The new mineral fluorbarytolamprophyllite, (Ba,Sr,K)2[(Na,Fe2+)3TiF2][Ti2(Si2O7)2O2] and chemical evolution of lamprophyllite-group minerals in agpaitic syenites of the Kola Peninsula

Abstract

Unusual agpaitic syenites containing up to 25 vol.% lamprophyllite-group minerals (lamprophyllite, fluorlamprophyllite, barytolamprophyllite, and the new mineral species fluorbarytolamprophyllite, IMA 2016–089) have been discovered in the Niva intrusion and Mokhnatye Roga alkaline dyke belonging to the Kola Alkaline Province, northwestern Russia. The other main components of the rocks are potassium feldspar, Ti-rich aegirine-augite, aenigmatite, alkaline amphiboles, astrophyllite, natrolite, and ferripyrophyllite. Three generations of lamprophyllite-group minerals can be distinguished based on their morphological features. The new mineral fluorbarytolamprophyllite is the F-dominant analogue of barytolamprophyllite and the Ba-dominant analogue of fluorlamprophyllite. It represents the early generation of lamprophyllite-group minerals (LGM) and forms brown prismatic crystals, their radial aggregates and marginal zones of fluorlamprophyllite crystals. The lustre of the new mineral is vitreous to pearly. Mohs hardness is 2.5. Dcalc is 3.662 g/cm3. The mineral is optically biaxial (+), α = 1.738(3), β = 1.745(4), γ = 1.777(4) (589 nm), 2 V (meas.) = 55(5)°, 2 V (calc.) = 51°. The chemical composition (electron microprobe, water determined by TGA, wt.%) is: Na2O 10.01, K2O 2.65, MgO 0.43, CaO 0.64, SrO 5.59, BaO 16.23, MnO 0.50, FeO 4.44, Al2O3 0.08, TiO2 27.31, ZrO2 0.22, Nb2O5 0.91, Ta2O5 0.15, SiO2 29.35, F 2.41, H2O 0.26, total 101.18. The empirical formula based on 18 anions is (Ba0.865Sr0.44K0.46Na0.26)Σ2.025(Na2.38Ca0.09Fe0.47Mn0.06)Σ3.00(Ti2.79Mg0.09Fe0.035Nb0.06Zr0.015Ta0.01)Σ3.00(Si3.99Al0.01)Σ4.00 O16[F1.04O0.72(OH)0.24]Σ2.00. The IR spectrum is given. The strongest lines of the powder X-ray diffraction pattern are [d, A (I,%) (hkl)]: 9.692 (40) (200), 3.726 (59) (−311), 3.414 (67) (311), 3.230 (96) (300), 3.013 (53) (−5–11), 2.780 (100) (221), 2.662 (52) (002). The crystal structure has been solved and refined to R1 = 5.07 based on 2897 independent reflections with I > 2σ(I). Fluorbarytolamprophyllite is monoclinic, space group C2/m. The unit-cell parameters refined from the powder data are: a = 19.520(5), b = 7.0995(17), c = 5.3896(20) A, β = 96.657(23)°; V = 741.86(24) A3, Z = 2. At Niva and Mokhnatye Roga, most of the LGM were formed during magmatic stage of syenite crystallization from alkaline melt enriched in Na, K, Ba, Fe, Ti and F. Compositional variation of the examined LGM and their textural relations show changes in the Sr/Ba ratio in the parental melt and increasing activity of F and Ba in derivatives fluids as the main factors driving this variation.