Pen field and Stanley — Chemical Composition of Amphibole. 23 



Aet. IY. — On the Chemical Composition of Amphibole • by 

 S. L. Penfield and F. C. Stanley.* 



Owing to the common occurrence of amphibole and the 

 important role it plays as a rock-making mineral, its chemical 

 composition has naturally been the subject of repeated investi- 

 gation, with the result that there has accumulated a large 

 amount of analytical data, yet there is still wanting a satisfac- 

 tory explanation of the variations in chemical composition 

 exhibited by the mineral. The present investigation was 

 undertaken, therefore, with the hope that by having a few 

 analyses made with the utmost possible care on material of 

 unquestionable purity a clue might be gained which would 

 help to elucidate some of the difficulties presented by the prob- 

 lem. And here at the outset it may be stated that the present 

 communication is only a preliminary one : It is intended to 

 make more analyses than are given in this paper, but since it 

 happens that the work must be interrupted for a time, it is 

 believed that data of sufficient interest have already been 

 accumulated to warrant publication. It is also intended to 

 make a careful study of the optical properties of the various 

 amphiboles which have been analyzed, but the carrying out of 

 that part of the investigation must be left for the future. 



Turning to text-books and treatises on Mineralogy for an 

 explanation of the chemical composition of amphibole, it is 

 found that the formulas suggested by Tschermakf are the ones 

 which are almost always given. Briefly stated, Tschermak's 



*Note : — Prof. Penfield had had the problem of the chemical composition 

 of amphibole in mind for a number of years, but the opportunity for carry- 

 ing out an investigation to solve it did not present itself until about two 

 years ago, when Mr. F. C. Stanley, a graduate student, commenced, under 

 his direction, the series of amphibole analyses which are quoted in the pres- 

 ent paper. These analyses formed the. basis of a thesis which Mr. Stanley 

 presented in 1905 for the degree of Doctor of Philosophy. The results of the 

 analyses were not published at that time, for it was hoped, as Prof. Penfield 

 states, that the investigation could be continued and made more complete, 

 but circumstances prevented Dr. Stanley from continuing the work, and hence 

 during May and June, 1906, Prof. Penfield wrote up the investigation as far 

 as it had proceeded. After his death in August last, the manuscript of the 

 article presented here was found in his desk. Fortunately it was in such an 

 advanced stage that it has been possible to print it almost exactly as Prof. 

 Penfield left it, the only changes being occasional verbal ones and the only 

 addition being the concluding paragraph, headed Summary and Conclusion. 



Prof. Penfield, had he lived, would undoubtedly have extended the article 

 somewhat by the discussion of still other analyses taken from the literature, 

 and wotild, as was his invariable custom, have subjected the whole manu- 

 script to rigorous revision. But as it stands it forms a clear, well developed 

 and illuminating contribution, not only to the problem of the chemical com- 

 position of amphibole itself, but also to the wider problem of the composi- 

 tion of pneumatolitic minerals in general. — W. E. Ford. 



f Mineralogische Mittheilungen, 1871, page 41. 



