28 



PROCEEDINGS OF THE NATIONAL MUSEUM 



VOL. 71 



The best crystals of titanite occur closely associated with the yel- 

 lowish-green epidote, pyrite and fine-scaly grayish chlorite in vugs 

 and cavities in whitish feldspar specimens the base of which con- 

 sists of anorthite, to some extent replaced by orthoclase (No. 84561). 

 The titanite crystals are small, most of them averaging only about 

 1 mm. and the largest not exceeding 3 mm. Many of them are pale 

 yellow in color and from this they grade into a greenish yellow 

 approaching that of the associated epidote. The crystals of this 

 lot of specimens consist of both simple individuals and twins. One 

 of the simple crystals of typical development is illustrated in Figure 

 12. As is shown these crystals are prismatic by elongation of the 

 positive hemipyramid ?i(lll), a not uncommon habit for this min- 

 eral. The crystals of this habit were measured in several positions 

 and that adopted in the drawing was finally found to be correct. 

 The angles measured were as follows : 



Measurements of titanite, Figure 12 



Quiility, description 



Measured 



Calculated 



Excellent 



Minute, poor 



Good 



Very good 



Poor 



89 55 



90 13 

 56 30 

 65 39 

 40 15 



29 54 



90 00 



90 00 



63 53 



48 28 



90 00 



90 00 



56 45 



65 30 



40 36 



90 00 



90 00 



64 06 



48 22 



The twinned crystals occur with the untwinned ones and appear, 

 except for the twinning, to have the same habit ; but measurement of 

 several twins in several positions showed that, while the direction of 

 elongation of the simple crystals is parallel to n{lll), the twins are 

 elongated parallel to the vertical axis, the twinning plane being 

 P(IOO), the commonest twinning plane for this species. A typical 

 twin is illustrated in Figure 13. This gave the following angles : 



Measurements of titanite (ticin) Figure IS 



Quality description 



Excellent 



Very good 



Excellent 



Very poor 



Excellent 



do - 



Measured 



90 05 



90 00 



56 49 



11 06 



40 29 



50 09 



29 25 



90 00 



90 00 



23 38 



48 13 



69 30 



Calculated 



90 00 



90 00 



56 45 



10 42 



40 36 



49 59 



90 00 



90 00 



23 29 



48 22 



69 22 



