Phenakite or phenacite is a fairly rare

nesosilicate Silicate minerals are rock-forming minerals made up of silicate groups. They are the largest and most important class of minerals and make up approximately 90 percent of Earth's crust. In mineralogy, silica (silicon dioxide, ) is usually consid ...

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. (2 ...

consisting of

beryllium Beryllium is a chemical element with the symbol Be and atomic number 4. It is a steel-gray, strong, lightweight and brittle alkaline earth metal. It is a divalent element that occurs naturally only in combination with other elements to for ...

orthosilicate In chemistry, orthosilicate is the anion , or any of its salts and esters. It is one of the silicate anions. It is occasionally called the silicon tetroxide anion or group.C. A. Kumins, and A. E. Gessler (1953), "Short-Cycle Syntheses of Ultra ...

, Be₂ Si O₄. Occasionally used as a

gemstone A gemstone (also called a fine gem, jewel, precious stone, or semiprecious stone) is a piece of mineral crystal which, in cut and polished form, is used to make jewelry or other adornments. However, certain rocks (such as lapis lazuli, opal, ...

, phenakite occurs as isolated

crystal 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, macro ...

s, which are

rhombohedral In geometry, a rhombohedron (also called a rhombic hexahedron or, inaccurately, a rhomboid) is a three-dimensional figure with six faces which are rhombi. It is a special case of a parallelepiped where all edges are the same length. It can be us ...

with parallel-faced hemihedrism, and are either lenticular or prismatic in habit: the lenticular habit is determined by the development of faces of several obtuse rhombohedra and the absence of prism faces. There is no cleavage, and the fracture is

conchoidal 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 ...

. The

Mohs hardness 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 ...

is high, being 7.5 – 8; the

specific gravity Relative density, or specific gravity, is the ratio of the density (mass of a unit volume) of a substance to the density of a given reference material. Specific gravity for liquids is nearly always measured with respect to water at its dens ...

is 2.96. The crystals are sometimes perfectly colorless and transparent, but more often they are greyish or yellowish and only translucent; occasionally they are pale rose-red. In general appearance the mineral is not unlike

quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical f ...

, for which indeed it has been mistaken. Its name comes from grc, φέναξ, phénax, meaning "deceiver" due to its close visual similarity to quartz, named by

Nils Gustaf Nordenskiöld Nils Gustaf Nordenskiöld (October 12, 1792 – February 2, 1866) was a Finnish mineralogist and traveller. He was the father of mineralogist and polar explorer Adolf Erik Nordenskiöld. Life Nordenskiöld was born on October 12, 1792, in M� ...

in 1833.

Largest phenakite

A large phenakite gemstone has been found in

Sri Lanka Sri Lanka (, ; si, ශ්‍රී ලංකා, Śrī Laṅkā, translit-std=ISO (); ta, இலங்கை, Ilaṅkai, translit-std=ISO ()), formerly known as Ceylon and officially the Democratic Socialist Republic of Sri Lanka, is an ...

. Found on November 18, 2021, this gemstone weighs 616.9 carats which makes it the largest of its kind. It is owned by a gem dealer from Beruwala, Sri Lanka, and reported to be worth around one billion in Sri Lankan rupees.

Occurrence

Phenakite is found in high-temperature pegmatite veins and in

mica Micas ( ) are a group of silicate minerals whose outstanding physical characteristic is that individual mica crystals can easily be split into extremely thin elastic plates. This characteristic is described as perfect basal cleavage. Mica is ...

schist Schist ( ) is a medium-grained metamorphic rock showing pronounced schistosity. This means that the rock is composed of mineral grains easily seen with a low-power hand lens, oriented in such a way that the rock is easily split into thin flakes ...

s associated with quartz,

chrysoberyl The mineral or gemstone chrysoberyl is an aluminate of beryllium with the formula Be Al2 O4. The name chrysoberyl is derived from the Greek words χρυσός ''chrysos'' and βήρυλλος ''beryllos'', meaning "a gold-white spar". Despite ...

apatite Apatite is a group of phosphate minerals, usually hydroxyapatite, fluorapatite and chlorapatite, with high concentrations of OH−, F− and Cl− ions, respectively, in the crystal. The formula of the admixture of the three most common ...

and

topaz Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al Si O( F, OH). It is used as a gemstone in jewelry and other adornments. Common topaz in its natural state is colorless, though trace element impurities can mak ...

