Mbocayaty

A group of small plugs are intruded into sediments of the Pennsylvanian Tubarao Series which are little known but appear to include essexite, shonkinite and nepheline syenite. The essexite consists of titanaugite, olivine, brown biotite, amphibole, alkali feldspar and altered nepheline, which is often in graphic intergrowth with the feldspar. Nepheline syenite contains alkali feldspar, abundant but altered nepheline and aegirine-augite; there is some analcime. Photomicrographs and two analyses are given by Milton and Eckel (1959) which indicate an Na2O:K2O ratio of about 1:3. J.H. Palmieri (personal communication, 1981) is of the opinion that there is only one intrusion, which comprises essexite and shonkinite with nepheline syenite differentiates.