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University of California Publications. 



[Geology 



loses luster in part and yields a little water but does not fuse. 

 Fuses readily in lower part of bunsen flame (2.5) with intumes- 

 cence to a brown glass, practically colorless in thin bubbles. 



It resists hot hydrochloric and nitric acids and may therefore 

 be separated from the natrolite matrix by these agents. It is 

 easily attacked by hydrofluoric acid which leaves a white film of 

 decomposition products on its surface. 



Qualitative chemical tests have shown the presence in reason- 

 able quantity of silica, titanium and calcium. Iron is also present 

 and probably determines the color. 



When first observed in small particles without definite crystal 

 form the mineral was thought by the writer to be titanite. It 

 answers to all the tests usually applied to titanite in small 

 irregular particles in thin sections. In particular may be men- 

 tioned its color, high refractive index, strong double refraction, 

 biaxial positive character, its pleochroic colors and absorption 

 scheme, the tests for silica, titanium, and calcium. Its fusibility 

 is exceptionally low and its density somewhat higher than the 

 usual range of titanite. The crystal form is most distinctive. It 

 has a characteristic orthorhombic habit unlike any of the titanites 

 hitherto described. So very different in their general appear- 

 ance however are the various habits of titanite that already in 

 its history it has been given a number of different names. It 

 seemed possible then that this might be a new and pseudo- 

 orthorhombic habit of this protean mineral. After considering 

 various possible orientations, the closest approximation was 

 found, in considering the apparent basal plane to be a; (102) and 

 the symmetry plane to bisect the obtuse angles of the pyramid. 

 This would give the proper optical orientation, as in titanite c is 

 almost perpendicular to x and lies in the symmetry plane. In 

 this arrangement the two back faces (p s , p 4 ) and the front faces 

 (p 1 , p- as described above) must belong to different forms and 

 would be expected to show some systematic difference in their 

 angular relation to x and in the angles where they meet in the 

 plane of symmetry. If orthorhombic each set of angles should 

 have the same values. An examination of the detailed figures 

 given above will show that the differences are only a few minutes 

 and that the slight variations are not systematic or symmetrical 



