ELLINGSENITE, Na5Ca6Si18O38(OH)13{middle dot}6H2O, A NEW MARTINITE-RELATED MINERAL SPECIES FROM PHONOLITE OF THE ARIS ALKALINE COMPLEX, NAMIBIA

Abstract

Ellingsenite, Na 5 Ca 6 Si 18 O 38 (OH) 13 ·6H 2 O, is a new calcium silicate hydrate (CSH) [triclinic, P 1, a 9.55(3), b 9.395(8), c 16.329(3) A, α 100.2(1), β 94.9(2), γ 117.8(2)°, V 1251(8) A 3 , Z = 1 (from powder-diffraction data) or a 9.576(11), b 9.577(11), c 16.438(19) A, α 85.85(2), β 75.23(2), γ 60.142(14)°, V 1262(3) A 3 , Z = 1 (from single-crystal diffraction data)], chemically and structurally related to minerals of the gyrolite–reyerite group. The mineral is found in a hydrothermally altered phonolite of the Aris alkaline complex, in Namibia, as snow-white spherules (up to 3 mm in diameter) of well-shaped rhomb-like crystals associated with aegirine, albite, manganoneptunite, microcline, natrolite and polylithionite. The mineral is transparent, colorless in separate crystals, white in aggregates, with a vitreous (separate crystals) to silky (aggregates) luster and a white streak. Cleavage is perfect on {001}, and the fracture is smooth. The Mohs hardness is 4. In transmitted light, the mineral is colorless; dispersion is not observed. Ellingsenite is biaxial (–): α 1.520(2), β 1.534(2), γ 1.536 (589 nm), 2 V meas 5°. Optical orientation: X = c . D calc = 2.38 g cm −3 , D meas = 2.32(5) g cm −3 . The mean chemical composition determined by electron microprobe is: Na 2 O 9.26, K 2 O 0.23, CaO 17.35, SiO 2 60.35, H 2 O 12.5 (Penfield method), for a total of 99.69 wt.%. The empirical formula calculated on the basis of 57 atoms of oxygen is (Na 4.95 K 0.09 ) ∑5.04 (Ca 5.57 Na 0.43 ) ∑6.00 Si 18.10 O 38 (OH) 13 ·6H 2 O. The simplified formula is Na 5 Ca 6 Si 18 O 38 (OH) 13 ·6H 2 O. The mineral does not effervesce in 1:1 HCl at room temperature. The strongest X-ray powder-diffraction lines [ d in A(I) hkl ] are: 15.50(100)001, 4.22(16)201, 3.159(30)005, 3.023(33)321, 2.791(24)214, and 1.827(27)511. The crystal-structure model of ellingsenite ( R 1 = 0.247) obtained from a crystal of poor quality displays layers of octahedra and tetrahedra of the T 2 OT 2 type with the composition {Na 1.90 Ca 5.10 [Si 8 O 20 ] X 2 }, where X = O, OH, with the interlayer content unresolved. Ellingsenite can be considered as a derivative of martinite, obtained from the latter by intercalation of some additional species (most probably, Na hydrosilicate) in between the adjacent layers of octahedra and tetrahedra. The principal absorption bands in the infrared spectrum include 3460, 1600, 1360 and 1025 cm −1 (shoulder at 1140 cm −1 ) plus four bands in the region of 780–380 cm −1 . The mineral is named in honor of Dr. Hans Vidar Ellingsen (born 1930), a well-known Norwegian mineral collector, who found this mineral during his expedition to the Aris complex.