Alacranite
Encyclopedia
Alacránite is an arsenic
sulfide mineral
first discovered in the Uzon
caldera
, Kamchatka, Russia
. It was named for its occurrence in the Alacrán silver
, arsenic
, antimony
mine
, Pampa Larga, Chile
. It is generally more rare than realgar
and orpiment
. Its origin is hydrothermal. It occurs as subhedral to euhedral
tabular orange to pale gray crystals that are transparent
to translucent. It has a yellow-orange streak with a hardness of 1.5. It crystallizes in the monoclinic crystal system
. It occurs with realgar and uzonite as flattened and prismatic
grains up to 0.5 mm across.
and calcite
veins in the Alacran silver mine. They also assumed that the mineral was identical to the species occurring in the Ag-As-Sb vein deposit at Alacran due to the similarity of its X-ray diffraction powder pattern. They discovered that alacranite was similar to the high-temperature α polymorph
of As4S4 in X-ray characteristics. In addition, alacranite was considered as realgar-like mineral. After that, they reported the composition of alacranite as As8S9 when they noticed another occurrence of Alacranite in Uzon Caldera associated with realgar
and uzonite as cement in sandy gravels. They reported the composition of alacranite to be As8S9 regarding to the electron-microprobe analyses. When they analyzed a mineral during seafloor sampling consisting of red and orange arsenic sulfides by X-ray diffraction, a mixture of realgar and alacranite was resulted and it was noticed that the mineral was identical to synthetic β-As4S4 supposed that the original chemical formula of alacranite (As8S9) was incorrect. They argued that it is identical to alacranite because of the similarity of the physical properties and the unit-cell dimensions but further studies argue that it is different with respect to chemical formula and unit-cell volume. The chemistry of the average of four analyses in Uzon caldera, Russia corresponding to alacranite results in 67.35% arsenic and 32.61% sulfur resulting in a total of 99.96%, yielding the formula As7.98S9.02, ideally As8S9.
volcanism and regional tectonics in Papua New Guinea collected samples that consists of clay minerals
, pyrite
, sphalerite
, galena
, chalcopyrite
, sulfosalts and arsenic-bearing sulfides like realgar and
alacranite. Data were gathered at different times using monochromatic X-radiation when a crystal of alacranite with approximate dimensions 0.14 × 0.10 × 0.06 mm was placed in a platform 3-circle goniometer equipped with a 1K charge-coupled device for 2θ up to 56.7°. The data shows reflection statistics and systematic absences that indicates space group C2/c referring to the monoclinic symmetry for alacranite. Final results of the experiment show that alacranite is isostructural with the compound α-As4S4 and it is the third mineral polymorph with the formula As4S4, the others are realgar and pararealgar.
The three minerals consist of covalently bonded As4S4 molecules. In alacranite, each arsenic atom is bonded to one arsenic atom and two sulfur atoms, while the sulfur atoms bonded only to two arsenic atoms.
The structure of molecules in alacranite is chemically the same as in realgar held together by van der Waals forces but different in the arrangement of the As4S4 in both structures. The differences between alacranite and realgar are demonstrated in the unit cell sizes and the packed structures of both minerals. Alacranite has a C-centered unit cell that is smaller than the primitive cell
of realgar and also the structure of alacranite has a closely and more orderly packed structure than realgar.
Further studies of a single crystal of alacranite with dimensions (30 × 60 × 120 mm was examined using direct methods, Fourier syntheses and structure refinement resulting in intensities violating the C lattice type and symmetries like h0l reflections with l = 2n + 1 were absent. These results confirm the P2/c space group of alacranite resulting in a structure of two different kinds of cage-like molecules that are packing closely and found in the β-phase.
