Chiappinoite-(Y), Y 2 Mn(Si 3 O 7 ) 4 , a new layer silicate found in peralkaline syenitic ejecta from the A ´ gua de Pau volcano, Azores ANTHONY R. KAMPF 1, * and ROBERT M. HOUSLEY 2 1 Mineral Sciences Department, Natural History Museum of Los Angeles County, 900 Exposition Blvd., Los Angeles, CA 90007, USA *Corresponding author, e-mail: akampf@nhm.org 2 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA 91125, USA Abstract: Chiappinoite-(Y) (IMA2014-040), ideally Y 2 Mn(Si 3 O 7 ) 4 , is a new mineral found in peralkaline syenitic ejecta from the A ´ gua de Pau volcano, Sa ˜n Miguel Island, Azores District, Portugal. It was discovered by collector Luigi Chiappino of Milan, Italy, after whom the mineral is named. It occurs as colourless, thin to thick prisms, up to 1 mm long, with blunt chisel-like terminations, and exhibits the forms {100}, {010}, {001}, {110} and {011}. Crystals are transparent with vitreous lustre. The Mohs hardness is about 6 and the mineral is brittle with uneven fracture and one perfect cleavage on {001}. The measured and calculated densities are 3.09(2) and 3.073 g/cm 3 , respectively. Chiappinoite-(Y) is biaxial () and the refractive indices (white light) are a ¼ 1.590(1), b ¼ 1.5978(10), g ¼ 1.5982(10); 2V meas ¼ 24(1)  and 2V calc ¼ 25  . The mineral exhibits no dispersion and is nonpleochroic. The optical orientation is X ¼ c, Y ¼ b, Z ¼ a. The empirical formula on 28 O apfu is (Y 1.17 Ce 0.18 Dy 0.10 Na 0.10 Nd 0.09 Er 0.08 La 0.07 Gd 0.07 Yb 0.06 Sm 0.03 Pr 0.02 ) Æ1.97 (Mn 2þ 0.61 Ca 0.25 Fe 2þ 0.09 ) Æ0.95 Si 12.07 O 28 . The eight strongest reflections in the X-ray diffraction pattern [d obs. in A ˚ (I)(hkl)] are: 9.84 (90) (002), 4.129 (52) (024), 3.977 (48) (114), 3.544 (100) (211,042,202), 3.203 (48) (222), 2.999 (71) (044,204), 2.478 (67) (310,046,206) and 2.065 (57) (multiple). The mineral is orthorhombic, Ibam, with a ¼ 7.5549(3), b ¼ 15.2342(5), c ¼ 19.6418(14) A ˚ , V ¼ 2260.6(2) A ˚ 3 and Z ¼ 4. The structure of chiappinoite-(Y) contains 3-tetrahedra-thick silicate layers consisting of sinuous batisite-like chains, above and below which are four-membered silicate rings. Eight-coordinate Y and Mn sites are located between the silicate layers. The structure is essentially identical to that of the synthetic phases PrNaSi 6 O 14 and NdNaSi 6 O 14 , except that the latter phases have an additional 12-coordinate Na site within the ‘‘cage’’ in the silicate layer. Key-words: Chiappinoite-(Y); new mineral; crystal structure; layer silicate; rare-earth elements; A ´ gua de Pau volcano; Azores. 1. Introduction The islands that make up the Portuguese archipelago of the Azores are volcanic in origin. They sit astride the Mid- Atlantic Ridge, along the East Azores Fracture Zone, and have experienced numerous eruptions in the Quaternary and into historic times. The volcanoes of the Azores are stratovolcanoes with rocks belonging to an alkali basalt/ trachyte suite. Sa ˜o Miguel Island, the largest and most populous of the islands, has three active, dominantly tra- chytic volcanoes, Sete Cidades, A ´ gua de Pau and Furnas (Booth et al., 1978; Moore, 1991). The syenitic ejecta, which have resulted from plinian eruptions of A ´ gua de Pau, and especially those from the most recent eruption in 1563, have been the subject of numerous papers (cf. Ternes, 1999; Ridolfi et al., 2003). The enrichment of volatiles, alkalis, HFSE (high field strength elements, e.g. Zr, Nb, Hf, Ta, Ti, U, Th) and rare-earth elements (REE) in the trachytic magmas has resulted in a variety of rare minerals in the ejecta. Cann (1967) reported the second world occurrence of dalyite in ejecta from A ´ gua de Pau, but until now, no new minerals have been described from the A ´ gua de Pau ejecta blocks. The new mineral chiappinoite-(Y), described herein, is named in honour of Luigi Chiappino (born 1950) of Milan, Italy. As a mineral collector since 1970, Mr. Chiappino has developed an excellent eye for unusual micro-species. He has submitted hundreds of self-collected specimens for scientific study, including many from A ´ gua de Pau volcano, a special focus of his collecting efforts. These efforts have resulted in the recognition of numerous species not previously recorded from the ejecta blocks of A ´ gua de Pau (bastna ¨site-(Ce), cerite-(Ce), ekanite, elpidite, epididymite, fluorcalciopyro- chlore, fluorite, hingganite-(Y), kentbrooksite, manganoeu- dialyte, narsarsukite, okanoganite-(Y), parisite-(Ce), petarasite, polycrase-(Y), pyrite, steacyite, vlasovite and a potentially new rosenbuschite-group mineral) and he is the discoverer of the new species chiappinoite-(Y). The new mineral and name have been approved by the Commission on New Minerals, Nomenclature and Classification 0935-1221/14/0026-2416 $ 3.15 DOI: 10.1127/ejm/2014/0026-2416 # 2014 E. Schweizerbart’sche Verlagsbuchhandlung, D-70176 Stuttgart Eur. J. Mineral. 2015, 27, 91–97 Published online 13 November 2014 eschweizerbart_xxx