Perrierite in siliceous lavas from Mt Amiata, central Italy

PERRIERITE and chevkinite are rare R E E T i silicates reported from granitic and syenitic pegmatites and igneous rocks, volcanic ash beds, ignimbrites, and anorthosite (e.g. Jaffe et al., 1956; Young and Powers, 1960; Mitchell, 1966; Izett and Wilcox, 1968; Raade, 1970; Brooks and Rucklidge, 1976; Segalstad and Larsen, 1978; McDowell, 1979; Harding et al., 1982). The minerals are easily confused because of their close chemical and structural relationships, which have been discussed by Bonatti and Gottardi (1954, 1966), Bonatti (1959), Gottardi (1960), Lima de Faria (1962), and Calvo and Faggiani (1974). Experimental studies revealed that perrierite and chevkinite exhibit thermal polymorphism within certain compositional ranges (Ito, 1967; Ito and Arem, 1971). The type locality of perrierite is Nettuno, Rome (Italy), where Bonatti and Gottardi (1950) found it as a new mineral in shore sands derived from pyroclastics. This communication describes an occurrence in siliceous lavas and, to the best of my knowledge, records the second discovery in Italy. Routine energy-dispersive microprobe work on the lavas of Mt Amiata, a Pleistocene volcanic complex in southern Tuscany, which consists of lava and domes with both calcalkaline and potassicalkaline affinities, revealed the presence of a REE-T i silicate with a strong yellowish brown-reddish brown-opaque pleochroism. The mineral was found in studied specimens of the so-called early rhyodacites, which are K-rich lavas containing some 66 wt. ~o SiO2. These mineralogically uniform rocks carry phenocrysts of plagioclase, sanidine, pyroxene (both orthoand clino-), and biotite, in addition to about 61 ~ perlitic glass. Minor and accessory minerals are ilmenite, apatite, zircon, iron sulphide, and embayed quartz. Chemical and mineralogical details about these volcanics are to be found in Mazzuoli and Pratesi (1963) and Van Bergen et al. (1983). Mazzuoli and Pratesi (1963) reported the ubiquitous presence of very rare crystals of non-metamict allanite with a strong brown-opaque pleochroism. In the thin sections available for microprobe analysis, all grains that conform to this description proved to have the composition of perrierite or chevkinite, whereas allanite was not found. As these minerals are difficult to distinguish optically, this suggests that the REE-T i silicate is a common accessory phase in all the siliceous lavas ofMt Amiata. Abundances do not exceed a few tiny grains per thin section, however. The perrierite typically occurs as stubby rounded crystals of less than 200 pm. Some are included in orthopyroxenes, but most occur along orthopyroxene rims or as discrete grains. Elongate crystals also occur but are extremely rare. Texturally the mineral occupies a position in the crystallization sequence comparable to that of ilmenite, and was probably among the early forming minerals. Young and Powers (1960) reached a similar conclusion for chevkinites in a number of siliceous volcanic ashes from the United States, whereas McDowell (1979) suggested that the chevkinite in the Little Chief Granite, California appeared late in the period of phenocryst crystallization. Several X-ray diffraction patterns of very small, unheated grains were made in a Gandolfi camera fitted with a two-axes rotating sample-holder. As pointed out by Bonatti and Gottardi (1954), Lima de Faria (1962), and Mitchell (1966), X-ray powder data should be sufficiently diagnostic to distinguish between natural perrierite, chevkinite, and allanite. The results correspond well to perrierite, although occasional reflections were found that could also be attributed to chevkinite. The determination as perrierite is based on the consistent presence of two characteristic strong reflections at d-values between 2.94 and 3.00 A.