290 E. S. Dana — Preliminary notice of Beryllonite. 



Art. XXXII. — Preliminary notice of Beryllonite, a new 

 mineral • by Edward S. Dana. 



A few weeks since the writer received for examination 

 some specimens of a mineral, the identity of which the Under 

 had been unable to establish. A brief study made it clear that 

 the mineral was new, and one offering a number of points of 

 more than usual interest. A complete account of the mineral 

 cannot be given until two or three months later, although the 

 essential characters now known prove beyond a doubt that it 

 is new ; a preliminary notice at this time, therefore, seems to 

 be desirable. 



The specimens of the mineral in hand consist for the most 

 part of isolated crystals, or parts of crystals, and broken frag- 

 ments from the size of a pea upwards. The largest crystal 

 found is nearly an inch across, and the largest broken mass has 

 a surface of 1J X 1J- inches and a thickness of £ inch. The 

 crystals belong to the orthorhombic system, or if they vary from 

 this the deviation is very small. They are short prismatic or 

 tabular in habit. They show one highly perfect cleavage, as 

 perfect as that of topaz ; a second nearly perfect but interrupted 

 at right angles to it (measured 90° 0'), a third very imperfect 

 corresponding to the third pinacoid plane apparently at right 

 angles to the others, and a fourth, in the zone of the last two 

 corresponding to a prism of very nearly 60°. Calling the 

 plane of perfect cleavage the base, c, and making the cleavage 

 prism, m, the unit prism, the second cleavage is brachydiagonal 

 Z>, the* third macrodiagonal a. The crystals are highly modi- 

 fied : in the prismatic zone there are seven prisms developed, 

 the unit prism m, two macro-prisms and four brachy-prisms. 

 The macrodome zone is also highly developed, the planes here 

 corresponding very nearly in angle to the several prismatic 

 planes measured from the same pinacoid plane a • in other 

 words, the axes o and c are nearly equal. Only one brachy- 

 dome has been noted. Of pyramids there are upwards of ten 

 forming several prominent zones. Many of the crystals are 

 twins and sometimes repeated twins with the cleavage prism, m, 

 of nearly 60° as the twinning plane — these are contact twins. 

 A few examples have also been noted of what are apparently 

 penetration-twins having a pyramid in the unit series and in- 

 clined on c about 60° as the twinning-plane. If we make this 

 the unit pyramid, p, the approximate axial ratio is : 



a: b\ c = 0-57: 1 : 0'94. 



