South Australian Dolomites. 495 



composition as in (160), and is undergoing alteration to a sericitic 

 aggregate, potash apparently being derived from neighbouring 

 microcline. Quartz is evenly distributed through the slide. The 

 remaining constituents include zoisite in small amount. This rock 

 formed a portion of a rifted bar of mica schist in a granite sill a few 

 hundred yards to the east of the diopside headland, and is clearly 

 related to the diopside-microcline rocks already described. 



Zoisite is plentifully distributed through a rock, otherwise very 

 similar to the type (160) already described. In this rock (123). 

 plagioclase is absent, and its place is taken by clear, colourless 

 grains of meionite. Zoisite is abundant in some parts of the rock, 

 but may disappear in others. This mineral is usually quite irregular 

 in outline, but has a well-developed single cleavage. It is commonly 

 twinned, and the anomalous interference colours are very striking. 

 The plane of the optic axes is at right angles to the cleavage trace, 

 and the extinction is often oblique, angles up to 25 degrees being 

 observed. The sign of the birefringence is positive. These 

 characters agree with the properties of clinozoisite. It is, however, 

 probably not of uniform composition, as judged by the variability 

 of the interference tint seen in one grain. The development of 

 higher interference colours suggests the intergrowth of epidote 

 molecules. Graphite may occur intergrown with the green pyroxene, 

 but it is more characteristically associated with the zoisite, often in 

 irregular bunched aggregates. Quartz is also present. 



An interesting rock (106) forms part of the rifted block of 

 mica schist already referred to above. In hand-specimens this 

 rock is characterized by the abundant development of mica, but 

 parallel to the foliation there are developed lenses of green pyroxene 

 and white felspar. The original rock was apparently an aluminous 

 sediment, and contained streaks and lenses of a more calcareous 

 nature. On metamorphism these have developed into diopside- 

 felspar lenses. Examined under the microscope the minerals seen 

 to be present are microcline, diopside, biotite, plagioclase, apatite, 

 and some secondary ragged grains of zoisite. 



Diopside is fairly uniformly distributed in colourless grains, with 

 occasional development of uralitic actinolite at the periphery of the 

 grains. Microcline is clear and unaltered, shomng the characteristic 

 microcline extinction. Biotite is abundant in flakes, strongly 

 pleochroic in brownish yellow to brown tints, and contains little 

 grains of zircon surrounded by pleochroic haloes. 



Water-clear grains of plagioclase felsjoar are jalentifully distributed. 

 The refractive index is considerably greater than C. balsam, and 

 sections showing positive and negative birefringence are present. 

 The extinctions in the symmetrical zone for albite lamellae, together 

 with the above, indicate a range of composition from labradorite- 

 bytownite to bytownite. Diablastic intergrowths of plagioclase 

 and quartz form bays of myrmekite-like nature into plates of the 

 potash felspar. 



