S. ±,. Penfield — Brookite from Magnet Cove, Ark. 387 



Art. XXXYIII. — Brookiie from Magnet Cove, Arkansas ; by 

 Samuel L. Penfield. 



Finding in the collection of Professor Geo. J. Brush a very 

 beautiful crystal of brookite from Magnet Cove, which differs 

 very much in habit from the ordinary crystals which I have 

 seen, T thought it would be of interest to give not only a de- 

 scription of it, but also to figure and describe some of the forms 

 which are common at the locality. The crystals are of the 

 variety called Arkansite by C. TJ. Shepard,* and as far as I can 

 learn very little has been written concerning them in our 

 American journals. The common forms, corresponding to fig- 

 ures 1 and 2 of this article have been figured and fully described 

 by G. vom Kath,f together with the pseudomorphs of rutile 

 after brookite which occur at the locality. My examination 

 has been confined to cabinet specimens in the collection of Pro- 

 fessor Geo. J. Brush and the Yale College cabinet, and nothing 

 can be said concerning the geological relations of the mineral. 

 The crystals are frequently loose or are attached to smoky 

 or milky quartz ; they vary in size up to 2 cm in diameter. 

 Although usually possessing polished faces, there are vicinal 

 faces occurring with both prisms and pyramids which make 

 accurate measurements with the goniometer difficult. The 

 best measurements agree closely with those given by N. von 

 Kokscharow4 The observed forms are : 



e, 122, 1-2 a, 100, i-l 



2, 112, i m, 110, 1 



X, 124, £-2 t, 021, 2-% 



Of these forms, e is the most common, occurring frequently 

 alone, usually, however, in combination with m. This latter 

 combination is especially interesting when the prism is of such 

 a size that it meets the four planes of the pyramid at the ex- 

 tremity of the b axis, forming there a solid angle of six faces ; 

 fig. 1. This combination is very common and appears like a 

 doubly terminated hexagonal pyramid. The brachydiagonal 

 pole edge of e is inclined 60° 42 r to the vertical axis, so that 

 the projection of the six faces upon the brachypinacoid would 

 be almost a perfect hexagon. As a rule, the faces of the prism 

 vary in lustre from the pyramid and the frequent truncation of 

 the vertical edge of m by the macropinacoid a serves as a ready 

 means of orientation. The next most frequently occurring 

 pyramid is z, which usually occurs beveling the brachydiagonal 

 pole edge of e ; fig. 2, showing also the prism m, a very common 

 combination. The brachydome t is not so common as the above 



* This Journal, II, ii, 250. f Pogg. Ann., clviii, p. 407. 



X Materialien zur Mineralogie Russlands, i, 61. 



