Fangite is a

sulfosalt Sulfosalt minerals are sulfide minerals with the general formula , where *A represents a metal such as copper, lead, silver, iron, and rarely mercury, zinc, vanadium *B usually represents semi-metal such as arsenic, antimony, bismuth, and rarely ...

first discovered in the Mercur

gold Gold is a chemical element with the symbol Au (from la, aurum) and atomic number 79. This makes it one of the higher atomic number elements that occur naturally. It is a bright, slightly orange-yellow, dense, soft, malleable, and ductile met ...

deposit located in Tooele County Utah. The specimen was found in a

boulder In geology, a boulder (or rarely bowlder) is a rock fragment with size greater than in diameter. Smaller pieces are called cobbles and pebbles. While a boulder may be small enough to move or roll manually, others are extremely massive. In c ...

in the southern

Oquirrh Mountains The Oquirrh Mountains is a mountain range that runs north-south for approximately 30 miles (50 km) to form the west side of Utah's Salt Lake Valley, separating it from Tooele Valley. The range runs from northwestern Utah County–centr ...

. The only available specimens of fangite are located in the

National Museum of Natural History The National Museum of Natural History is a natural history museum administered by the Smithsonian Institution, located on the National Mall in Washington, D.C., United States. It has free admission and is open 364 days a year. In 2021, with 7 ...

, Smithsonian Institution. The International Mineralogical Association Commission approved the name Fangite after Dr. Jen-Ho Fang, a crystal chemist affiliated with the

University of Alabama The University of Alabama (informally known as Alabama, UA, or Bama) is a Public university, public research university in Tuscaloosa, Alabama. Established in 1820 and opened to students in 1831, the University of Alabama is the oldest and la ...

, in honor of his significant contributions to

crystallography Crystallography is the experimental science of determining the arrangement of atoms in crystalline solids. Crystallography is a fundamental subject in the fields of materials science and solid-state physics (condensed matter physics). The wor ...

crystal chemistry Crystal chemistry is the study of the principles of chemistry behind crystals and their use in describing structure-property relations in solids. The principles that govern the assembly of crystal and glass structures are described, models of many ...

, and

geostatistics Geostatistics is a branch of statistics focusing on spatial or spatiotemporal datasets. Developed originally to predict probability distributions of ore grades for mining operations, it is currently applied in diverse disciplines including petro ...

Occurrence

Fangite was detected in an isolated

vug A vug, vugh, or vugg ( ) is a small- to medium-sized cavity inside rock. It may be formed through a variety of processes. Most commonly, cracks and fissures opened by tectonic activity (folding and faulting) are partially filled by quartz, ...

sulfide ore Sulfide (British English also sulphide) 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 chemical compounds la ...

stockpile within the Morrin Hill pit at the Mercur gold deposit. Fangite is associated with subhedral

pyrite The mineral pyrite (), or iron pyrite, also known as fool's gold, is an iron sulfide with the chemical formula Iron, FeSulfur, S2 (iron (II) disulfide). Pyrite is the most abundant sulfide mineral. Pyrite's metallic Luster (mineralogy), lust ...

and fine-grained sulfide indicating partial replacement of 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 ...

, and other sulfide minerals. Based on surrounding materials, it can be concluded that Fangite forms as a result of alteration of thallium sulfates or by substituting fluids that are Ti-rich for

realgar Realgar ( ), also known as "ruby sulphur" or "ruby of arsenic", is an arsenic sulfide mineral with the chemical formula α-. It is a soft, sectile mineral occurring in monoclinic crystals, or in granular, compact, or powdery form, often in asso ...

orpiment Orpiment is a deep-colored, orange-yellow arsenic sulfide mineral with formula . It is found in volcanic fumaroles, low-temperature hydrothermal veins, and hot springs and is formed both by sublimation and as a byproduct of the decay of another a ...

Physical properties

Fangite is a deep red to maroon, transparent mineral with a vitreous luster and a metallic luster when tarnished. The original sample of fangite was too small to determine hardness. Synthetic Tl₃ As S₄ exhibits a hardness of 2-2.5 on the

Mohs hardness scale The Mohs scale of mineral hardness () is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. The scale was introduced in 1812 by th ...

and a density of 6.185 g/cm³. Synthetic Tl₃ As S₄ was also used for assessing streak, revealing an orange color. Fangite exhibits a blue-gray luster with low

birefringence Birefringence is the optical property of a material having a refractive index that depends on the polarization and propagation direction of light. These optically anisotropic materials are said to be birefringent (or birefractive). The birefring ...

and red internal reflections when polished. Due to no well-formed

crystals A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macrosc ...

in the original specimen collected, cleavage could not be determined. However, synthetic fangite exhibits

conchoidal fracture Conchoidal fracture describes the way that brittle materials break or fracture when they do not follow any natural planes of separation. Mindat.org defines conchoidal fracture as follows: "a fracture with smooth, curved surfaces, typically slig ...

