MINERALOGY AND TEXTURES OF ‘CASWELLITE’, A PSEUDOMORPH AFTER MICA FROM FRANKLIN, NEW JERSEY

Caswellite is an informal textural term that has been used for the past 125 years to describe pseudomorphs of original mica, chiefly hendricksite, with grossular as the most common replacement product. This material has yet to be examined using modern analytical techniques. Nine samples of caswellite from the Franklin Mineral Museum were cut normal to the foliation and analyzed with SEM-EDS. Caswellite is micaceous in appearance, but presents a broad spectrum of textures at the microscopic scale. Some specimens are fine-grained and heterogeneous with grains 1-5 μm thick and 5-200 μm long; some are much coarser-grained and heterogeneous with grains 10-100 μm thick and 20-500 μm long; and some are homogeneous on the millimeter scale. All specimens display clear foliation, presumably along the precursor mica’s relict cleavage planes. The major mineral constituents are garnet (~Grs₆₂And₂₆Sps₁₂), a chlorite mineral rich in Mn and Zn, willemite, and tephroite. To our knowledge, this is the first time that tephroite has been reported as a significant component of caswellite. Of the first five samples analyzed, no relict mica was observed. Cross-cutting relationships and compositional zoning provide evidence of multiple episodes of alteration. We conclude that caswellite does not simply describe a garnet pseudomorph after mica, but a wide range of replacement textures and compositions.