The first molecule is identical to the As4S4 which is realgar where every As atom links one As and two S atoms and that is determined in the structure of the β-phase and realgar. The second molecule in the structure of alacranite is found to be chemically and structurally identical to As4S5 which is uzonite. Upon exposure to light, As4S4 molecule expands its unit cell volume and hence, transfers to As4S5. This transformation could be described to the variation of the intermolecular distances. These coherent orders of both molecules along [110] attributes to the change of the translation symmetry from C (β-phase) to P which is alacranite. In both molecules, the distances of the As-S bond are about 2.205 to 2.238 angstrom. However, the As-As bond distance in the As4S4 is longer than the As-As bond contained in As4S5 molecule within alacranite structure. It was also shown that the unit-cell volume increases proportionally with increasing S content of minerals with ranging composition from As4S4 to As8S9 where alacranite has the biggest unit-cell in this range. Evidence supports that alacranite has a P2/c space group
whereas the high-temperature and less content of S form is considered to be a new mineral that corresponds to the species from alacran previously described by which is As4S4 that crystallizes with the space group C2/c and has a smaller volume.
. Its crystal faces occur as pinacoidal, prismatic
and flattened on [100]. Its major forms are {100}, {111}, and {111}. Other forms are weak, dull or tarnished. It shows an orange to pale gray with rose-yellow internal reflections (shown in figure 1) with a yellow-orange streak
. It has adamantine, vitreous, resinous and greasy lustre
and it is transparent. The weak chemical bonding in its structure, gives the mineral a low hardness around 1.5 and weak forms. It has an imperfect cleavage on {100} and its fracture is conchoidal and very brittle. Its density
is measured to be around 3.43. When it reacts with 5M KOH, alacranite changes color to brown, it changes into brown-gray flakes when it is heated and boiling it should return
its color to brown. However, if it is mixed with HCl or HNO3, it does not show any activity.
. The Uzon caldera is located near the eastern volcanic belt of the Kamchatka peninsula
. The area is a basalt
ic shield volcano
with lacustrine
sediments, faulting and extension, dome formation and hydrothermal fluids from hot springs within the caldera
. Amounts of realgar, stibnite
, cinnabar
and pyrite
are contained in sediments near active hot springs.
Alacranite occurs in the condensation zone of a hydrothermal Hg-Sb-As system in the Uzon caldera. Alacranite could also be found in hydrothermal As-S viens.
It was named alacranite after its occurrence in the Alacran mine in Chile
due to its similarities in XRD patterns of the samples from the Uzon caldera to those in the Alacan mine.
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
sulfide mineral
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...
first discovered in the Uzon
Uzon
Uzon is a 9 by 12 km volcanic caldera located in the eastern part of Kamchatka Peninsula, Russia. Together with the Geyzernaya caldera it hosts the largest geothermal field in the Kamchatka Peninsula. The calderas were formed in the mid-Pleistocene in several large eruptions that deposited...
caldera
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...
, Kamchatka, Russia
Russia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
. It was named for its occurrence in the Alacrán silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
, arsenic
Arsenic
Arsenic is a chemical element with the symbol As, atomic number 33 and relative atomic mass 74.92. Arsenic occurs in many minerals, usually in conjunction with sulfur and metals, and also as a pure elemental crystal. It was first documented by Albertus Magnus in 1250.Arsenic is a metalloid...
, antimony
Antimony
Antimony is a toxic chemical element with the symbol Sb and an atomic number of 51. A lustrous grey metalloid, it is found in nature mainly as the sulfide mineral stibnite...
mine
Mining
Mining is the extraction of valuable minerals or other geological materials from the earth, from an ore body, vein or seam. The term also includes the removal of soil. Materials recovered by mining include base metals, precious metals, iron, uranium, coal, diamonds, limestone, oil shale, rock...
, Pampa Larga, Chile
Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...
. It is generally more rare than realgar
Realgar
Realgar, α-As4S4, is an arsenic sulfide mineral, also known as "ruby sulphur" or "ruby of arsenic". It is a soft, sectile mineral occurring in monoclinic crystals, or in granular, compact, or powdery form, often in association with the related mineral, orpiment . It is orange-red in colour, melts...
and orpiment
Orpiment
Orpiment, As2S3, is a common monoclinic arsenic sulfide mineral. It has a Mohs hardness of 1.5 to 2 and a specific gravity of 3.46. It melts at 300 °C to 325 °C...
