Alacránite
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Alacránite (As8S9) is an arsenic sulfide mineral first discovered in the
Uzon Uzon (russian: Узон) 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 ...
caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
, Kamchatka, Russia. It was named for its occurrence in the Alacrán silver/ arsenic/ antimony
mine Mine, mines, miners or mining may refer to: Extraction or digging * Miner, a person engaged in mining or digging *Mining, extraction of mineral resources from the ground through a mine Grammar *Mine, a first-person English possessive pronoun ...
. Pampa Larga, Chile. It is generally more rare than realgar and orpiment. Its origin is hydrothermal. It occurs as
subhedral Euhedral crystals (also known as idiomorphic or automorphic crystals) are those that are well-formed, with sharp, easily recognised faces. The opposite is anhedral (also known as ''xenomorphic'' or ''allotriomorphic''): a rock with an anhedral ...
to
euhedral Euhedral crystals (also known as idiomorphic or automorphic crystals) are those that are well-formed, with sharp, easily recognised faces. The opposite is anhedral (also known as ''xenomorphic'' or ''allotriomorphic''): a rock with an anhedral ...
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 An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are ''not'' prisms. The most familiar type of optical ...
grains up to 0.5 mm across.


Composition

When alacranite was first discovered in 1970 by Clark, it was associated with
barite Baryte, barite or barytes ( or ) is a mineral consisting of barium sulfate ( Ba S O4). Baryte is generally white or colorless, and is the main source of the element barium. The ''baryte group'' consists of baryte, celestine (strontium sulfate), ...
and
calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...
veins in the Alacran silver mine.
Burns, P. and Pervival, J. (2001) Alacranite, As4S4: A new occurrence, new formula and determination of the crystal structure. Canadian Mineralogist, 39, 809-818.
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 X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
powder pattern. They discovered that alacranite was similar to the high-temperature α
polymorph Polymorphism, polymorphic, polymorph, polymorphous, or polymorphy may refer to: Computing * Polymorphism (computer science), the ability in programming to present the same programming interface for differing underlying forms * Ad hoc polymorphi ...
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 Uzon (russian: Узон) 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 ...
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.Bonazzi P., Bindi L., Popova V., Pratesi G. and Menchetti S. 2003: Alacranite, As8S9: structural study of the holotype and re-assignment of the original chemical formula. American Mineralogist, 88(11-12), pp. 1796-1800/> 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


X-ray crystallography

The structure of alacranite remained unsolved until further studies collected specimens containing crystals of alacranite. A group that studies seafloor hydrothermal,
submarine A submarine (or sub) is a watercraft capable of independent operation underwater. It differs from a submersible, which has more limited underwater capability. The term is also sometimes used historically or colloquially to refer to remotely op ...
volcanism and regional tectonics in Papua New Guinea collected samples that consists of
clay minerals Clay minerals are hydrous aluminium phyllosilicates (e.g. kaolin, Al2 Si2 O5( OH)4), sometimes with variable amounts of iron, magnesium, alkali metals, alkaline earths, and other cations found on or near some planetary surfaces. Clay mineral ...
, pyrite,
sphalerite Sphalerite (sometimes spelled sphaelerite) is a sulfide mineral with the chemical formula . It is the most important ore of zinc. Sphalerite is found in a variety of deposit types, but it is primarily in Sedimentary exhalative deposits, sedimen ...
,
galena Galena, also called lead glance, is the natural mineral form of lead(II) sulfide (PbS). It is the most important ore of lead and an important source of silver. Galena is one of the most abundant and widely distributed sulfide minerals. It cryst ...
, 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 Pararealgar is an arsenic sulfide mineral with the chemical formula , also represented as AsS. It forms gradually from realgar under exposure to light. Its name derives from the fact that its elemental composition is identical to realgar, . It is ...
. 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 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 ...
of realgar and also the structure of alacranite has a closely and more orderly packed structure than realgar.


Fourier analysis

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 inter molecular distances.
Bonnazzi, P. (2006) Light-induced changes in molecular arsenic sulfides: State of the art and new evidence by single-crystal X-ray diffraction. American Mineralogist, 91,1323.
These coherent orders of both molecules along 10attributes 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.
Bonnazzi, P. and Bindi, L. (2008) A crystallographic review of arsenic sulfides: effects of chemical variations and changes induced by exposure to light. Mineralogical Crystallography. 223, 132-147.
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.


Physical properties

Alacranite occurs as cement in sandy gravel and in hydrothermal As-S
veins Veins are blood vessels in humans and most other animals that carry blood towards the heart. Most veins carry deoxygenated blood from the tissues back to the heart; exceptions are the pulmonary and umbilical veins, both of which carry oxygenated b ...
. It occurs with a grain size up to 0.5 mm as flattened and
prismatic An optical prism is a transparent optical element with flat, polished surfaces that are designed to refract light. At least one surface must be angled — elements with two parallel surfaces are ''not'' prisms. The most familiar type of optical ...
crystals. Some forms are weak, dull or tarnished. It appears an orange to pale gray crystals with rose-yellow internal reflections with a yellow-orange streak. It has adamantine, vitreous, resinous and greasy luster and it is transparent. The weak chemical bonding in its structure gives the mineral a low Mohs hardness of around 1.5 and weak forms. It has an imperfect cleavage and its fracture is conchoidal and very brittle. Its specific gravity is measured to be around 3.43. When it reacts with 5 molar potassium hydroxide, 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 hydrochloric acid or nitric acid, it does not show any activity.
Hawthorne, F., Burke, E., Ercit, T., Grew, E., Grice, J., Jambor, J., Puziewicz, J., Roberts, A., and Vanko, D. (1988) New Mineral Names. American Mineralogist. 73, 189.


Geologic occurrence

Alacranite was first found in the
Uzon Caldera Uzon (russian: Узон) 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 ...
, Russia. The Uzon caldera is located near the eastern volcanic belt of the Kamchatka peninsula. The area is a basaltic
shield volcano A shield volcano is a type of volcano named for its low profile, resembling a warrior's shield lying on the ground. It is formed by the eruption of highly fluid (low viscosity) lava, which travels farther and forms thinner flows than the more v ...
with lacustrine sediments, faulting and extension, dome formation and hydrothermal fluids from hot springs within the
caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
. 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 veins. It was named alacranite after its occurrence in the Alacran mine in Chile due to its similarities in
X-ray diffraction X-ray crystallography is the experimental science determining the atomic and molecular structure of a crystal, in which the crystalline structure causes a beam of incident X-rays to diffract into many specific directions. By measuring the angles ...
patterns of the samples from the Uzon caldera to those in the Alacan mine.


References


Handbook of Mineralogy
and references therein (original description by Popova et al. 1986)

* Bonazzi P., Bindi L., Popova V., Pratesi G. and Menchetti S. 2003: Alacranite, As8S9: structural study of the holotype and reassignment of the original chemical formula. American Mineralogist, 88(11-12), pp. 1796–1800

* Athens, G., Gennadii A. Geologic setting of the uzon caldera, Kamchatka, far east Russia.

*Jambor, J. and Roberts, A (2004) New Minerals Names. American Mineralogist. 89, 249-253. *Sorrel, C. and Sandstrom, G. (1973) Rocks and Minerals: A Guide to Field Identification. Golden Field Guides. 82-111. {{DEFAULTSORT:Alacranite Arsenic minerals Sulfide minerals Monoclinic minerals Minerals in space group 13