of sulfur with the chemical formula S2− or a compound containing one or more S2− ions. Solutions of sulfide salts are corrosive. ''Sulfide'' also refers to large families o ...

structure were determined to be 64 percent Pt, 27 percent Pd, and 14 percent Ni. When based on 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 In mathematics, a unit vector i ...

content, this approximates to Pt₅Pd₂NiS₈.

Geologic occurrence

Platinum group minerals occur many places throughout the world in layered

mafic A mafic mineral or rock is a silicate mineral or igneous rock rich in magnesium and iron. Most mafic minerals are dark in color, and common rock-forming mafic minerals include olivine, pyroxene, amphibole, and biotite. Common mafic rocks include ...

and

ultramafic Ultramafic rocks (also referred to as ultrabasic rocks, although the terms are not wholly equivalent) are igneous and meta-igneous rocks with a very low silica content (less than 45%), generally >18% MgO, high FeO, low potassium, and are usua ...

intrusions formed at high magmatic temperatures, such as the Great Merensky Reef deposits of the Bushveld Igneous Complex in

Transvaal Province The Province of Transvaal (), commonly referred to as the Transvaal (; ), was a province of South Africa from 1910 until 1994, when a new constitution subdivided it following the end of apartheid. The name "Transvaal" refers to the province's ...

, South Africa and the Precambrian Stillwater Complex in

Montana Montana ( ) is a landlocked U.S. state, state in the Mountain states, Mountain West subregion of the Western United States. It is bordered by Idaho to the west, North Dakota to the east, South Dakota to the southeast, Wyoming to the south, an ...

. Braggite has also been found as

euhedral Euhedral and anhedral are terms used to describe opposite properties in the formation of crystals. Euhedral (also known as idiomorphic or automorphic) crystals are those that are well-formed, with sharp, easily recognised faces. The opposite i ...

grains in platinum-iron nuggets from

alluvial deposits Alluvium (, ) is loose clay, silt, sand, or gravel that has been deposited by running water in a stream bed, on a floodplain, in an alluvial fan or beach, or in similar settings. Alluvium is also sometimes called alluvial deposit. Alluvium is ...

in remote regions of eastern

Madagascar Madagascar, officially the Republic of Madagascar, is an island country that includes the island of Madagascar and numerous smaller peripheral islands. Lying off the southeastern coast of Africa, it is the world's List of islands by area, f ...

. The possible sources for these nuggets can be traced to ultramafic

facies In geology, a facies ( , ; same pronunciation and spelling in the plural) is a body of rock with distinctive characteristics. The characteristics can be any observable attribute of rocks (such as their overall appearance, composition, or con ...

composed primarily of

pyroxenite Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite, diopside, hypersthene, bronzite or enstatite. Pyroxenites are classified into clinopyroxenites, orthopyroxenites, and the w ...

peridotite Peridotite ( ) is a dense, coarse-grained igneous rock consisting mostly of the silicate minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium (Mg2+), reflecting the high pr ...

and serpentine, and

tremolite Tremolite is a member of the amphibole group of silicate minerals with composition Ca2(Mg5.0-4.5Fe2+0.0-0.5)Si8O22(OH)2. Tremolite forms by metamorphism of sediments rich in dolomite and quartz, and occurs in two distinct forms, crystals and fib ...

and

soapstone Soapstone (also known as steatite or soaprock) is a talc-schist, which is a type of metamorphic rock. It is composed largely of the magnesium-rich mineral talc. It is produced by dynamothermal metamorphism and metasomatism, which occur in sub ...

. In solution, cooperite forms the first solid at just above the 1100 °C threshold, braggite at around 1000 °C, and lastly vysotskite at temperatures below 900 °C. The amount of braggite varies in any given PGM complex. It averages 35.9 volume percent in the Western Transvaal while the Atok Platinum mine in Bushveld boasts 60 volume percent.

Atomic structure

Braggite is a tetragonal mineral with lattice spacing a = 6.38, c = 6.57 Å, Z = 8 and angles between axes α = β = γ = 90° with space group symmetry P4₁/m.

Ionic bonds Ionic bonding is a type of chemical bonding that involves the electrostatic attraction between oppositely charged ions, or between two atoms with sharply different electronegativities, and is the primary interaction occurring in ionic compound ...

form between X and Z sites, but braggite also tends to exhibit

metallic bonding Metallic bonding is a type of chemical bonding that arises from the electrostatic attractive force between conduction electrons (in the form of an electron cloud of delocalized electrons) and positively charged metal ions. It may be desc ...

characteristics. The general sulfide structure is X_mZ_n, where X represents the

metal A metal () is a material that, when polished or fractured, shows a lustrous appearance, and conducts electrical resistivity and conductivity, electricity and thermal conductivity, heat relatively well. These properties are all associated wit ...

lic elements and Z the nonmetallic element. In the braggite structure, the Z site is always filled by sulfur with a charge of 2⁻. Due to distortions in the structure the X site can vary in size and is filled by Pd, Pt, or Ni each having a 2⁺ charge.

Physical properties

Braggite appears steel grey to silvery white to the naked eye. When viewed through a

petrographic microscope A petrographic microscope is a type of optical microscope used to identify Rock (geology), rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descr ...

, under plane

polarized light , or , is a property of transverse waves which specifies the geometrical orientation of the oscillations. In a transverse wave, the direction of the oscillation is perpendicular to the direction of motion of the wave. One example of a polarize ...

, braggite is white and slightly bireflectant and lacks reflectance

pleochroism Pleochroism is an optical phenomenon in which a substance has different colors when observed at different angles, especially with Polarization (waves), polarized light. Etymology The roots of the word are from Greek (). It was first made compou ...

. Observing between crossed polars, its

anisotropy Anisotropy () is the structural property of non-uniformity in different directions, as opposed to isotropy. An anisotropic object or pattern has properties that differ according to direction of measurement. For example, many materials exhibit ve ...

is distinct in air and is characterized by a purplish-grey to brown-grey tint. Relatively large crystals (up to 8 mm long) are not unusual for braggite and

fracturing Fracture is the appearance of a crack or complete 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 sur ...

is common. Braggite has a measured

of 10 and calculated specific gravity of 9.383. Twinning is rarely observed. Braggite was found to have microindentation VHN values ranging from 973–1015.

Discovery and naming

Bannister and Hey discovered and named braggite in 1932. It is the first mineral to be discovered by X-ray methods alone, and thus it was named to honour Sir

and Sir

Cambridge University The University of Cambridge is a Public university, public collegiate university, collegiate research university in Cambridge, England. Founded in 1209, the University of Cambridge is the List of oldest universities in continuous operation, wo ...

; both were pioneers in the x-ray investigation of crystals.

References

{{Reflist Platinum minerals Nickel minerals Palladium minerals Sulfide minerals Tetragonal minerals Minerals in space group 84 Minerals described in 1932