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

Townendite, ideally Na₈ZrSi₆O₁₈, earlier considered as an extremely rare mineral, is found in large amount in a hyperagpaitic pegmatoid aegirine-nepheline-sodalite-microcline rock with eudialyte, kazakovite, villiaumite, lomonosovite and alkali sulfides in deep levels at Mt. Kar-nasurt, Lovozero alkaline pluton, Kola peninsula, Russia. Townendite forms transparent lilac to light violet segregations up to 6 × 4 cm. Its crystal structure was studied 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) ų, and Z = 3. The simplified crystal chemical formula derived from the structure refinement is: ^ANa₃^B(Na_2.7□_0.3)_Σ3^C(Na_1.8Mn_0.2)_Σ2^MZrSi₆O₁₈. The IR spectrum shows only trace amount of OH groups. Townendite serves as a proto-phase for the formation in nature of Ca- and Mn-poor (Ca + Mn < 0.5 apfu) OH-bearing lovozerite-group zirconosilicates. Two different in crystal chemical mechanism natural evolution series are found: (1) townendite ^ANa₃^BNa₃^CNa₂^MZrSi₆O₁₈ → kapustinite ^ANa₃^B(Na_2–3□_0–1)^C□₂^MZrSi₆O_15–16(OH)_2–3 → → litvinskite ^A(Na_2–3□_0–1)^B(□,H₂O)₃^C□₂^MZrSi₆O_13–14(OH)_5–6; (2) townendite → zolotarevite ^ANa₃^B(H₂O,□)₃^CNa₂^MZrSi₆O₁₅(OH)₃ → litvinskite. The abundance of products of townendite alteration in near-surface rocks of the Lovozero complex shows 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, sporadically main concentrator of Zr and Hf in them. The major amount of townendite occurs in unaltered rocks of the fifth intrusive phase of the Lovozero pluton which are protholith rocks of the well-known porphyry-like lovozerite lujavrites.