Braggite
Encyclopedia
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
. 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.
It occurs as magmatic segregations in layered igneous intrusions such as Bushveld, the Stillwater igneous complex
, the Lac des Îles igneous complex
, the Isle of Rum intrusive, the Great Dyke
and many others. It is one of the most common platinum group
minerals.
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.
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
. Braggite has also been found as euhedral
grains in platinum-iron nuggets from alluvial deposits in remote regions of eastern Madagascar
. The possible sources for these nuggets can be traced to ultramafic facies
composed primarily of pyroxenite
s, peridotite
and serpentine, and tremolite
and soapstone
. 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.
, 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.
and Sir William Lawrence Bragg
of Cambridge University, both were pioneers in the x-ray investigation of crystals.
Sulfide mineral
The sulfide minerals are a class of minerals containing sulfide as the major anion. Some sulfide minerals are economically important as metal ores. The sulfide class also includes the selenides, the tellurides, the arsenides, the antimonides, the bismuthinides, the sulfarsenides and the sulfosalts...
of platinum
Platinum
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78. Its name is derived from the Spanish term platina del Pinto, which is literally translated into "little silver of the Pinto River." It is a dense, malleable, ductile, precious, gray-white transition metal...
, palladium
Palladium
Palladium is a chemical element with the chemical symbol Pd and an atomic number of 46. It is a rare and lustrous silvery-white metal discovered in 1803 by William Hyde Wollaston. He named it after the asteroid Pallas, which was itself named after the epithet of the Greek goddess Athena, acquired...
and nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
with chemical formula: (Pt
PT
PT, Pt, or pt may stand for:* Pacific Time zone* Pro tempore, a Latin phrase "for the time being"/temporary* Physical Training, an abbreviation in the United States military- Units :...
,Pd
PD
-Political parties:*Democratic Party of Albania *Democratic Party *Democratic Liberal Party *Democratic Party...
,Ni
Ni
- Geography :* Ni River, a river of New Caledonia* Ni River , a tributary of the Mattaponi River* Mount Ni, a hill in Shandong, China- Letters :* Ni , romanisation of the Japanese kana に and ニ...
)S
S
S is the nineteenth letter in the ISO basic Latin alphabet.-History: Semitic Šîn represented a voiceless postalveolar fricative . Greek did not have this sound, so the Greek sigma came to represent...
. It is a dense (specific gravity
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
of 10), steel grey, opaque mineral which crystallizes in the tetragonal crystal system
Crystal system
In crystallography, the terms crystal system, crystal family, and lattice system each refer to one of several classes of space groups, lattices, point groups, or crystals...
. It is the central member in the platinum group end-members cooperite
Cooperite
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 S. It is a dimorph of braggite....
and vysotskite.
It was first described in 1932 for an occurrence in the Bushveld Igneous Complex
Bushveld igneous complex
The Bushveld Igneous Complex is a large layered igneous intrusion within the Earth's crust which has been tilted and eroded and now outcrops around what appears to be the edge of a great geological basin, the Transvaal Basin. Located in South Africa, the BIC contains some of the richest ore...
of South Africa
South Africa
The Republic of South Africa is a country in southern Africa. Located at the southern tip of Africa, it is divided into nine provinces, with of coastline on the Atlantic and Indian oceans...
. Its name came from William Henry Bragg
William Henry Bragg
Sir William Henry Bragg OM, KBE, PRS was a British physicist, chemist, mathematician and active sportsman who uniquely shared a Nobel Prize with his son William Lawrence Bragg - the 1915 Nobel Prize in Physics...
(1862–1942) and his son, William Lawrence Bragg
William Lawrence Bragg
Sir William Lawrence Bragg CH OBE MC FRS was an Australian-born British physicist and X-ray crystallographer, discoverer of the Bragg law of X-ray diffraction, which is basic for the determination of crystal structure. He was joint winner of the Nobel Prize for Physics in 1915. He was knighted...
(1890–1971). It was the first mineral that was discovered with the assistance of X rays.
It occurs as magmatic segregations in layered igneous intrusions such as Bushveld, the Stillwater igneous complex
Stillwater igneous complex
The Stillwater igneous complex is a large layered intrusion located in southern Montana in Stillwater, Sweet Grass and Park Counties. The complex is exposed across 30 miles of the north flank of the Beartooth Mountain Range. The complex has extensive reserves of chromium ore and has a history of...
, the Lac des Îles igneous complex
Lac des Îles igneous complex
The Lac des Îles igneous complex of northwestern Ontario, Canada is a layered gabbroic intrusion which is the host for the largest palladium orebody in Canada...
, the Isle of Rum intrusive, the Great Dyke
Great Dyke
The Great Dyke 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 approximately . The hills become taller as the range goes north, and...
and many others. It is one of the most common platinum group
Platinum group
The platinum group metals is a term used sometimes to collectively refer to six metallic elements clustered together in the periodic table.These elements are all transition metals, lying in the d-block .The six...
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 oreOre
An ore is a type of rock that contains minerals with important elements including metals. The ores are extracted through mining; these are then refined to extract the valuable element....
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
Microprobe
A microprobe is an instrument that applies a stable and well-focused beam of charged particles to a sample.-Types:When the primary beam consists of accelerated electrons, the probe is termed an electron microprobe, when the primary beam consists of accelerated ions, the term Ion Microprobe is used...
analyses now available. Using electron probe analyses
Electron microprobe
An electron microprobe , also known as an electron probe microanalyzer or electron micro probe analyzer , is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials...
the average proportions for metal in the sulfide
Sulfide
A sulfide is an anion of sulfur in its lowest oxidation state of 2-. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.- Properties :...
