﻿316 E. S. Dana — Brookite from Magnet Cove, Ark. 



In figures 9 and 10 a common type of crystal is represented 

 in which the macro-prism I predominates. These crystals are 

 often quite flat parallel to the macropinacoid from the oscilla- 

 tory combination of the two prisms I and m, and the termina- 

 tion is often formed by narrow faces of the pyramid e. In fig. 

 18 the prism I is alone and the appearance of the crystal is 

 strongly in contrast to the stout nearly square prisms of figs. 

 1, 2, etc. 



The remainder of the crystals figured are essentially pyram- 

 idal in type. The most interesting of these is figure 17, in 

 which the planes m and t are "in equilibrium" and thus form 

 a nearly regular hexagonal pyramid. This is one of the most 

 common and striking types occurring at the locality; many of 

 the crystals occurring implanted upon massive quartz have this 

 habit. The front edge of the prism m is not infrequently trun- 

 cated by the macropinacoid a, and sometimes the planes c, z, % 

 and t are also present as shown in figs. 20, 22, 23 : these more 

 complex forms still preserve the hexagonal aspect by the pre- 

 domination of e and m. Fig. 19 is a form, observed by Pen- 

 field, of small crystals implanted thickly on a porous siliceous 

 gangue. 



In fig. 21 a form is shown in which the pyramid e predomi- 

 nates, only modified by z and m ; in fig. 24 m is absent and 

 only the pyramid e present with its front edge beveled by z. 

 The ordinary projection, used in fig. 21, fails to give the true 

 effect of this type, which is that of an acute nearly square 

 pyramid, elongated in the direction of the a axis. The shape 

 is better exhibited in the basal projection of fig. 24. This 

 form is interesting in the same way as fig. 1, in that it approxi- 

 mates in form and angle to the common acute octahedron of 

 the third form of titanium dioxide, octahedrite. The pyram- 

 idal angles of the latter species are 82° 9', 82° 9' and 43° 24' 

 (basal), while the corresponding angles of brookite are 84° 38', 

 78° 57 7 and 44° 23'. This type of crystal is often altered to 

 rutile. It may be added that the crystal figured in 24 was 

 most strikingly like the pseudomorphs of wolfram after scheelite 

 from Monroe, Connecticut. 



As has been remarked by Penfield, the brookite from Mag- 

 net Cove is ill adapted for close measurements. Many of the 

 planes are striated, as those of the prismatic zone, and those of 

 the zone z ) %, %', z'. Even when the planes are seemingly 

 smooth they give uncertain or multiple reflections. The fol- 

 lowing measurements obtained from the best of the crystals 

 under examination, are however, worth recording, though not 

 deserving of great weight. 



