gems, jade, pyroxene jade, fei cui, Guatemala, Mesoamerica, Nicaragua, Mexico, Honduras, Costa Rica, archaeogemology, Cuba, Dominican Republic, geology

Jadeitite occurs along with albitite (ex-jadeitite) in blocks in tectonised serpentinite, or more often in river pebbles, along the Motagua River Valley and its tributaries (MRV) in Guatemala (e.g. McBirney et al., 1967; Harlow, 1994). The MRV is generally considered to be the source area for most, if not all, of the jade used widely in the prehispanic civilisations ranging from the Olmecs through the Mayas to the Aztecs (e.g. Hargett, 1990; Harlow, 1991). Harlow (1994) mentioned the evident spatial relationships of known jadeitite or eclogite occurrences with the major suture contact of the North American and Caribbean Plates (which runs W.S.W.-E.N.E. precisely along the MRV), and he emphasised that jadeitite and eclogite are respectively restricted to the North and South sides of the MRV. Raman Microprobe (RMP) analysis on the unprepared small (~4×2×2 cm) green river pebble n° FG96/27, collected from the South side of the MRV, showed it to be a near monomineralic jadeitite with a composition of 90 ± 10 mol% Jd, with a characteristic T-O-T vibration at 697–701 cm⁻¹ (cf. Smith & Boyer, 1985; Smith & Gendron, 1997). Both macro- and micro-mode RMP also revealed local enrichments of rutile, a mineral not listed in the comprehensive study of MRV jadeitites by Harlow (1994). A subsequently manufactured thin section revealed: an irregular granular microcrystalline texture with the pyroxene (~97 vol%, i.e. true jadeite jade) sometimes showing optically-zoned grain boundaries; abundant rutile (~2 vol%); rare titanite and zircon; and more importantly numerous micron-sized inclusions in the cores of a few of the larger equant to elongate jadeite grains. EMP analysis then proved 77 to 97 mol% Jd. All analysed microinclusions are quartz (also not listed by Harlow, 1994), thus indicating a trapped core assemblage on the higher P side of the Ab = Jd + Qz reaction for the given compositions. No other mineral species have been found, in particular no albite, nepheline nor analcime, and hence no retrogression at all. Jadeitite n° FG96/27 is thus very different from all the varieties of jadeitite from the North side of the MRV described by Harlow (1994). Therefore it seems that its occurrence on the South side is a geological reality, rather than a human-caused anomaly (e.g. prehispanic commerce). Hence it represents a new locality and a new kind of jadeitite with a higher-P different P-T-X-t trajectory, but most probably one formed by the same overall petrogenetical process and geodynamical event. It thus also presents an extra possibility for the geological provenancing of Mesoamerican artefacts in jade which, in fact, collectively reveal several distinct kinds of jade.