Chesnokovite, Na2[SiO2(OH)2] · 8H2O, the first natural sodium orthosilicate from the Lovozero alkaline pluton, Kola Peninsula: Description and crystal structure of a new mineral species

Abstract

family were described for the first time only 40 years ago after the discovery of magadiite and kenyaite in the soda-bearing sediments of Lake Magadi, Kenya (Eugster, 1967). A few years later, makatite and kanemite also found in the sediments of African soda lakes were described by Sheppard et al. (1970) and Johan and Maglione (1972). Other findings in a similar environment showed that hydrous sodium silicates are characteristic of alkali carbonate supergene, foremost, the sedex-type mineral assemblages. Subsequently, the members of this family were described from a quite different setting. Makatite and new minerals-revdite and grumantite-were found in the salt mineralization of the late, low-temperature derivates of hyperalkaline igneous rocks in the Lovozero alkaline pluton, Kola Peninsula, Russia (Khomyakov et al.Abstract -Chesnokovite, a new mineral species, is the first natural sodium orthosilicate. It has been found in an ussingite vein uncovered by underground mining at Mt. Kedykverpakhk, Lovozero alkaline pluton, Kola Peninsula, Russia. Natrolite, sodalite, vuonnemite, steenstrupine-(Ce), phosinaite-(Ce), natisite, gobbinsite, villiaumite, and natrosilite are associated minerals. Chesnokovite occurs as intergrowths with natrophospate in pockets up to 4 × 6 × 10 cm in size consisting of chaotic segregations of coarse lamellar crystals (up to 0.05 × 1 × 2 cm in size) flattened along [010]. The crystals are colorless and transparent. The aggregates are white to pale brownish yellowish, with a white streak and a vitreous luster. The cleavage is perfect parallel to (010) and distinct to (100) and (001). The fracture is stepped. The Mohs' hardness is 2.5. The measured density is 1.68 g/cm 3 ; the density calculated on the basis of an empirical formula is 1.60 g/cm 3 and 1.64 g/cm 3 on the basis of an idealized formula. The new mineral is optically biaxial, positive, α = 1.449, β = 1.453, γ = 1.458, 2 V meas = 80 ° , and Z = b . The infrared spectrum is given. The chemical composition (Si determined with electron microprobe; Na, K, and Li, with atomic emission analysis; and H 2 O, with the Alimarin method) is as follows, wt %: 21.49 Na 2 O, 0.38 K 2 O, 0.003 Li 2 O, 21.42 SiO 2 , 54.86 H 2 O, total is 98.153. The empirical formula calculated on the basis of O 2 (OH) 2 is as follows: (Na 1.96 K 0.02 ) Σ 1.98 Si 1.005 O 2 (OH) 2 · 7.58H 2 O. The simplified formula ( Z = 8) is Na 2 [SiO 2 (OH) 2 ] · 8H 2 O. The new mineral is orthorhombic, and the space group is Ibca . The unit-cell dimensions are: a = 11.7119, b = 19.973, c = 11.5652 Å, and V = 2299.0 Å 3 . The strongest reflections in the X-ray powder pattern [ d , Å ( I , %)( hkl )] are: 5.001(), and 2.774(100)(341, 143, 114). The crystal structure was studied using the Rietveld method, R p = 5.77, R wp = 7.77, R B = 2.07, and R F = 1.74. The structure is composed of isolated [SiO 2 (OH) 2 ] octahedrons and the chains of edge-shared [Na[H 2 O) 6 ] octahedrons. The Si and Na polyhedrons are linked only by H-bonds, and this is the cause of the low stability of che...

Paleontology of Earth's Mantle

“…SiO 2 is also soluble and, in this hypothesis, stabilizes ribose. Both anhydrous and hydrous sodium silicate minerals exist naturally in alkaline plutonic rocks (e.g., Pekov et al 2007). Their formation requires CO 2 -rich and alkali-rich magma but does not involve products of photosynthesis in any apparent way.…”