Abstract:
Labyrinthite was found on the Nyorkpakhk Mt (Khibiny massif), occurring in hypcragpaitic pegmatite as anhcdral rounded grains 0.5-1 cm in cross-size. It associates with potassium feldspar, sodalitc, alkaline amphi-bolc, acgirinc, pectolitc, lamprophyllite, lomonosovitc, villiaumitc, etc. At the surface, grains of the mineral are replaced by aggregates of zirsinalite, lovozcritc and thcrmonatritc. Color bright pink, streak white, transparent, luster vitreous; brittle, with conchoidal fracture. Mohs' hardness 5-6. D(mcas.) = 2.88(2) g/cm3 (volumetric method), D(calc.) = 2.87 g/cm3. Optically uniaxial, positive, «0 = 1-597(1), «„= 1.601(1). Trigonal, space group R3,a= 14.239(1), с = 60.733(7) A, V= 10664(3) A. Z= 3. The strongest lines of the powder diffraction pattern [d, A (I)(hkl)] arc: 2.977(100)(1.3.10), 2.853(88)(048), 4.324(68)(0.2.10), 3.230(44)(0.2.16), 3.550(39)(220), 2.685(38)(140), 3.049(36)(1.1.18), 2.605(36)(0.3.18), 5.70(34)(024), 3.173(34)(1.2.14). The IR data arc given. Chemical composition (wt%, electron probe, H2O content by the Pcnficld method): Na2O 16.77, K2O 1.11, CaO 10.73, SrO 1.24, FcO 2.56, MnO 1.00, Ce2O3 0.27, SiO2 50.04, ZrO2 11.90, Ti02 0.67, Cl 1.70, F011,H2O 1.58, -O=(CI,F)2 0.43, total 99.25. Empirical formula (based on total of cations Si + Zr + Ti = 57.7 as determined by crystal structure analysis) is (Na333oK|.45Sro.74k35.49(Ca]ij7Cc(nobii.87(Fc2j9Mno.s7)i3.o6Zr5.94(Ti()32Sio.26)5:<>.7s Si51Oi44.4s(OH)4.8, C12.,5FIU6 3H2O. Simplified formula: (Na,K,Sr)35Cai2Fc3Zr6TiSi5iOi44(O,OH,H2O)9Cl3. The mineral is named labyrinthite (from the Greek 'labyrinthos' [labyrinth]) for its unusually complex crystal structure, whose unit cell contains about 800 cations and anions distributed over more than 100 crystallographic sites. The crystal structure is characterized by the presence of two types of modules (alluaivite-likc and cudialyte-likc) alternating along the triad axis, causing labyrinthite to have a double с period of its unit cell relatively to cudialy-tc, and a close crystal chemical relationship to rastsvctacvitc (ZRMO, 2006, No. 1). According to the authors' crystal chemical systcmatics of the cudialytc group, the mineral belongs to one of its three principal subgroups characterized by the 24-laycrcd structural array. Labyrinthite is a latc-pcgmatitic mineral formed during the final stages of hyperagpaitic pegmatite formation. Type material is deposited at the Fersman Mineralogical Museum, Moscow.