. Its origin is hydrothermal. It occurs as subhedral to 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...
tabular orange to pale gray crystals that are transparent
Transparency and translucency
In the field of optics, transparency is the physical property of allowing light to pass through a material; translucency only allows light to pass through diffusely. The opposite property is opacity...
to translucent. It has a yellow-orange streak with a hardness of 1.5. It crystallizes in the monoclinic 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 occurs with realgar and uzonite as flattened and prismatic
Prism (geometry)
In geometry, a prism is a polyhedron with an n-sided polygonal base, a translated copy , and n other faces joining corresponding sides of the two bases. All cross-sections parallel to the base faces are the same. Prisms are named for their base, so a prism with a pentagonal base is called a...
grains up to 0.5 mm across.
Composition
When alacranite was first discovered in 1970 by Clark, it was associated with bariteBarite
Baryte, or barite, is a mineral consisting of barium sulfate. The baryte group consists of baryte, celestine, anglesite and anhydrite. Baryte itself is generally white or colorless, and is the main source of barium...
and calcite
Calcite
Calcite is a carbonate mineral and the most stable polymorph of calcium carbonate . The other polymorphs are the minerals aragonite and vaterite. Aragonite will change to calcite at 380-470°C, and vaterite is even less stable.-Properties:...
veins in the Alacran silver mine. They also assumed that the mineral was identical to the species occurring in the Ag-As-Sb vein deposit at Alacran due to the similarity of its X-ray diffraction powder pattern. They discovered that alacranite was similar to the high-temperature α polymorph
Polymorphism (materials science)
Polymorphism in materials science is the ability of a solid material to exist in more than one form or crystal structure. Polymorphism can potentially be found in any crystalline material including polymers, minerals, and metals, and is related to allotropy, which refers to chemical elements...
of As4S4 in X-ray characteristics. In addition, alacranite was considered as realgar-like mineral. After that, they reported the composition of alacranite as As8S9 when they noticed another occurrence of Alacranite in Uzon Caldera associated with realgar
Realgar
Realgar, α-As4S4, is an arsenic sulfide mineral, also known as "ruby sulphur" or "ruby of arsenic". It is a soft, sectile mineral occurring in monoclinic crystals, or in granular, compact, or powdery form, often in association with the related mineral, orpiment . It is orange-red in colour, melts...
and uzonite as cement in sandy gravels. They reported the composition of alacranite to be As8S9 regarding to the electron-microprobe analyses. When they analyzed a mineral during seafloor sampling consisting of red and orange arsenic sulfides by X-ray diffraction, a mixture of realgar and alacranite was resulted and it was noticed that the mineral was identical to synthetic β-As4S4 supposed that the original chemical formula of alacranite (As8S9) was incorrect. They argued that it is identical to alacranite because of the similarity of the physical properties and the unit-cell dimensions but further studies argue that it is different with respect to chemical formula and unit-cell volume. The chemistry of the average of four analyses in Uzon caldera, Russia corresponding to alacranite results in 67.35% arsenic and 32.61% sulfur resulting in a total of 99.96%, yielding the formula As7.98S9.02, ideally As8S9.
Structure
The structure of alacranite remained unsolved until further studies collected specimens containing crystals of alacranite. A group that studies seafloor hydrothermal, submarineSubmarine
A submarine is a watercraft capable of independent operation below the surface of the water. It differs from a submersible, which has more limited underwater capability...
volcanism and regional tectonics in Papua New Guinea collected samples that consists of clay minerals
Clay minerals
Clay minerals are hydrous aluminium phyllosilicates, sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations. Clays have structures similar to the micas and therefore form flat hexagonal sheets. Clay minerals are common weathering products and low...
, pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
, sphalerite
Sphalerite
Sphalerite is a mineral that is the chief ore of zinc. It consists largely of zinc sulfide in crystalline form but almost always contains variable iron. When iron content is high it is an opaque black variety, marmatite. It is usually found in association with galena, pyrite, and other sulfides...
, galena
Galena
Galena is the natural mineral form of lead sulfide. It is the most important lead ore mineral.Galena is one of the most abundant and widely distributed sulfide minerals. It crystallizes in the cubic crystal system often showing octahedral forms...
