Labyrinthite

Labyrinthite
General
Category Silicate mineral, Cyclosilicate
Formula
(repeating unit)		 (Na,K,Sr)35Ca12Fe3Zr6TiSi51O144(O,OH,H2O)9Cl3
Crystal system Trigonal
Crystal class Pyramidal (3)
H-M symbol: (3)
Space group R3
Unit cell a = 14.24, c = 60.73 [Å] (approximated); Z = 3
Identification
Color Pink
Crystal habit anhedral grains
Cleavage None
Fracture Conchoidal
Tenacity Brittle
Mohs scale hardness 5-6
Luster Vitreous
Streak White
Diaphaneity Transparent
Density 2.88 g/cm3 (measured)
Optical properties Uniaxial (+)
Refractive index n0=1.60, ne=1.60 (approximated)
Pleochroism None
Alters to surface alteration to lovozerite, zirsinalite and thermonatrite
References [1][2]

Labyrinthite is a very rare mineral of the eudialyte group.[1] When compared to other species in the group, its structure is extremely complex - with over 100 sites and about 800 cations and anions - hence its name. Its complexity being expressed in its formula: (Na,K,Sr)35Ca12Fe3Zr6TiSi51O144(O,OH,H2O)9Cl3.[2][1] The formula is simplified as it does not show the presence of cyclic silicate groups. Complexity of the structure results in symmetry lowering (likely due to ordering of cations) from the typical centrosymmetrical group to R3 space group.[2] Other eudialyte-group representatives with such symmetry lowering include aqualite, oneillite, raslakite, voronkovite.[1] Labyrinthite is the second dual-nature (both zircono- and titanosilicate) representative of the group after dualite[3] and third with essential titanium after dualite and alluaivite.[1]

Occurrence and association

Labyrinthite was discovered in hyperagpaitic (ultra-alkaline) pegmatite at Mt. Nyorkpakhk, Khibiny massif, Kola Peninsula, Russia, where it represents a late-stage mineral. It coexists with aegirine, alkaline amphibole, lamprophyllite, lomonosovite, potassium feldspar, pectolite, sodalite, and villiaumite.[2]

Notes on chemistry

Beside that given in the formula, other elements substituting the main ones in labyrinthite are manganese, fluorine, and cerium.[2]

Notes on crystal structure

Labyrinthite is two-modular - its structure contains alluaivite-like and eudialyte-like module. The framework of labyrinthite is 24-layered. It also characterize in doubled c unit cell parameter.[2]

References

  1. 1 2 3 4 5 Mindat, http://www.mindat.org/min-27420.html
  2. 1 2 3 4 5 6 Khomyakov, A.P., Nechelyustov, G.N., and Rastsvetaeva, R.K., 2006. Labyrinthite (Na,K,Sr)35Ca12Fe3Zr6TiSi51O144(O,OH,H2O)9Cl3, a new mineral with a modular eudialyte-like structure from Khibiny Alkaline Massif, Kola Peninsula, Russia. Zapiski Vserossiyskogo Mineralogicheskogo Obshchestva 135(2), 38-49
  3. ↑ Khomyakov, A.P., Nechelyustov, G.N., and Rastsvetaeva, R.K., 2009: Dualite, Na30(Ca,Na,Ce,Sr)12(Na,Mn,Fe,Ti)6Zr3Ti3MnSi51O144(OH,H2O,Cl)9, a new zircono-titanosilicate with a modular eudialyte-like structure from the Lovozero alkaline Pluton, Kola Peninsula, Russia. Geology of Ore Deposits 50(7), 574-582

Further reading

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