Optical properties

Fangite has a vitreous luster that turns metallic when tarnished. When examining fangite in a polished section, only one grain could be used to test

optical properties The optical properties of a material define how it interacts with light. The optical properties of matter are studied in optical physics, a subfield of optics. The optical properties of matter include: *Refractive index * Dispersion *Transmittance a ...

due to the abundant presence of internal red reflections. Due to the angles of

fracture Fracture is the 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 surfaces within the solid. If a displa ...

in synthetic material, grains are present that reflect away unwanted components. It is concluded that fangite has a very low birefringence with a difference of Y% of 0.4. The consistent decrease in

reflectance The reflectance of the surface of a material is its effectiveness in reflecting radiant energy. It is the fraction of incident electromagnetic power that is reflected at the boundary. Reflectance is a component of the response of the electronic ...

from the blue to red end of the

visible spectrum The visible spectrum is the portion of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' or simply light. A typical human eye wil ...

aligns with the blue-gray appearance of fangite in polished sections. This appearance corresponds to its red and

translucent In the field of optics, transparency (also called pellucidity or diaphaneity) is the physical property of allowing light to pass through the material without appreciable scattering of light. On a macroscopic scale (one in which the dimensions a ...

characteristics in hand specimen or

thin section In optical mineralogy and petrography, a thin section (or petrographic thin section) is a thin slice of a rock or mineral sample, prepared in a laboratory, for use with a polarizing petrographic microscope, electron microscope and electron mic ...

. Comparing these reflectance spectra with those in the Quantitative Data File for ore minerals reveals similarities between R1 and R2 of fangite and the R2 values of

proustite Proustite is a sulfosalt mineral consisting of silver sulfarsenide, Ag3 As S3, known also as light red silver or ruby silver ore, and an important source of the metal. It is closely allied to the corresponding sulfantimonide, pyrargyrite, from whi ...

(Ag₃AsS₃). However, proustite is distinctly bireflectant and appears more purplish-red due to its somewhat higher reflectance at the blue end of the spectrum.

Structure

The ideal chemical formula for fangite is Tl₃AsS₄. When examining the crystal structure of fangite along the a-axis, it is apparent that fangite exhibits a polyhedral arrangement. Arsenic (As) atoms in this orientation display perfect

tetrahedral In geometry, a tetrahedron (plural: tetrahedra or tetrahedrons), also known as a triangular pyramid, is a polyhedron composed of four triangular faces, six straight edges, and four vertex corners. The tetrahedron is the simplest of all the o ...

coordination with Sulfur (S) featuring an average bond length of 2.172 Å. A distorted

trigonal In crystallography, the hexagonal crystal family is one of the six crystal families, which includes two crystal systems (hexagonal and trigonal) and two lattice systems (hexagonal and rhombohedral). While commonly confused, the trigonal crysta ...

dipyramid with an average bond length of 3.17 Å is formed with the Thallium (Tl) atom, coordinated in a fivefold manner with S. The Thallium (Tl) atom is also coordinated sevenfold with Sulfur (S), creating a monocapped

octahedron In geometry, an octahedron (plural: octahedra, octahedrons) is a polyhedron with eight faces. The term is most commonly used to refer to the regular octahedron, a Platonic solid composed of eight equilateral triangles, four of which meet at ea ...

with an average bond length of 3.288 Å. Chains, comprising alternating Tl₂ dipyramids and As tetrahedra, are linked through corner-sharing at S1 and S2. These chains align parallel to the c axis, and the structure repeats with each alternating chain undergoing a 180° rotation and shifting up and down the A axis. Fangite has an ideal chemical formula of Tl₃AsS₄. When examining an isolated chain from the b-axis, it is apparent that the previously described chain is a Tl₂-As polyhedral layer. This layer consists of Tl₂ trigonal dipyramids arranged in zigzag chains along the a-axis, connected in the c-direction by As tetrahedra. These tetrahedra share an edge and two vertices with the dipyramids, creating nearly planar Tl₂-As polyhedral layers aligned with the (010) plane. The linkage between these layers is in the b-direction by interlayer Tl atoms.

Chemical composition

X-ray crystallography

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 ...

was collected from synthetic fangite using a Gondolfi pattern, using an automated powder diffractometer. Synthetic material was supplied by M. Gottlieb, associated with the Westinghouse Research Laboratories in

Pittsburgh, Pennsylvania Pittsburgh ( ) is a city in the Commonwealth (U.S. state), Commonwealth of Pennsylvania, United States, and the county seat of Allegheny County, Pennsylvania, Allegheny County. It is the most populous city in both Allegheny County and Wester ...

. Through analysis, it is concluded that fangite is a part of the orthorhombic

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. Space groups are classified into crystal systems according to their po ...

with a

space group In mathematics, physics and chemistry, a space group is the symmetry group of an object in space, usually in three dimensions. The elements of a space group (its symmetry operations) are the rigid transformations of an object that leave it unchan ...

of Pnma. Unit cell, dimensions were also analyzed, revealing: a=8.894(8) Å, b=10.855(9) Å, c=9.079(9) Å, z=4.

References

{{Authority control Wikipedia Student Program Arsenic minerals Thallium minerals Orthorhombic minerals