Heyerdahlite, Na3Mn7Ti2(Si4O12)2O2(OH)4F(H2O)2, a new mineral of the astrophyllite supergroup from the Larvik Plutonic complex, Norway: Description and crystal structure. (28th February 2018) - Sokolova, Elena
ABSTRACT: Heyerdahlite, ideally Na3 Mn7 Ti2 (Si4 O12 )2 O2 (OH)4 F(H2 O)2, is a new astrophyllite-supergroup mineral from the Larvik Plutonic complex, Norway. Heyerdahlite was found in a nepheline-syenite pegmatite with albite, aegirine, hastingsite/magnesio-hastingsite, kupletskite, lorenzenite and pyrophanite. Heyerdahlite is colourless to pale brown, transparent and has a vitreous lustre. Mohs hardness is 3, D calc. = 3.245 g/cm 3 . Heyerdahlite is biaxial (+), α = 1.694(2), β = 1.710(5), γ = 1.730(5), 2Vmeas. = 80(4)° and 2Vcalc. = 84.5°. Cleavage is perfect parallel to {001}. The empirical formula based on 32.18 (O + F) apfu is (Na1.18 K0.68 Rb0.12 Cs0.01 Pb0.01 )Σ2 Na1.00 (Mn6.29 Zn0.23 Mg0.07 Zr0.04 ${\rm Fe}_{{\rm 0}{\rm. 03}}^{{\rm 2 +}} $ Ca0.01 Na0.34 )Σ7.01 (Ti1.78 Nb0.17 Mg0.03 Zr0.02 )Σ2 (Si8.03 O24 )O2 [(OH)3.92 F0.08 ]Σ4 F1.00 [(H2 O)1.18 □0.82 ]Σ2, for Z = 1. Heyerdahlite is triclinic, space group P $\bar 1$, a = 5.392(2), b = 11.968(4), c = 11.868(4) Å, α = 112.743(8), β = 94.816(7), γ = 103.037(8)° and V = 675.6(7) Å 3 . The crystal structure was refined to R 1 = 4.44% for 3577 unique ( F o > 4σ F ) reflections. In the crystal structure of heyerdahlite, there are four [4] T sites occupied by Si. The [6] D site is occupied mainly by Ti. The T4 O12 astrophyllite ribbons composed of SiO4 tetrahedra and D octahedra constitute the H (Heteropolyhedral) sheet. In the O (Octahedral) sheet, there are four Mn-dominant [6] M (1–4) sites. Two H sheets and one central O sheet form the HOH block, and adjacent HOH blocks link via a common ${\rm X}_{\rm D}^{\rm P} $ anion of the two D octahedra. In the I (Intermediate) block between HOH blocks, there are two interstitial cation sites, A and B, and a W A site, partly occupied by H2 O. The A site splits into two sites, [12] A (1) and [6] A (2), partly occupied by K and Na, respectively, with A(1)–A(2) = 0.754 Å. The aggregate content of the A site is ideally Na2 apfu. The [10] B site is occupied by Na. The W A site is ideally occupied by (H2 O)2 pfu. The mineral is named after the Norwegian explorer Thor Heyerdahl (1914–2002), who was born in Larvik.