“ag ot eal ae 
Penfield and Pratt—Optical Properties, ete. 387 
Art. XL.—Hffect of the Mutual Replacement of Man- 
ganese and Lron on the Optical Properties of Lithiophilite 
and Triphylite ; by S. L. PENFIELD and J. H. Prarv. 
THE numerous complete analyses of lithiophilite and_tri- 
phylite which have been made by Penfield* and Wellst have 
shown in a striking way the transition from essentially 
LiMnPO, to the isomorphous LiFePO,. The pure end products 
are not known, the lithiophilite richest in manganese being 
‘that from Branchville, Conn., with MnO=40°9 per cent and 
FeO=4-0 per cent, and the triphylite richest in iron that from 
Rabenstein and Bodenmais, Bavaria, with FeO=36-2 per cent 
and MnO=9-0 per cent. 
Our knowledge of the optical properties of these minerals is 
confined to the results obtained by E. §. Danat on the lithi- 
ophilite from Branchville, who found that the plane of the 
optical axes was 001, the acute bisectrix at right angles to 010, 
the divergence of the optical axes large, 2Ha (n for the oil 
being 1°47) =74° 45’ for red and 79° 30’ for blue, the disper- 
sion therefore strong o<v. The character of the double 
refraction was found to be positive. 
The present investigation has been made upon different 
varieties of lithiophilite and triphylite and was undertaken in 
order to determine the variations in the optical properties due 
to the mutual replacement of manganese and iron. The 
material was selected from specimens in the Brush collection 
and from the following localities: Branchville, Conn., three 
varieties of lithiophilite showing a considerable range in com- 
position ; Grafton, N. H., a variety about midway in composi- 
tion between lithiophilite and triphylite; and Rabenstein near 
Zwiesel, Bavaria, the nearest approach to theoretically pure 
triphylite. None of the material showed crystal faces, and the 
sections and prisms that were prepared were orientated by the 
cleavages, of which there are two, one fairly good parallel to 
001, the other less distinct parallel to 010. The material was 
not always well adapted for optical work, and it was often 
quite difficult to obtain sections that were sufficiently trans- 
parent. This was due, however, not to any decomposition of 
the mineral but to numerous crackled and opaque portions of 
the mass, caused perhaps by crushing. Liquid inclusions were 
abundant in the material from Branchville. 
The cleavage was in no case so perfect that a fragment of 
suitable orientation for optical purposes could be broken out, 
and in order to prepare sections and surfaces parallel to a given 
* This Journal, xiii, p. 425, 1877; xvii, p. 226, 1879, and xxvi, p. 176, 1883. 
+ Ib., xvi, p. 119, 1878. t Ib., xvi, p. 119, 1878. 
