I08 MINERALOGY AND LITHOLOGY. 



67. Cyanite [AI2 Si O5]. 



The ordinary blue, bladed crystals of this mineral abound in many 

 localities in our state. They are conspicuous, and easily recognized. 

 These blades are made by the meeting of prismatic planes, which make 

 an angle with one another of 106°, and which are also the cleavage 

 planes ; hence, in thin sections of the rocks, cyanite, when cut parallel to 

 the base, appears as composed of many little rhombs. In thin sections it 

 can easily be recognized as triclinic, since none of its sections are dark 

 when the prism is parallel with the plane of a Nicol. This can also be 

 tried with cleavage bits, that are so easily obtained ; but it is to be noted, 

 that unless they are very thin, no result is obtained, since the large crys- 

 tals are so often composed of little crystals in twinned positions that the 

 splinters, if not very thin, are always colored when revolved between the 

 Nicol prisms. Bellows Falls, Lyme (in the north-west part), Jaffrey (Mt. 

 Monadnock), Orford, Warren, Hanover, and Norwich, Vt., are places 

 where good specimens of cyanite can be obtained. 



6S. TiTANiTE (Sphene) [Ca Ti Si O']. 



This is not of much mineralogical interest, save as a rock constituent, 

 in which condition it is widely distributed, though rarely visible except 

 with the microscope. It commonly presents itself in rounded grains, but 

 sometimes the rhombic and six- or eight-sided forms, which are obtained 

 by cutting its common crystals, are seen. The sections are usually 

 greenish-yellow and dichroic, but, owing to the faintness of its color in 

 thin sections, it is not always possible to observe this. In color and 

 dichroism it looks like epidote, from which it is easily distinguished, 

 because, between crossed Nicols it gives scarcely any colors, while epi- 

 dote gives very brilliant interference colors. 



In our rocks titanite appears usually to be a product resulting from 

 the decomposition of titanic iron. For example : it has been explained 

 that the titanic iron in our greenstones is often subjected to a peculiar 

 kind of decomposition. Now, where the skeletons of titanic iron are 

 most abundant, there the sphene is found in the largest amounts. As 

 was stated, the iron of titanic iron being removed, the product remaining 

 has been shown by Prof. Lasaulx to be titanate of lime, but, if silica takes 



