Aguilarite is an uncommon

sulfosalt mineral 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 ...

with formula Ag₄ Se S. It was described in 1891 and named for discoverer Ponciano Aguilar.

Description

Aguilarite is bright lead-gray on fresh surfaces but becomes dull iron black when exposed to air. The mineral occurs with massive

habit A habit (or wont, as a humorous and formal term) is a routine of behavior that is repeated regularly and tends to occur subconsciously. A 1903 paper in the '' American Journal of Psychology'' defined a "habit, from the standpoint of psychology, ...

, as elongated pseudododecahedral crystals up to , or as intergrowths with acanthite or naumannite.

Discovery

In the late 19th century, Ponciano Aguilar, superintendent of the San Carlos mine in Guanajuato, Mexico, found several specimens of a mineral thought to be

naumannite Silver selenide (Ag2Se) is the reaction product formed when selenium toning analog silver gelatine photo papers in photographic print toning. The selenium toner contains sodium selenite (Na2SeO3) as one of its active ingredients, which is the sourc ...

. The samples were given to F. A. Genth for identification, who, along with S. L. Penfield, discovered that it was a new mineral. The mineral was described in the ''

American Journal of Science The ''American Journal of Science'' (''AJS'') is the United States of America's longest-running scientific journal, having been published continuously since its conception in 1818 by Professor Benjamin Silliman, who edited and financed it himself ...

'' in 1891 and named ''aguilarite'' in honor of Ponciano Aguilar. When the

International Mineralogical Association Founded in 1958, the International Mineralogical Association (IMA) is an international group of 40 national societies. The goal is to promote the science of mineralogy and to standardize the nomenclature of the 5000 plus known mineral species. ...

was founded, aguilarite was grandfathered as a valid mineral species.

Occurrence

Aguilarite is uncommon, and forms at relatively low temperatures in hydrothermal deposits rich in silver and selenium but deficient in sulfur. The mineral is known from a number of countries in North and South America, Europe, Asia, and Australasia. Aguilarite occurs in association with

acanthite Acanthite is a form of silver sulfide with the chemical formula Ag2S. It crystallizes in the monoclinic system and is the stable form of silver sulfide below . Argentite is the stable form above that temperature. As argentite cools below that temp ...

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 ...

, naumannite, pearceite,

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

silver Silver is a chemical element; it has Symbol (chemistry), symbol Ag () and atomic number 47. A soft, whitish-gray, lustrous transition metal, it exhibits the highest electrical conductivity, thermal conductivity, and reflectivity of any metal. ...

stephanite Stephanite is a silver antimony sulfosalt mineral with formula: Ag5 Sb S4. It is composed of 68.8% silver, and sometimes is of importance as an ore of this metal. History Under the name Schwarzerz it was mentioned by Georgius Agricola in 1546, a ...

, and

quartz Quartz is a hard, crystalline mineral composed of silica (silicon dioxide). The Atom, atoms are linked in a continuous framework of SiO4 silicon–oxygen Tetrahedral molecular geometry, tetrahedra, with each oxygen being shared between two tet ...

Chemistry and structure

In 2013, aguilarite's chemistry and crystal structure were reexamined by Bindi and Pingitore. The significant reevaluation of aguilarite did not discredit its status as a valid mineral, but it was established as the selenium analogue of acanthite instead of sulfur-rich naumannite. The sample primarily studied came from the Gem and Mineral Collection of the Department of Geosciences at

Princeton University Princeton University is a private university, private Ivy League research university in Princeton, New Jersey, United States. Founded in 1746 in Elizabeth, New Jersey, Elizabeth as the College of New Jersey, Princeton is the List of Colonial ...

. The work of Petruk ''et al''. in 1974 formed the basis of knowledge regarding the silver–sulfur–selenium system for about forty years. They indexed their

x-ray diffraction X-ray diffraction is a generic term for phenomena associated with changes in the direction of X-ray beams due to interactions with the electrons around atoms. It occurs due to elastic scattering, when there is no change in the energy of the waves. ...

patterns of aguilarite on an

orthorhombic In crystallography, the orthorhombic crystal system is one of the 7 crystal systems. Orthorhombic Lattice (group), lattices result from stretching a cubic crystal system, cubic lattice along two of its orthogonal pairs by two different factors, res ...

cell similar to naumannite. Bindi and Pingitore determined that aguilarite is, in fact,

monoclinic In crystallography, the monoclinic crystal system is one of the seven crystal systems. A crystal system is described by three Vector (geometric), vectors. In the monoclinic system, the crystal is described by vectors of unequal lengths, as in t ...

and is isostructural to acanthite and not naumannite. Bindi and Pingitore believe that Petruk ''et al''. were unable to resolve closely spaced peaks due to low resolution equipment, making aguilarite appear similar to naumannite. Additionally, a number of inconsistencies in

unit 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 In mathematics, a unit vector i ...

dimensions in the 1974 work show that aguilarite does not have the same structure as naumannite. The

crystal structure In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat ...

of aguilarite consists of planes nearly parallel to (010) composed of tetrahedrally coordinated nonmetal atoms and AgX₃ triangles (where X is a nonmetal). The planes are joined by twofold-coordinated silver atoms. Aguilarite is part of the acanthite-like

solid solution A solid solution, a term popularly used for metals, is a homogeneous mixture of two compounds in solid state and having a single crystal structure. Many examples can be found in metallurgy, geology, and solid-state chemistry. The word "solutio ...

series Ag₂S–Ag₂S_0.4Se_0.6. The mineral comprises the range from 50

atomic percent The atomic ratio is a measure of the ratio of atoms of one kind (i) to another kind (j). A closely related concept is the atomic percent (or at.%), which gives the percentage of one kind of atom relative to the total number of atoms. The molecula ...

selenium up to the transition from monoclinic to orthorhombic.

References

;Bibliography * *

External links

{{commons category-inline Monoclinic minerals Sulfosalt minerals Silver minerals Sulfide minerals Selenide minerals Minerals in space group 14