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.
Geologic occurrence
Platinum group minerals occur many places throughout the world in layered maficMafic
Mafic is an adjective describing a silicate mineral or rock that is rich in magnesium and iron; the term is a portmanteau of the words "magnesium" and "ferric". Most mafic minerals are dark in color and the relative density is greater than 3. Common rock-forming mafic minerals include olivine,...
and ultramafic intrusions formed at high magmatic temperatures, such as the Great Merensky Reef deposits of the Bushveld Igneous Complex in Transvaal Province
Transvaal
The Transvaal is the name of an area of northern South Africa. The land originally comprised most of the independent Boer South African Republic, which had existed since 1856, despite two previous attempts by the British of varying success to establish supremacy...
, South Africa and the Precambrian Stillwater Complex in Montana
Montana
Montana is a state in the Western United States. The western third of Montana contains numerous mountain ranges. Smaller, "island ranges" are found in the central third of the state, for a total of 77 named ranges of the Rocky Mountains. This geographical fact is reflected in the state's name,...
. Braggite has also been found as euhedral
Euhedral
Euhedral crystals are those that are well-formed with sharp, easily recognised faces. Normally, crystals do not form smooth faces or sharp crystal outlines. Many crystals grow from cooling liquid magma...
grains in platinum-iron nuggets from alluvial deposits in remote regions of eastern Madagascar
Madagascar
The Republic of Madagascar is an island country located in the Indian Ocean off the southeastern coast of Africa...
. The possible sources for these nuggets can be traced to ultramafic facies
Facies
In geology, facies are a body of rock with specified characteristics. Ideally, a facies is a distinctive rock unit that forms under certain conditions of sedimentation, reflecting a particular process or environment....
composed primarily of pyroxenite
Pyroxenite
Pyroxenite is an ultramafic igneous rock consisting essentially of minerals of the pyroxene group, such as augite and diopside, hypersthene, bronzite or enstatite. They are classified into clinopyroxenites, orthopyroxenites, and the websterites which contain both pyroxenes...
s, peridotite
Peridotite
A peridotite is a dense, coarse-grained igneous rock, consisting mostly of the minerals olivine and pyroxene. Peridotite is ultramafic, as the rock contains less than 45% silica. It is high in magnesium, reflecting the high proportions of magnesium-rich olivine, with appreciable iron...
and serpentine, and tremolite
Tremolite
Tremolite is a member of the amphibole group of silicate minerals with composition: Ca2Mg5Si8O222. Tremolite forms by metamorphism of sediments rich in dolomite and quartz. Tremolite forms a series with actinolite and ferro-actinolite. Pure magnesium tremolite is creamy white, but the color grades...
and soapstone
Soapstone
Soapstone is a metamorphic rock, a talc-schist. It is largely composed of the mineral talc and is thus rich in magnesium. It is produced by dynamothermal metamorphism and metasomatism, which occurs in the areas where tectonic plates are subducted, changing rocks by heat and pressure, with influx...
. 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 form between X and Z sites, but braggite also tends to exhibit metallic bonding characteristics. The general sulfide structure is XmZn, where X represents the metallic 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 microscopePetrographic microscope
A petrographic microscope is a type of optical microscope used in petrology and optical mineralogy to identify rocks and minerals in thin sections. The microscope is used in optical mineralogy and petrography, a branch of petrology which focuses on detailed descriptions of rocks...
, under plane polarized light, braggite is white and slightly bireflectant and lacks reflectance pleochroism
Pleochroism
Pleochroism is an optical phenomenon in which a substance appears to be different colors when observed at different angles with polarized light.- Background :Anisotropic crystals will have optical properties that vary with the direction of light...
. Observing between crossed polars, its anisotropy
Anisotropy
Anisotropy is the property of being directionally dependent, as opposed to isotropy, which implies identical properties in all directions. It can be defined as a difference, when measured along different axes, in a material's physical or mechanical properties An example of anisotropy is the light...
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
Specific gravity
Specific gravity is the ratio of the density of a substance to the density of a reference substance. Apparent specific gravity is the ratio of the weight of a volume of the substance to the weight of an equal volume of the reference substance. The reference substance is nearly always water for...
of 10 and calculated specific gravity of 9.383. Twinning
Crystal twinning
Crystal twinning occurs when two separate crystals share some of the same crystal lattice points in a symmetrical manner. The result is an intergrowth of two separate crystals in a variety of specific configurations. A twin boundary or composition surface separates the two crystals....
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 honor Sir William Henry BraggWilliam Henry Bragg
Sir William Henry Bragg OM, KBE, PRS was a British physicist, chemist, mathematician and active sportsman who uniquely shared a Nobel Prize with his son William Lawrence Bragg - the 1915 Nobel Prize in Physics...
and Sir William Lawrence Bragg
William Lawrence Bragg
Sir William Lawrence Bragg CH OBE MC FRS was an Australian-born British physicist and X-ray crystallographer, discoverer of the Bragg law of X-ray diffraction, which is basic for the determination of crystal structure. He was joint winner of the Nobel Prize for Physics in 1915. He was knighted...
of Cambridge University, both were pioneers in the x-ray investigation of crystals.