Labyrinthite

Labyrinthite is a very rare mineral of the eudialyte group.^[2] 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, with its complexity expressed in its chemical formula (Na,K,Sr)₃₅Ca₁₂Fe₃Zr₆TiSi₅₁O₁₄₄(O,OH,H₂O)₉Cl₃.^[2][3] 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.^[3] Other eudialyte-group representatives with such symmetry lowering include aqualite, oneillite, raslakite, voronkovite.^[2] Labyrinthite is the second dual-nature (both zircono- and titanosilicate) representative of the group after dualite^[4] and third with essential titanium after dualite and alluaivite.^[2]

Labyrinthite
General
Category	Silicate mineral, cyclosilicate
Formula	(Na,K,Sr)35Ca12Fe3Zr6TiSi51O144(O,OH,H2O)9Cl3
IMA symbol	Lby[1]
Crystal system	Trigonal
Crystal class	Pyramidal (3) (same H-M symbol)
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	[2][3]

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.^[3]

Notes on chemistry

Beside that given in the formula, other elements substituting the main ones in labyrinthite are manganese, fluorine, and cerium.^[3]

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.^[3]