Zincian micas from peralkaline phonolites of the Oktyabrsky massif, Azov Sea region, Ukrainian Shield

Abstract

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%): SiO 2 (37.7–38.7), TiO 2 (up to 0.3), Al 2 O 3 (10.1–10.9), FeO t (0.3–1.4), ZnO (21.5–25.8), MgO (6.9–9.7), MnO (6.4–7.7), K 2 O (8.4–9.1), Rb 2 O (0.5–0.8), Li 2 O (up to 0.3), F (1.5–2.3) and H 2 O (2.1–2.5 wt.%). The simplified average formula of this hendricksite can be expressed as K(Zn 1.5 Mg 0.9 Mn 0.5 Li 0.1 )(Al 0.9 Si 3.1 O 10 )(OH) 1.5 F 0.5 . Zn-Mn-bearing fluorophlogopite and fluorian phlogopite occur in porphyritic phonolite. These micas are richer in SiO 2 (41.5–46.9), MgO (13.2–18.5), Li 2 O (0.3–1.3) and F (3.1–5.9), but poorer in Al 2 O 3 (7.5–10.0) and ZnO (5.3–15.4 wt%) than hendricksite. Their formulae can be expressed as K(Mg 2 Li 0.4 Zn 0.3 Mn 0.3 )(Al 0.6 Si 3.4 O 10 )F 1.3 (OH) 0.7 and K(Mg 1.6 Zn 0.8 Mn 0.4 Li 0.2 )(Al 0.8 Si 3.2 O 10 )(OH) 1.1 F 0.9 , respectively, indicating that these minerals are compositionally intermediate with respect to fluorophlogopite KMg 3 (AlSi 3 O 10 )F 2 , phlogopite KMg 3 (AlSi 3 O 10 )(OH) 2 , hendricksite KZn 3 (AlSi 3 O 10 )(OH,F) 2 and tainiolite KLiMg 2 (Si 4 O 10 )F 2 . The appearance of [6] Zn-containing minerals (kupletskite, Zn-rich micas, perraultite) in the Oktyabrsky phonolites indicates low f S 2 , high f O 2 , 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 H 2 O- or F-bearing minerals; their Fe content is mainly concentrated as Fe 3+ in aegirine.