Electron microprobe study of the Tehuitzingo chromite-bearing serpentinite, Southern Mkxico

G. Gonzalez M.*, F.Ortega**

* Departamento de Ingenieria Metalhrgica, Fat. de Qufmica, UNAM. M&ico D.F. 04210, M&XICO ** Instituto de Geologia, UNAM, Mkico D.F. 04210, MI?XICO

The studied rock was collected near the town of Tehuitzingo in the State of from the . largest ultramafic body of forming a unit of the Paleozoic Acatlan Complex [ 11. The body is formed by a completely serpentinized mantle peridotite with common schleiren and podiform segregations of chromite. Under the petrographic microscope the analyzed sample consists of a mat of serpentine and chloritic phyllosilicates in which several spots of opaque Cr-Fe ore are included (figure 1) Under polarized light this ore developed a wide reaction rim of a greenish mineral formed between the ore and the serpentine matrix. The present study was aimed at resolving the composition of the mineral phases involved in the reaction structure and the chemical nature of the reaction using standard petrographic and EDS electron microprobe techniques. The opaque ore, eventhough of homogeneous appearance, under high magnification (1000 x) consists of a symplectitic aggregate of two domains differing in composition and physical constitution. Domain A (figure 2) is homogeneous, which from its microprobe analysis it corresponds to the mineral chromian magnetite; the second domain B is composite and includes an unresolvable aggregate of a cryptocrystalline silicate (?), and a Cr-Fe phase richer in Cr than the homogeneous phase. The greenish phyllosilicate forming the rind around the opaque core corresponds to an aluminous member of the chlorite-amesite solid solution series, whereas the external bluish phyllosilicate may correspond to the mineral lizardite because of its lower contents of aluminum [2]. Since Cr was not incorporated into the reacting phyllosilicates the process was complex and apparently involved only the rearrangement of Si and Al between Fe-Cr oxides and silicates, with the possible net transfer of Al from an original aluminous chromite into the rind of chlorite and loss of Si from the external serpentine into the cryptocrystalline domain of the internal Cr-Fe zone. It is probably more realistic to consider two different reactions separated in time; a first one forming the chlorite between the oxide and the serpentine, and a second process of symplectitic exsolution forming the internal opaque domain upon cooling of the system.

References 1. Ortega, G.F., Estratigrafia de1 Complejo Acatlan en la region de Baja, Estado de Puebla, Univ. Nal. Auton. Mexico, Inst. Geologia, Revista, V. 2, ,1978, p. 112-13 1 2. O’Hanley, D.S., , Serpentinites, p. 277, New York, Oxford University Press,. 1996. FIG 1 Opaque-&e (I’e-Cr oxide) grains (0) rimmed directly by Mg-rich chlorite (C) followed by Al rich serpentine (S), as labeled in the photograph.

FIG.2 Interior of the opaque phase showing two domains: A is a homo lgeneous chromian magnetite and B, an unresolvable symplectitic aggregate of cryptocrystalline silicate :s and a chromium-rich oxide phase.