APPRAISAL OF THE NEW OCCURRENCE OF GOTZENITE(SS), KHIBINSKITE AND APOPHYLLITE IN KALSILITE-BEARING LAVAS FROM SAN VENANZO AND CUPAELLO (UMBRIA), ITALY

Minerals corresponding in chemical composition to probable gotzenite s olid solutions, gotzenite(ss), ideally (Ca, Na)(3)a (Ti, Zr) (Si2O7) ( F, O, OH)(2)a, khibinskite (K4Zr2Si4O14) and apophyllite (KCa4Si8O20(O H, F)) were first reported in an aphyric, glass-bearing kalsilitite fr om Cupaello (Umbria), Italy, by Cundari and Ferguson (1991). To the au thors knowledge, this occurrence of khibinskite is only the second rec orded. Gotzenite(ss) was also found in a leucite-, melilite-rich pegma toid in an olivine-melilitite from San Venanzo, S. Venanzo, in the sam e region. The S. Venanzo phase, with significant Zr (0.18 atoms per fo rmula unit) is transitional to rosenbuschite, which is the Zr-rich end -member of the isostructural series gotzenite-rosenbuschite, while tha t from Cupaelo, virtually Zr free, is close to the ideal gotzenite end -member. The S. Venanzo phase crystallized as the preferred Zr phase f rom the relatively low-Zr (Zr=335 ppm) S. Venanzo lava. This probably inhibited khibinskite crystallization in the S. Venanzo lava, whereas khibinskite is the preferred Zr phase in the Cupaello lava (Zr=760 ppm ), coexisting with low-Zr gotzenite(ss). The structural formula of the Cupaello khibinskite, based on 10 cations, yielded: K3.89N[Zr1.99Mg0. 01Fe0.042+Ti0.02]2.06Si4.05O14.1 which is very close to the compositio n first reported by Khomiakov et al. (1974). Apophyllite, also occurri ng in the Cupaello assemblages and in cavities in the lava, yielded a composition close to the ideal formula and supports the important role of alkali-rich fluids in the crystallization history of this lava. Th e rare records of these minerals reflect, at least in part, the diffic ulty in identifying them. Their role as potential accepters of both la rge ion lithophile and high field strength elements in peralkaline roc k compositions is important in understanding the geochemistry of lampr oitic rocks and in detecting possible genetic links with related rocks .