Davidsmithite

Davidsmithite is a Ca-bearing nepheline-group mineral with the formula (Ca,◻)₂Na₆Al₈Si₈O₃₂, with the "◻" being a vacancy in the formula. It is a feldspathoid mineral that occurs as a rock forming in the Liset eclogite pod (Norwegian Caledonides). Commonly found alongside lisetite, davidsmithite is found overgrowing this other mineral in corona textures.^[1]

Davidsmithite
General
Category	Tectosilicate minerals, feldspathoid group
Formula	(Ca,◻)2Na6Al8Si8O32
IMA symbol	Dvs
Strunz classification	9.FA.05
Crystal system	Hexagonal
Unit cell	a = 9.982(1) Å; c = 8.364(2) Å; V = 732.74 Å3; Z=1
Identification
Color	Transparent, Colorless
Crystal habit	Irregular
Twinning	None
Cleavage	None
Fracture	N/A
Mohs scale hardness	5.5
Luster	Vitreous, Greasy
Streak	White
Density	2.597g cm−3
Optical properties	Uniaxial Negative
Birefringence	low
Ultraviolet fluorescence	None
References	[1][2]

Name

Davidsmithite is named after David Christopher Smith, who has made significant contributions to the world of mineralogy and petrology. 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).^[1]

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.^[1] It has also been reported in melted rocks (clinkers and paralavas) in Eastern Mongolia.^[2]

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, plagioclase and lisetite.^[1] 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, 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) and SEM (scanning electron microscope).^[1]

X-ray crystallography

Davidsmithite is in the hexagonal crystal system with the space group of P6₃. The unit cell 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 ų, and Z = 1. No polarization or Lorentz corrections were applied.^[1]

Chemical composition

Oxide	Weight %
CaO	3.16
K2O	0.69
Na2O	16.89
Al2O3	35.52
Fe2O3	0.13
SiO2	43.63
MgO	0.02
MnO	0.04
NiO	0.01
TiO2	0.04

^[1]