114 0. Bowles — Crystal JTorms of Pyromorphite. 



Art. XL — Crystal Forms of Pyromorphite • by O. Bowles. 



During the past winter a detailed study was made of the 

 crystal forms of the mineral pyromorphite, the museum and 

 laboratory of the University of Minnesota affording a number 

 of excellent specimens from various localities. In addition to 

 these, examination was made of a number of loaned specimens 

 obtained through the kindness of the Foote Mineral Company 

 of Philadelphia. 



In 1876 H. Baumhaner,* by etching pyromorphite with 

 nitric acid, obtained depressions possessing the symmetry of 

 pyramidal hemihedrism, and for this reason the mineral was 

 placed in the hexagonal bipyramidal class. Up to the present 

 time no evidence has been presented, other than that sup- 

 plied by etch figures, for placing it in this class. The present 

 investigations, however, supply corroborative evidence and 

 establish the mineral in its crystallographic class on the basis 

 of morphology. This is made possible through the observation 

 of many crystal faces representing a prism of the third order, 

 and a small number of faces of a bipyramid of the third order. 



Measurements of about forty crystals were made by means of 

 a Goldschmidt two-circle goniometer of the 1910 model. 



Several aggregates of pale, wax-yellow crystals were from 

 the well-known locality, Ems, Nasseau, Germany. The largest 

 crystals are approximately three to five millimeters in diameter, 

 and five to ten millimeters in length, and are somewhat barrel- 

 shaped. The most perfect crystals are smaller, more tabular 

 inhabit, and show most prominently the forms J1010J, |1011£, 

 and JOOOlf. In many cases the form ^1120[ is well developed, 

 and on such crystals distinct third-order prism faces are usually 

 present. 



The latter form occurs at times as mere line faces, or may 

 be as fully developed as the second order forms. Fig. 1 illus- 

 trates a crystal with four distinct faces of this form. It is l-7 mm 

 in diameter and l mm in length. Care has been taken to repre- 

 sent accurately the relative size of each face. The form was 

 observed on many crystals, and, for about fifty per cent of the 

 faces observed, the reflection figure in the goniometer was clear 

 and definite, giving reliable readings. The results obtained 

 point conclusively to the symbol { 2130 ^ . The average of read- 

 ings for eighteen faces gives a value of 19° 11' for the angle 0, 

 the calculated angle being 19° 06'. 



The most perfect crystals of pyromorphite yet observed by 

 the writer are those obtained from the Broken Hill Mines of 

 *Neues Jahrbuch fiir Mineralogie, 1876, p. 411. 



