South Australian Dolomites. 453 



(ii) Dolomite. — As remarked above, this mineral has been 

 distinguished by its unstained character when the section is treated 

 with Lemberg's solution. Whenever it shows any definite outline, 

 this outline is always idioblastic towards calcite, the reverse not 

 being seen. The marked limpidity is a notable feature. Contrary to 

 the usual statements in the literature, the dolomite in these slides 

 is characteristically twinned, often showing rather wide hemitrope 

 bands. In fact, it may be said that dolomite is more often twinned 

 than calcite. The twinning law seems also to be of a different type 

 to that of calcite. The plane Oll2 in dolomite is not a glide plane, 

 and the twinning in these slides would correspond to twins on 

 0221. Where a section shows the two cleavages intersecting at the 

 rhombohedral angle, the twin bands intersect the obtuse angle 

 between the cleavages and are parallel to the shorter diagonal of 

 the rhomb. This point serves to distinguish it from calcite. In 

 addition, then, to the staining differentiation for these rocks, we can 

 apply the following criteria : — 



(1) Twinning in dolomite is at least as common as in calcite, 

 and is often more so. Moreover, the twinning is of a different 

 type. In dolomite the twin plane is 0221, and in calcite 0112. 

 In favourable sections, such as those showing the cleavages 

 intersecting at the rhombohedral angle, the lamellae cut the 

 intercleavage angle, such that in dolomite they are usually 

 parallel to the shorter diagonal of the rhomb, and in calcite 

 are parallel to the longer diagonal. The twinning in dolomite 

 would appear to indicate that the plane 0221 is also a glide plane. 



(2) Where the one mineral exerts its form against the other 

 dolomite is always the idioblastic mineral. 



(3) Dolomite is usually of a remarkably limpid character, 

 whilst calcite is invariably turbid, owing to tlie presence of minute 

 inclusions. The significance of this will be dealt with later. 

 This feature can be safely used to differentiate calcite and dolomite 

 in the rocks under discussion, and is the most important of the 

 three criteria. 



(iii) For sterile. — Builds rounded grains or crystals with elongation 

 parallel to the c axis in size up to 1'5 mm., rarely larger. In some 

 grains the 010 cleavage, the trace being parallel to the elongation, 

 is very distinct, but it may be completely absent. Apart from its 

 characteristic decomposition products, it is distinguished by its 

 large optic axial angle, positive^ sign, and the position of the optic 

 axial plane with reference to the 010 cleavage. The characteristic 

 cracks and the serpentinous decomposition parallel the occurrence 

 of its ferriferous member in the eruptive rocks. In some sections 

 (115) the forsterite is now represented by aggregates of opal and 

 calcite. The determination of calcite within the forsterite outline 

 was settled by a staining test. 



