DOUBLY-TERMINATED EUHEDRAL K-RICH MARIALITIC SCAPOLITE IN THE CAMPANIAN IGNIMBRITE, ITALY: FORMATION BY VAPOR-PHASE GROWTH

The Campanian volcanic zone, located west of Naples, Italy, has been active for more than 300 ky. The Campanian ignimbrite, a trachyte-phonolite pyroclastic deposit, erupted at 39 ka with an estimated volume > 200 km³ and spread predominantly eastward overlapping the nearby western Apennines. Recent studies have distinguished a complex stratigraphy of at least two cooling units with variable degrees of welding. We have examined more than 80 samples of scoria, pumice, and massive gray and yellow facies ignimbrite with scanning electron microscopy and electron microprobe techniques in conjunction with detailed analytical chemical. Euhedral marialitic scapolite, in addition to other Cl-bearing minerals (cancrinite, sodalite, halite, and sylvite) have been identified in the massive yellow and gray facies ignimbrite and in the proximal facies (piperno). Eudhedral marialitic scapolite contains ~ 4.1 wt% Cl, 10-11 wt% Na₂0, and is K-rich, 2-3 wt% K₂O; Me = ~16.5%. The scapolite usually occurs as doubly-terminated euhedral crystals with a typical aspect ratio of 2:1 (l:w) and varies in size from 100 µm to 2 mm. The crystals are elongated parallel to the c-axis and typically have (100) prism faces; face (110) can be absent, but is commonly developed to varying degrees. The crystals display pyramidal terminations and on these (111) faces are slight to deep reentrants filled with various mineral phases, such as titanomagnetite, clinopyroxene, titanite, apatite, and zircon. Scapolite in most of our samples, has a grey color due to the presence of solid inclusions such as clinopyroxene, apatite, titanite, zircon, and Mn-bearing titanomagnetite. We did not observe any silicate melt or liquid (at room temperature) aqueous inclusions, however, “vapor” inclusions, commonly with a negative crystal morphology, are very common, and range in size from about 1 µm to > 100 µm. Based on the details of the textures, contained vapor and solid inclusions, and crystal habit, we have concluded that the euhedral scapolite formed from hot aqueous vapor-phase fluids post-ignimbrite emplacement. Cooling, fluid migration, and perhaps post-emplacement fluid exsolution mobilized sufficient material to form a variety of volatile-rich phases, alter glass, and to form overgrowths on feldspars.