WOLFF AND PALACHE. — APATITE FROM MINOT, ME. 525 



dary lines were sharp and the readings generally good. From the 

 average of a large number of readings the following values were 

 obtained : 



w^-„ = 1.G3353 ojLi = 1.63067 



t^_c = 0.00191 a,-€ = 0.0020 



£jj,==: 1.63162 eLi= 1.62865 



The crystal was then heated to 320° C. or until decolorized and the 

 indices again determined as follows : 



wj,.^ = 1.63346 ej,, = 1.63165 w — c = 0.00181 



The change in the bi-refringence and in both indices is within the limits 

 of error. 



The pleochroism is strong and the ray vibrating parallel to c (c) red- 

 dish purple, perpendicular to c (w) deep violet blue. In converging 

 light the thick clear crystals show on the basal plane a marked bi-axial 

 character with the vertical axis the acute (negative) bisectrix and a 

 division of the base into six sectors, in each of which the axial plane 

 is parallel to a prism of the second order (or perpendicular to a lateral 

 axis). These sectors come out clearly with the sensitive tint of the 

 gypsum plate ; while some are almost perfect, others merge together 

 and overlap at the centre of the crystal. The angle of the optic axes 

 in one of these sectors was measured iu the optic angle apparatus : 



2E,-, = 20°, 



but it appears to vary in different sectors of the same crystal. These 

 phenomena of apparent orthorhombic symmetry were described and fig- 

 ured by Mallard * for the violet apatite from Schlaggenwald, but appear 

 to be even more distinct in the Maine apatite. While driving off the 

 colonng matter destroys the pleochroism the anomalous bi-axial characters 

 are not affected. 



Conclusion. 



The Minot apatite is a pure fluor-apatite with a fluorine content lower 

 than that necessary for the formula Ca5F(P04)3 and indicating Groth's 

 formula Ca5P3(F.OH)Oi2 as also deduced by Jannasch from the analysis 

 quoted above. Rammelsberg f explained the low content of fluorine 

 (and chlorine) in certain apatites as due to removal of these elements 



* Annales des Mines, VIL 10, 1876, p. 147. 

 t N. J. M., 1897, 2, p. 38. 



