gems, jade, pyroxene jade, fei cui, jadeite, omphacite, kosmochlor, Guatemala, Costa Rica, Mexico, Maya, Olmec, Aztec, serpentine

In the central part of the San Augustin Acasaguastlan Quadrangle north of the Metagua River Valley, Guatemala, jadeite-rich rocks occur as inclusions in serpentine. The associated minerals are albite and in minor amounts, muscovite, actinolite and zoisite. The country rock is generally highly sheared serpentine, although massive serpentine is also present in lesser amounts. Most of the jadalte in the area is white or slightly greenish, having only small amounts of diopside, acmite and hedenbergite molecules. The green variety has a higher content of these pyroxene molecules and the variation in optical properties of the different specimens of jadeite analyzed is related to the impurities present. Jadeite seems to have formed before serpentinization of the ultrabasic body and later reacted with silica-bearing aqueous solutions forming secondary albite. Albitites are quite abundant in the area, occurring as large included bodies in the serpentine. From field relationship, chemical and mineral composition, they seem to have originated from a pegmatite which underwent desilication.