, chalcopyrite
Chalcopyrite
Chalcopyrite is a copper iron sulfide mineral that crystallizes in the tetragonal system. It has the chemical composition CuFeS2. It has a brassy to golden yellow color and a hardness of 3.5 to 4 on the Mohs scale. Its streak is diagnostic as green tinged black.On exposure to air, chalcopyrite...
, sulfosalts and arsenic-bearing sulfides like realgar and
alacranite. Data were gathered at different times using monochromatic X-radiation when a crystal of alacranite with approximate dimensions 0.14 × 0.10 × 0.06 mm was placed in a platform 3-circle goniometer equipped with a 1K charge-coupled device for 2θ up to 56.7°. The data shows reflection statistics and systematic absences that indicates space group C2/c referring to the monoclinic symmetry for alacranite. Final results of the experiment show that alacranite is isostructural with the compound α-As4S4 and it is the third mineral polymorph with the formula As4S4, the others are realgar and pararealgar.
The three minerals consist of covalently bonded As4S4 molecules. In alacranite, each arsenic atom is bonded to one arsenic atom and two sulfur atoms, while the sulfur atoms bonded only to two arsenic atoms.
The structure of molecules in alacranite is chemically the same as in realgar held together by van der Waals forces but different in the arrangement of the As4S4 in both structures. The differences between alacranite and realgar are demonstrated in the unit cell sizes and the packed structures of both minerals. Alacranite has a C-centered unit cell that is smaller than the primitive cell
Primitive cell
Used predominantly in geometry, solid state physics, and mineralogy, particularly in describing crystal structure, a primitive cell is a minimum cell corresponding to a single lattice point of a structure with translational symmetry in 2 dimensions, 3 dimensions, or other dimensions...
of realgar and also the structure of alacranite has a closely and more orderly packed structure than realgar.
Further studies of a single crystal of alacranite with dimensions (30 × 60 × 120 mm was examined using direct methods, Fourier syntheses and structure refinement resulting in intensities violating the C lattice type and symmetries like h0l reflections with l = 2n + 1 were absent. These results confirm the P2/c space group of alacranite resulting in a structure of two different kinds of cage-like molecules that are packing closely and found in the β-phase.
The first molecule is identical to the As4S4 which is realgar where every As atom links one As and two S atoms and that is determined in the structure of the β-phase and realgar. The second molecule in the structure of alacranite is found to be chemically and structurally identical to As4S5 which is uzonite. Upon exposure to light, As4S4 molecule expands its unit cell volume and hence, transfers to As4S5. This transformation could be described to the variation of the intermolecular distances. These coherent orders of both molecules along [110] attributes to the change of the translation symmetry from C (β-phase) to P which is alacranite. In both molecules, the distances of the As-S bond are about 2.205 to 2.238 angstrom. However, the As-As bond distance in the As4S4 is longer than the As-As bond contained in As4S5 molecule within alacranite structure. It was also shown that the unit-cell volume increases proportionally with increasing S content of minerals with ranging composition from As4S4 to As8S9 where alacranite has the biggest unit-cell in this range. Evidence supports that alacranite has a P2/c space group
Space group
In mathematics and geometry, a space group is a symmetry group, usually for three dimensions, that divides space into discrete repeatable domains.In three dimensions, there are 219 unique types, or counted as 230 if chiral copies are considered distinct...
whereas the high-temperature and less content of S form is considered to be a new mineral that corresponds to the species from alacran previously described by which is As4S4 that crystallizes with the space group C2/c and has a smaller volume.
Physical properties
Alacranite occurs as cement in sandy gravel and in hydrothermal As-S veins. It occurs with a grain size up to 0.5 mm as flattened and prismaticPrism (geometry)
In geometry, a prism is a polyhedron with an n-sided polygonal base, a translated copy , and n other faces joining corresponding sides of the two bases. All cross-sections parallel to the base faces are the same. Prisms are named for their base, so a prism with a pentagonal base is called a...
