Townendite, Na8ZrSi6O18, an Indicator of Extremely High Agpaicity and Important Zirconium Concentrator in Peralkaline Rocks of the Lovozero Pluton, Kola Peninsula

Townendite, ideally Na8ZrSi6O18, earlier considered an extremely rare mineral, was found in abundance in a hyperagpaitic pegmatoid aegirine–nepheline–sodalite–microcline rock with eudialyte, kazakovite, villiaumite, lomonosovite, and alkali sulfides in deep levels at Mt. Karnasurt, Lovozero alkaline pluton, Kola Peninsula, Russia. Townendite forms transparent lilac to light violet segregations up to 6 × 4 cm in size. Its crystal structure was studied based on single-crystal XRD data, R1 = 2.29%. The mineral is trigonal, R‑3m, a = 10.2910(3), c = 13.1577(4) Å, V = 1206.77(7) Å3, and Z = 3. The simplified crystal chemical formula derived from the structure refinement is: $$^{A}{\text{Na}}_{3}^{B}({\text{N}}{{{\text{a}}}_{{2.7}}}{{\square }_{{0.3}}})_{{\Sigma 3}}^{C}\left( {{\text{N}}{{{\text{a}}}_{{{\text{1}}{\text{.8}}}}}{\text{M}}{{{\text{n}}}_{{{\text{0}}{\text{.2}}}}}} \right)_{{\Sigma 2}}^{M}{\text{ZrS}}{{{\text{i}}}_{{\text{6}}}}{{{\text{O}}}_{{18}}}$$ . The IR spectrum shows only a trace amount of OH groups. Townendite serves as a protophase for the formation in nature of low-Ca and Mn (Ca + Mn < 0.5 apfu) OH-bearing lovozerite-group zirconosilicates. Two natural evolution series different in a crystal chemical mechanism were found: 1) townendite $$^{A}{\text{Na}}_{3}^{B}{\text{Na}}_{3}^{C}{\text{Na}}_{2}^{M}{\text{ZrS}}{{{\text{i}}}_{{\text{6}}}}{{{\text{O}}}_{{18}}}$$ → kapustinite $$^{A}{\text{Na}}_{3}^{B}{{\left( {{\text{N}}{{{\text{a}}}_{{2-3}}}{{\square }_{{0-1}}}} \right)}^{C}}\square _{2}^{M}{\text{ZrS}}{{{\text{i}}}_{{\text{6}}}}{{{\text{O}}}_{{15-16}}}{{\left( {{\text{OH}}} \right)}_{{2-3}}}$$ → litvinskite $$^{A}{{\left( {{\text{N}}{{{\text{a}}}_{{2-3}}}{{\square }_{{0-1}}}} \right)}^{B}}\left( {\square ,{{{\text{H}}}_{{\text{2}}}}{\text{O}}} \right)_{3}^{C}\square _{2}^{M}{\text{ZrS}}{{{\text{i}}}_{{\text{6}}}}{{{\text{O}}}_{{13-14}}}{{\left( {{\text{OH}}} \right)}_{{5-6}}}$$ ; 2) townendite → zolotarevite $$^{A}{\text{Na}}_{3}^{B}\left( {{{{\text{H}}}_{{\text{2}}}}{\text{O}},\square } \right)_{3}^{C}{\text{Na}}_{2}^{M}{\text{ZrS}}{{{\text{i}}}_{6}}{{{\text{O}}}_{{15}}}{{\left( {{\text{OH}}} \right)}_{3}}$$ → litvinskite. The abundance of townendite alteration products in near-surface rocks of the Lovozero complex is indicative of the fact that this mineral is widespread in deep levels of the pluton in “fresh” peralkaline intrusive rocks and pegmatites and is 1) a bright indicator of extremally high agpaicity and 2) an important, occasionally main Zr and Hf concentrator in them. Townendite is major in unaltered rocks of the fifth intrusive phase of the Lovozero pluton, which are protolith rocks of well-known porphyry-like lovozerite lujavrites.