Oriented chlorite lamellae in chromite from the Pedra Branca Mafic-Ultramafic Complex, Ceará, Brazil

Chromite grains in chromitite from the Pedra Branca Mafic-Ultramafic Complex, Ceará, Brazil, contain lamellar inclusions of chlorite oriented parallel to {111}. The chromite grains have a core of aluminian chromite and a broad margin of ferrian chromite, formerly called “ferritchromit.” The chlorite is a Cr-bearing IIb clinochlore, and the inclusions occur preferentially within “ferritchromit.” The orientation relationship is $cchlorile*$||[111]_chromite, a_chlorite||[hh0]_chromite, and b_chlorite||[hh2h]_chromite. This is equivalent to the orientation of chlorite-olivine intergrowths and corresponds to topotaxial sharing of a layer of closest-packed anions at the interface between the two phases. Because of the considerable misfit in the strain-free equivalent lattices, the interface structures in both chlorite-chromite and chlorite-olivine intergrowths may consist of an interlayer sheet (brucite layer) of H bonded to a layer of closest-packed O of the host substrate. Chloritization of the chromite was broadly contemporaneous with the alteration to “ferritchromit.”