Braggite | |
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General | |
Category | Sulfide mineral |
Formula (repeating unit) | (Pt,Pd,Ni)S |
IMA symbol | Bg [1] |
Strunz classification | 2.CC.35a |
Dana classification | 2.8.5.3 |
Crystal system | Tetragonal |
Crystal class | Dipyramidal (4/m) H-M symbol: (4/m) |
Space group | P42/m |
Unit cell | a = 6.367 Å, c = 6.561 Å; Z = 8 |
Identification | |
Color | Steel grey; white in reflected light |
Crystal habit | Prismatic crystals and rounded grains |
Twinning | Rarely observed |
Cleavage | None |
Mohs scale hardness | 1.5 |
Luster | Metallic |
Diaphaneity | opaque |
Specific gravity | 10 (measured) 9.383 (calculated) |
Optical properties | Anisotropism distinct in polished section |
References | [2] [3] [4] [5] |
Braggite is a sulfide mineral of platinum, palladium and nickel with chemical formula: (Pt, Pd, Ni)S. It is a dense (specific gravity of 10), steel grey, opaque mineral which crystallizes in the tetragonal crystal system. [3] 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 of South Africa. Its name came from William Henry Bragg (1862–1942) and his son, William Lawrence Bragg (1890–1971). It was the first mineral that was discovered with the assistance of X rays. [4] [5]
It occurs as magmatic segregations in layered igneous intrusions such as Bushveld, the Stillwater igneous complex, the Lac des Îles igneous complex, the island of Rùm intrusive, the Great Dyke and many others. [4] [5] It is one of the most common platinum group minerals.
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 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 analyses now available. Using electron probe analyses the average proportions for metal in the sulfide structure were determined to be 64 percent Pt, 27 percent Pd, and 14 percent Ni. When based on the unit cell content, this approximates to Pt5Pd2NiS8. [6]
Platinum group minerals occur many places throughout the world in layered mafic and ultramafic intrusions formed at high magmatic temperatures, such as the Great Merensky Reef deposits of the Bushveld Igneous Complex in Transvaal Province, South Africa and the Precambrian Stillwater Complex in Montana. [6] Braggite has also been found as euhedral grains in platinum-iron nuggets from alluvial deposits in remote regions of eastern Madagascar. [7] The possible sources for these nuggets can be traced to ultramafic facies composed primarily of pyroxenites, peridotite and serpentine, and tremolite and soapstone. [7] 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. [8] 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. [6]
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. [3] Ionic bonds form between X and Z sites, but braggite also tends to exhibit metallic bonding characteristics. [9] The general sulfide structure is XmZn, where X represents the metallic elements and Z the nonmetallic element. [10] 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. [9]
Braggite appears steel grey to silvery white to the naked eye. When viewed through a petrographic microscope, under plane polarized light, braggite is white and slightly bireflectant and lacks reflectance pleochroism. Observing between crossed polars, its anisotropy 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 is common. Braggite has a measured specific gravity 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. [11]
Bannister and Hey discovered and named braggite in 1932. [12] It is the first mineral to be discovered by X-ray methods alone, and thus it was named to honour Sir William Henry Bragg and Sir William Lawrence Bragg of Cambridge University; both were pioneers in the x-ray investigation of crystals. [9]
Pentlandite is an iron–nickel sulfide with the chemical formula (Fe,Ni)9S8. Pentlandite has a narrow variation range in nickel to iron ratios (Ni:Fe), but it is usually described as 1:1. In some cases, this ratio is skewed by the presence of pyrrhotite inclusions. It also contains minor cobalt, usually at low levels as a fraction of weight.
The platinum-group metals (PGMs), also known as the platinoids, platinides, platidises, platinum group, platinum metals, platinum family or platinum-group elements (PGEs), are six noble, precious metallic elements clustered together in the periodic table. These elements are all transition metals in the d-block.
Cooperite is a grey mineral consisting of platinum sulfide, generally in combinations with sulfides of other elements such as palladium and nickel. Its general formula is (Pt,Pd,Ni)S. It is a dimorph of braggite.
Laurite is an opaque black, metallic ruthenium sulfide mineral with formula: RuS2. It crystallizes in the isometric system. It is in the pyrite structural group. Though it's been found in many localities worldwide, it is extremely rare.
Sperrylite is a platinum arsenide mineral with the chemical formula PtAs2 and is an opaque metallic tin white mineral which crystallizes in the isometric system with the pyrite group structure. It forms cubic, octahedral or pyritohedral crystals in addition to massive and reniform habits. It has a Mohs hardness of 6–7 and a very high specific gravity of 10.6.
Troilite is a rare iron sulfide mineral with the simple formula of FeS. It is the iron-rich endmember of the pyrrhotite group. Pyrrhotite has the formula Fe(1-x)S which is iron deficient. As troilite lacks the iron deficiency which gives pyrrhotite its characteristic magnetism, troilite is non-magnetic.
The Bushveld Igneous Complex (BIC) is the largest layered igneous intrusion within the Earth's crust. It has been tilted and eroded forming the outcrops around what appears to be the edge of a great geological basin: the Transvaal Basin. It is approximately 2 billion years old and is divided into four different limbs: the northern, southern, eastern, and western limbs. The Bushveld Complex comprises the Rustenburg Layered suite, the Lebowa Granites and the Rooiberg Felsics, that are overlain by the Karoo sediments. The site was first publicised around 1897 by Gustaaf Molengraaff who found the native South African tribes residing in and around the area.