. Its crystal faces occur as pinacoidal, prismatic
Prism (geometry)
In geometry, a prism is a polyhedron with an n-sided polygonal base, a translated copy , and n other faces joining corresponding sides of the two bases. All cross-sections parallel to the base faces are the same. Prisms are named for their base, so a prism with a pentagonal base is called a...
and flattened on [100]. Its major forms are {100}, {111}, and {111}. Other forms are weak, dull or tarnished. It shows an orange to pale gray with rose-yellow internal reflections (shown in figure 1) with a yellow-orange streak
Streak (mineralogy)
The streak of a mineral is the color of the powder produced when it is dragged across an unweathered surface. Unlike the apparent color of a mineral, which for most minerals can vary considerably, the trail of finely ground powder generally has a more consistent characteristic color, and is thus...
. It has adamantine, vitreous, resinous and greasy lustre
Lustre (mineralogy)
Lustre is a description of the way light interacts with the surface of a crystal, rock, or mineral. The word lustre traces its origins back to the Latin word lux, meaning "light", and generally implies radiance, gloss, or brilliance....
and it is transparent. The weak chemical bonding in its structure, gives the mineral a low hardness around 1.5 and weak forms. It has an imperfect cleavage on {100} and its fracture is conchoidal and very brittle. Its density
Density
The mass density or density of a material is defined as its mass per unit volume. The symbol most often used for density is ρ . In some cases , density is also defined as its weight per unit volume; although, this quantity is more properly called specific weight...
is measured to be around 3.43. When it reacts with 5M KOH, alacranite changes color to brown, it changes into brown-gray flakes when it is heated and boiling it should return
its color to brown. However, if it is mixed with HCl or HNO3, it does not show any activity.
Geologic occurrence
Alacranite was first found in the Uzon caldera, RussiaRussia
Russia or , officially known as both Russia and the Russian Federation , is a country in northern Eurasia. It is a federal semi-presidential republic, comprising 83 federal subjects...
. The Uzon caldera is located near the eastern volcanic belt of the Kamchatka peninsula
Kamchatka Peninsula
The Kamchatka Peninsula is a peninsula in the Russian Far East, with an area of . It lies between the Pacific Ocean to the east and the Sea of Okhotsk to the west...
. The area is a basalt
Basalt
Basalt is a common extrusive volcanic rock. It is usually grey to black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey...
ic shield volcano
Shield volcano
A shield volcano is a type of volcano usually built almost entirely of fluid lava flows. They are named for their large size and low profile, resembling a warrior's shield. This is caused by the highly fluid lava they erupt, which travels farther than lava erupted from more explosive volcanoes...
with lacustrine
Lake
A lake is a body of relatively still fresh or salt water of considerable size, localized in a basin, that is surrounded by land. Lakes are inland and not part of the ocean and therefore are distinct from lagoons, and are larger and deeper than ponds. Lakes can be contrasted with rivers or streams,...
sediments, faulting and extension, dome formation and hydrothermal fluids from hot springs within the caldera
Caldera
A caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...
. Amounts of realgar, stibnite
Stibnite
Stibnite, sometimes called antimonite, is a sulfide mineral with the formula Sb2S3. This soft grey material crystallizes in an orthorhombic space group. It is the most important source for the metalloid antimony...
, cinnabar
Cinnabar
Cinnabar or cinnabarite , is the common ore of mercury.-Word origin:The name comes from κινναβαρι , a Greek word most likely applied by Theophrastus to several distinct substances...
and pyrite
Pyrite
The mineral pyrite, or iron pyrite, is an iron sulfide with the formula FeS2. This mineral's metallic luster and pale-to-normal, brass-yellow hue have earned it the nickname fool's gold because of its resemblance to gold...
are contained in sediments near active hot springs.
Alacranite occurs in the condensation zone of a hydrothermal Hg-Sb-As system in the Uzon caldera. Alacranite could also be found in hydrothermal As-S viens.
It was named alacranite after its occurrence in the Alacran mine in Chile
Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...
due to its similarities in XRD patterns of the samples from the Uzon caldera to those in the Alacan mine.