gems, jade, jadeite, fei cui, Mesoamerica, Mexico, Guatemala, Costa Rica, Mayans, Olmecs, Aztecs, archaeology, history, archaeogemology

McBirney, Bass, and Aoki report the discovery and mineralogical study of a large detrital jadeite boulder from Manzanal in the Motagua Valley of Guatemala, found during a 1963 reconnaissance of the metamorphic and ultramafic rocks associated with the Motagua fault zone. The approximately 100 kg boulder was recovered from an alluvial terrace beside the Atlantic highway, about 100 m east of Manzanal. Although jadeite had previously been found as pebbles and detrital material in the Motagua drainage, no occurrence in situ was then known. From the distribution of jadeite fragments, the authors infer a nearby source in a zone of intense shearing where metamorphic rocks of the Chuacús Series—including amphibolites, garnet-biotite gneisses, and actinolite schists—are faulted against serpentinite. Associated rocks include assemblages of albite-actinolite, albite-white mica, dolomite-chalcedony-quartz, and albite-quartz with blue-green amphibole. Chemical, optical, and X-ray studies of carefully separated material establish jadeite as the principal pyroxene, while the mineralogical associations indicate formation under high-pressure metamorphic conditions related to the Motagua fault zone. The occurrence provides further geological evidence for a local Guatemalan source of the jadeite used by the pre-Columbian cultures of Mesoamerica.