Zincian micas from peralkaline phonolites of the Oktyabrsky massif, Azov Sea region, Ukrainian Shield
Sharygin, Victor V.
European Journal of Mineralogy Volume 27 Number 4 (2015), p. 521 - 533
published: Aug 1, 2015
Hendricksite, Zn-Mn-bearing fluorophlogopite and phlogopite were observed in the groundmass of two types of peralkaline phonolite from the Oktyabrsky massif, eastern Azov Sea region, Ukraine. These micas are associated with nepheline, potassium feldspar, albite, sodalite, Zn-bearing kupletskite, aegirine, perraultite, fluorite, catapleiite, serandite, REE-rich eudyalite, fluorapatite, fluorbritholite-(Ce), a pyrochlore-group mineral, cryolite, thorite, thorianite and a mineral of the cancrinite group. Fine-grained phonolite from the Oktyabrsky massif represents the first known occurrence of hendricksite in peralkaline igneous rocks. This mica is variable in composition (in wt%): SiO2 (37.7–38.7), TiO2 (up to 0.3), Al2O3 (10.1–10.9), FeOt (0.3–1.4), ZnO (21.5–25.8), MgO (6.9–9.7), MnO (6.4–7.7), K2O (8.4–9.1), Rb2O (0.5–0.8), Li2O (up to 0.3), F (1.5–2.3) and H2O (2.1–2.5 wt.%). The simplified average formula of this hendricksite can be expressed as K(Zn1.5Mg0.9Mn0.5Li0.1)(Al0.9Si3.1O10)(OH)1.5F0.5. Zn-Mn-bearing fluorophlogopite and fluorian phlogopite occur in porphyritic phonolite. These micas are richer in SiO2 (41.5–46.9), MgO (13.2–18.5), Li2O (0.3–1.3) and F (3.1–5.9), but poorer in Al2O3 (7.5–10.0) and ZnO (5.3–15.4 wt%) than hendricksite. Their formulae can be expressed as K(Mg2Li0.4Zn0.3Mn0.3)(Al0.6Si3.4O10)F1.3(OH)0.7 and K(Mg1.6Zn0.8Mn0.4Li0.2)(Al0.8Si3.2O10)(OH)1.1F0.9, respectively, indicating that these minerals are compositionally intermediate with respect to fluorophlogopite KMg3(AlSi3O10)F2, phlogopite KMg3(AlSi3O10)(OH)2, hendricksite KZn3(AlSi3O10)(OH,F)2 and tainiolite KLiMg2(Si4O10)F2. The appearance of Zn-containing minerals (kupletskite, Zn-rich micas, perraultite) in the Oktyabrsky phonolites indicates low fS2, high fO2, high alkalinity and high volatiles content of their parental magma. These rocks represent the latest derivatives of magma evolution for the Oktyabrsky massif. They lack sulphide mineralization and contain abundant H2O- or F-bearing minerals; their Fe content is mainly concentrated as Fe3+ in aegirine.