Eudialyte

Eudialyte, whose name derives from the Greek phrase Εὖ διάλυτος, eu dialytos, meaning "well decomposable", is a somewhat rare, nine-member-ring cyclosilicate mineral, which forms in alkaline igneous rocks, such as nepheline syenites. Its name alludes to its ready solubility in acid.^[2][4]

Eudialyte
General
Category	Cyclosilicate
Formula	Na15Ca6(Fe,Mn)3Zr3SiO(O,OH,H2O)3 (Si3O9)2(Si9O27)2(OH,Cl)2
IMA symbol	Eud[1]
Strunz classification	9.CO.10
Crystal system	Trigonal
Crystal class	Hexagonal scalenohedral (3m) H-M symbol: (3 2/m)
Space group	R3m
Unit cell	a = 14.31, c = 30.15 [Å]; Z = 12
Identification
Color	Red, magenta, brown; also blue and yellow
Crystal habit	Crystals short rhombohedral to long prismatic, granular, irregular masses
Cleavage	Distinct on {0001} imperfect on {1120}
Fracture	Uneven
Tenacity	Brittle
Mohs scale hardness	5–6
Luster	Vitreous
Streak	White
Diaphaneity	Transparent to translucent
Specific gravity	2.74–3.10
Optical properties	Uniaxial (+/−)
Refractive index	nω = 1.606–1.610 nε = 1.610–1.613
Birefringence	δ = 0.004
Pleochroism	Weak: O= colorless, pale yellow, pink; E= pink to colorless
Solubility	H2SO4
Other characteristics	Mildly Radioactive
References	[2][3][4]

Pink eudialyte in syenite (lujavrite) from Poços de Caldas, Brazil. The white mineral is alkali feldspar, the black is aegirine, and the little brown bits are biotite.

Eudialyte was first described in 1819 for an occurrence in nepheline syenite of the Ilimaussaq intrusive complex of southwest Greenland.^[2]

Uses

Eudialyte is used as a minor ore of zirconium. Another use of eudialyte is as a minor gemstone, but this use is limited by its rarity, which is compounded by its poor crystal habit. These factors make eudialyte of primary interest as a collector's mineral. Eudialyte typically has a significant content of U, Pb, Nb, Ta, Zr, Hf, and rare earth elements (REE). Because of this, geoscientists use eudialyte as a geochronometer to date and investigate the genesis of the host rocks.^[5]

Associated minerals

Eudialyte is found associated with other alkalic igneous minerals, in addition to some minerals common to most igneous material in general.

Associate minerals include: microcline, nepheline, aegirine, lamprophyllite, lorenzenite, catapleiite, murmanite, arfvedsonite, sodalite, aenigmatite, rinkite, låvenite, titanite and titanian magnetite.^[3]

Alternative names

Alternative names of eudialyte include: almandine spar, eudalite, Saami blood.^[2] Eucolite is the name of an optically negative variety, more accurately the group member: ferrokentbrooksite.^[6]

Notes for identification

Eudialyte's rarity makes locality useful in its identification. Prominent localities of eudialyte include Mont Saint-Hilaire in Canada, Kola Peninsula in Russia and Poços de Caldas in Brazil, but it is also found in Greenland, Norway, and Arkansas. The lack of crystal habit, associated with color, is also useful for identification, as are associated minerals. A pink-red mineral with no good crystals associated with other alkaline igneous material, especially nepheline and aegirine, is a good indication a specimen is eudialyte. Iron (Fe²⁺) provides the color.^[7]

Eudialyte group

Main article: Eudialyte group

Microchemical (by electron microprobe) and structural analyses of different eudialyte (and related) samples have revealed the presence of many new eudialyte-like minerals. These minerals are structurally and chemically related and joined into the eudialyte group. The group includes Zr⁻, OH⁻, Cl⁻, F⁻, CO₃⁻ and possibly also SO₄-bearing silicates of Na, K, H₃O, Ca, Sr, REEs, Mn, Fe, Nb and W. Electron vacancies can be present in their structure, too.