W. E. Ford — Optical Study of the Amphiboles. 179 



Art. XIV. — A Contribution to the Optical Study of the 

 Amphiboles / by W. E. Ford. 



The late Prof. S. L. Penfield was engaged at the time of his 

 death upon an investigation of the chemical composition of the 

 minerals of the Amphibole Group. A series of eleven analyses 

 for this work had been made under his guidance, by Dr. F. C. 

 Stanley, of amphiboles which ranged in composition from trem- 

 olite to hornblende. The specimens selected for analysis afforded 

 unexcelled material for chemical investigation, and it is safe to 

 say that few such series of authoritative analyses of the amphi- 

 bole minerals can be fonnd elsewhere. The article, as far as 

 the discussion of the composition of the amphiboles was con- 

 cerned, was practically complete at the time of Prof. Penfield's 

 death and was subsequently published in this Journal.* It 

 had been the intention of Prof. Penfield to supplement the 

 discussion of the chemical composition of these amphiboles by 

 a study of their optical properties. Unfortunately he was not 

 able to do this, but it has seemed of considerable importance 

 that this work should be done. In the majority of cases the 

 identical specimens from which material for the analyses was 

 taken were preserved in the Brush Mineral Collection. In the 

 few cases where it was impossible to positively identify the 

 specimen which had furnished the material, a specimen in the 

 Collection and undoubtedly of the same suite, as the one ana- 

 lyzed, was used. 



In studying the literature of the last twenty-five years it is 

 surprising to find in how few cases both an analysis and an 

 optical description of the same amphibole have been recorded. 

 There are plenty of instances where one or the other is given, 

 but unfortunately the two are seldom found combined in the 

 same description. Therefore, if only as a matter of record, it 

 seemed advisable to complete the investigation of these amphi- 

 boles by a determination of their optical characters. This 

 work was started a. number of years ago in the laboratory of 

 Prof. Rosenbusch at Heidelberg, but through various causes it 

 has only recently been possible to carry it to completion. 



As far as possible the following optical facts were deter- 

 mined in each case : (1) the indices of refraction and the aver- 

 age index of refraction, (2) the angle of extinction on b (010) 

 measured with the trace of the prismatic cleavage, and (3) the 

 pleochroism. The value of the optical angle, 2 V, was deter- 

 mined whenever possible by calculation, but no direct meas- 

 urements of it were made. The indices of refraction were 

 measured upon a total refractometer, at least two differently 

 *Vol. xxiii, p. 23, 1907. 



