Davidsmithite is a Ca-bearing

nepheline Nepheline, also called nephelite (), is a rock-forming mineral in the feldspathoid groupa silica-undersaturated aluminosilicate, Na3 K Al4 Si4 O16, that occurs in intrusive and volcanic rocks with low silica, and in their associated pegmatit ...

-group mineral with the formula (Ca,◻)₂Na₆Al₈Si₈O₃₂, with the "◻" being a vacancy in the formula. It is a

feldspathoid The feldspathoids are a group of tectosilicate minerals which resemble feldspar Feldspar ( ; sometimes spelled felspar) is a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, pota ...

mineral that occurs as a rock forming in the Liset

eclogite Eclogite () is a metamorphic rock containing garnet ( almandine- pyrope) hosted in a matrix of sodium-rich pyroxene ( omphacite). Accessory minerals include kyanite, rutile, quartz, lawsonite, coesite, amphibole, phengite, paragonite, zoisit ...

pod (Norwegian Caledonides). Commonly found alongside lisetite, davidsmithite is found overgrowing this other mineral in corona textures.

Name

Davidsmithite is named after David Christopher Smith, who has made significant contributions to the world of

mineralogy Mineralogy is a subject of geology specializing in the scientific study of the chemistry, crystal structure, and physical (including optical mineralogy, optical) properties of minerals and mineralized artifact (archaeology), artifacts. Specific s ...

and

petrology Petrology () is the branch of geology that studies rocks, their mineralogy, composition, texture, structure and the conditions under which they form. Petrology has three subdivisions: igneous, metamorphic, and sedimentary petrology. Igneous ...

. As Emeritus professor at the Muséum National d’Histoire Naturelle (MNHN) in Paris, France, he has discovered minerals like nyböite, lisetite, taramite and ferro-taramite, in Norway. His contributions towards petrology include the recognition of the sub-discipline of Ultra High Pressure Metamorphism (UHPM).

Occurrence

Davidsmithite was discovered at the Liset eclogite pod, Liset, Selje, Western Gneiss Region, Vestlandet, Norway. It is found near the closely related Lisetite, which has a similar composition (CaNa₂Al₄Si₄O₁₆) and structure, being a

tectosilicate 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, the crystalline forms of silica (silicon dio ...

. It has also been reported in melted rocks (clinkers and paralavas) in Eastern Mongolia.

Properties

Davidsmithite occurs as anhedral and sometimes skeletal or lath-like crystals ranging between 80 and 100μm in length and about 30μm wide. It is indistinguishable to surrounding minerals such as

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 ...

plagioclase Plagioclase ( ) is a series of Silicate minerals#Tectosilicates, tectosilicate (framework silicate) minerals within the feldspar group. Rather than referring to a particular mineral with a specific chemical composition, plagioclase is a continu ...

and lisetite. The optical properties of Davidsmithite show it to be colorless to pale yellow in thin sections. It is optically uniaxial negative. It has a low

birefringence Birefringence, also called double refraction, 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 described as birefrin ...

, which is similar to lisetite and plagioclase. This low birefringence is the main way to distinguish it, otherwise it can only be confirmed by EMP (

electron microprobe An electron microprobe (EMP), also known as an electron probe microanalyzer (EPMA) or electron micro probe analyzer (EMPA), is an analytical tool used to non-destructively determine the chemical composition of small volumes of solid materials. I ...

) and SEM (

scanning electron microscope A scanning electron microscope (SEM) is a type of electron microscope that produces images of a sample by scanning the surface with a focused beam of electrons. The electrons interact with atoms in the sample, producing various signals that ...

X-ray crystallography

Davidsmithite is in the

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

with the

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

of P6₃. The

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 ...

was found using x-ray-powder diffraction. The unit cell dimensions are as follows: a = 9.982(1) Å, c = 8.364(2) Å, V = 721.74 Å³, and Z = 1. No polarization or Lorentz corrections were applied.

Chemical composition

References

{{reflist Wikipedia Student Program Silicate minerals Feldspathoid Hexagonal minerals