Various theories of ore genesis explain how the various types of mineral deposits form within Earth's crust. Ore-genesis theories vary depending on the mineral or commodity examined.
A layered intrusion is a large sill-like body of igneous rock which exhibits vertical layering or differences in composition and texture. These intrusions can be many kilometres in area covering from around 100 km2 (39 sq mi) to over 50,000 km2 (19,000 sq mi) and several hundred metres to over one kilometre (3,300 ft) in thickness. While most layered intrusions are Archean to Proterozoic in age, they may be any age such as the Cenozoic Skaergaard intrusion of east Greenland or the Rum layered intrusion in Scotland. Although most are ultramafic to mafic in composition, the Ilimaussaq intrusive complex of Greenland is an alkalic intrusion.
Cumulate rocks are igneous rocks formed by the accumulation of crystals from a magma either by settling or floating. Cumulate rocks are named according to their texture; cumulate texture is diagnostic of the conditions of formation of this group of igneous rocks. Cumulates can be deposited on top of other older cumulates of different composition and colour, typically giving the cumulate rock a layered or banded appearance.
Gersdorffite or Nickel glance is a nickel arsenic sulfide mineral with formula NiAsS. It crystallizes in the isometric system showing diploidal symmetry. It occurs as euhedral to massive opaque, metallic grey-black to silver white forms. Gersdorffite belongs to a solid solution series with cobaltite, CoAsS. Antimony freely substitutes also leading to ullmannite, NiSbS. It has a Mohs hardness of 5.5 and a specific gravity of 5.9 to 6.33.
A native metal is any metal that is found pure in its metallic form in nature. Metals that can be found as native deposits singly or in alloys include aluminium, antimony, arsenic, bismuth, cadmium, chromium, cobalt, indium, iron, manganese, molybdenum, nickel, niobium, rhenium, selenium, tantalum, tellurium, tin, titanium, tungsten, vanadium, and zinc, as well as the gold group and the platinum group. Among the alloys found in native state have been brass, bronze, pewter, German silver, osmiridium, electrum, white gold, silver-mercury amalgam, and gold-mercury amalgam.
Heazlewoodite, Ni3S2, is a rare sulfur-poor nickel sulfide mineral found in serpentinitized dunite. It occurs as disseminations and masses of opaque, metallic light bronze to brassy yellow grains which crystallize in the trigonal crystal system. It has a hardness of 4, a specific gravity of 5.82. Heazlewoodite was first described in 1896 from Heazlewood, Tasmania, Australia.
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 Mountain Range. The complex has extensive reserves of chromium ore and has a history of being mined for chromium. More recent mining activity has produced palladium and other platinum group elements.
Louis Jean-Pierre Cabri (born February 23, 1934 in Cairo) is an eminent Canadian scientist in the field of platinum group elements (PGE) mineralogy with expertise in precious metal mineralogy and base metals at the Canada Centre for Mineral and Energy Technology (CANMET). First as Research Scientist and later as Principal Scientist (1996–1999). In the 1970s he discovered two new Cu–Fe sulfide minerals, "mooihoekite" and "haycockite". In 1983 Russian mineralogists named a new mineral after him: cabriite (Pd2SnCu).
Mooihoekite is a copper iron sulfide mineral with chemical formula of Cu9Fe9S16. The mineral was discovered in 1972 and received its name from its discovery area, the Mooihoek mine in Transvaal, South Africa.
Millerite is a nickel sulfide mineral, NiS. It is brassy in colour and has an acicular habit, often forming radiating masses and furry aggregates. It can be distinguished from pentlandite by crystal habit, its duller colour, and general lack of association with pyrite or pyrrhotite.
Köttigite is a rare hydrated zinc arsenate which was discovered in 1849 and named by James Dwight Dana in 1850 in honour of Otto Friedrich Köttig (1824–1892), a German chemist from Schneeberg, Saxony, who made the first chemical analysis of the mineral. It has the formula Zn3(AsO4)2·8H2O and it is a dimorph of metaköttigite, which means that the two minerals have the same formula, but a different structure: köttigite is monoclinic and metaköttigite is triclinic. There are several minerals with similar formulae but with other cations in place of the zinc. Iron forms parasymplesite Fe2+3(AsO4)2·8H2O; cobalt forms the distinctively coloured pinkish purple mineral erythrite Co3(AsO4)2·8H2O and nickel forms annabergite Ni3(AsO4)2·8H2O. Köttigite forms series with all three of these minerals and they are all members of the vivianite group.
Ferronickel platinum is a very rarely occurring minerals from the mineral class of elements (including natural alloys, intermetallic compounds, carbides, nitrides, phosphides and silicides) with the chemical composition Pt2FeNi and thus is chemically seen as a natural alloy, more precisely an intermetallic compound of platinum, nickel and iron in a ratio of 2:1:1.
Palladium(II) sulfide is a chemical compound of palladium and sulfur with the chemical formula PdS. Like other palladium and platinum chalcogenides, palladium(II) sulfide has complex structural, electrical and magnetic properties.