. It has long been known from the

emerald Emerald is a gemstone and a variety of the mineral beryl (Be3Al2(SiO3)6) colored green by trace amounts of chromium or sometimes vanadium.Hurlbut, Cornelius S. Jr. and Kammerling, Robert C. (1991) ''Gemology'', John Wiley & Sons, New York, p ...

and chrysoberyl mine on the Takovaya stream, near

Yekaterinburg Yekaterinburg ( ; rus, Екатеринбург, p=jɪkətʲɪrʲɪnˈburk), alternatively romanized as Ekaterinburg and formerly known as Sverdlovsk ( rus, Свердло́вск, , svʲɪrˈdlofsk, 1924–1991), is a city and the administra ...

in the

Urals The Ural Mountains ( ; rus, Ура́льские го́ры, r=Uralskiye gory, p=ʊˈralʲskʲɪjə ˈɡorɨ; ba, Урал тауҙары) or simply the Urals, are a mountain range that runs approximately from north to south through western ...

Russia Russia (, , ), or the Russian Federation, is a transcontinental country spanning Eastern Europe and Northern Asia. It is the largest country in the world, with its internationally recognised territory covering , and encompassing one-ei ...

, where large crystals occur in mica-schist. It is also found with topaz and

amazonite Amazonite, also known as Amazonstone, is a green tectosilicate mineral, a variety of the potassium feldspar called microcline. Its chemical formula is KAlSi3O8, which is polymorphic to orthoclase. Its name is taken from that of the Amazon Rive ...

in the

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies un ...

of the Ilmen Mountains in the southern Urals and of the

Pikes Peak Pikes Peak is the highest summit of the southern Front Range of the Rocky Mountains, in North America. The ultra-prominent fourteener is located in Pike National Forest, west of downtown Colorado Springs, Colorado. The town of Manitou S ...

region in Colorado, USA. Additionally in Colorado, phenakite is found in the Mount Antero area with aquamarine,

bertrandite Bertrandite is a beryllium sorosilicate hydroxide mineral with composition: Be4Si2O7(OH)2. Bertrandite is a colorless to pale yellow orthorhombic mineral with a hardness of 6-7. It is commonly found in beryllium rich pegmatites and is in part an ...

, and

fluorite Fluorite (also called fluorspar) is the mineral form of calcium fluoride, CaF2. It belongs to the halide minerals. It crystallizes in isometric cubic habit, although octahedral and more complex isometric forms are not uncommon. The Mohs sca ...

. Small, gem grade individual crystals of phenakite showing a prismatic habit are noted in

beryl Beryl ( ) is a mineral composed of beryllium aluminium silicate with the chemical formula Be3Al2Si6O18. Well-known varieties of beryl include emerald and aquamarine. Naturally occurring, hexagonal crystals of beryl can be up to several ...

dissolution cavities at the Noumas II Pegmatite, part of the Orange River pegmatite belt in the Northern Cape of

South Africa South Africa, officially the Republic of South Africa (RSA), is the southernmost country in Africa. It is bounded to the south by of coastline that stretch along the South Atlantic and Indian Oceans; to the north by the neighbouring coun ...

. Large crystals of prismatic habit have been found in a

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...

quarry at

Kragerø Kragerø () is a town and municipality in Vestfold og Telemark county, Norway. It is part of the traditional regions of Grenland and the smaller Vestmar. The administrative centre of the municipality is the town of Kragerø. The city of Kra ...

Norway Norway, officially the Kingdom of Norway, is a Nordic countries, Nordic country in Northern Europe, the mainland territory of which comprises the western and northernmost portion of the Scandinavian Peninsula. The remote Arctic island of ...

. Framont near Schirmeck in

Alsace Alsace (, ; ; Low Alemannic German/ gsw-FR, Elsàss ; german: Elsass ; la, Alsatia) is a cultural region and a territorial collectivity in eastern France, on the west bank of the upper Rhine next to Germany and Switzerland. In 2020, it had ...

is another well-known locality. Still larger crystals, measuring in diameter and weighing . have been found at Greenwood in

Maine Maine () is a state in the New England and Northeastern regions of the United States. It borders New Hampshire to the west, the Gulf of Maine to the southeast, and the Canadian provinces of New Brunswick and Quebec to the northeast and nor ...

, but these are

pseudomorph In mineralogy, a pseudomorph is a mineral or mineral compound that appears in an atypical form ( crystal system), resulting from a substitution process in which the appearance and dimensions remain constant, but the original mineral is replaced ...

s of quartz after phenakite. For gem purposes the stone is cut in the brilliant form, of which there are two fine examples, weighing 34 and 43 carats (6.8 and 8.6 g), in the

British Museum The British Museum is a public museum dedicated to human history, art and culture located in the Bloomsbury area of London. Its permanent collection of eight million works is among the largest and most comprehensive in existence. It docum ...

. The indices of refraction are higher than those of quartz,

or topaz; a faceted phenakite is consequently rather brilliant and may sometimes be mistaken for

diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, b ...

References

{{Reflist Beryllium minerals Nesosilicates Trigonal minerals Minerals in space group 148 Gemstones