K F. Ayres — Notes on the Crystallization of Trona. 65 



Art. YII. — Notes on the Crystallization of Trona (Urao); 

 by Edward F. Ayres. 



An examination has been made by the writer of a series of 

 crystals of trona, in part natural crystals from Borax Lake, 

 San Bernardino Co., California, and in part artificial crystals, 

 furnished through the kindness of Dr. Thomas M. Chatard. 



The natural crystals are of considerable size, up to 15 mm in 

 length, but they were rough and so covered with saline incrus- 

 tations that they afforded no good measurements. In habit 

 they were flat and tabular with the basal plane largely devel- 

 oped and some indistinct orthodomes all deeply striated ; they 

 were terminated by the usual pyramidal planes (o). 



The artificial crystals gave much better opportunity for accu- 

 rate crystallographic work. These are slender acicular crystals 

 very much elongated in the direction of the orthodiagonal 

 axis. They average from 8 to 15 mm in length and about l mm in 

 diameter ; they are usually grouped in little radiating clusters. 

 The different samples received from Dr. Chatard vary among 

 themselves chieflv in size ; some of them being excessively 

 slender. Those which were subjected to measurement were 

 from one of the samples mentioned by Dr. Chatard. The sym- 

 metry of these crystals may be viewed as almost orthorhombic, 

 the angles ae, ec, and ao", co respectively varying but little 

 from each other. This is shown in fig. 2, a projection on the 

 clinopinacoid plane. 



The crystallization of trona was first described by Haidinger* 

 in 1825, and recently Zepharovichf has given a new determina- 

 tion of the form with a number of new planes ; he gives as the 

 composition Na 6 C 4 O u + 5H 2 0. The position here adopted 

 is that of Zepharovich. The planes observed are : 



a(100, i-i), c(001, 0), e(101, -1-i), s(302, §*), jp(Ill, -1), o(lll, 1), r(211, -2-2). 



The two planes j? (HI) and r (211) are new ; p (111) is quite 

 brilliant though small, and r (211) gives angles close enough 

 for identification. The habit of the more complex crystals is 

 shown in fig. 1. There is very perfect cleavage parallel to the 



*Pogg. Ann., v, 367, 1825. f Zeitschrif t I Kryst., xiii, 135, 1887. 



Am. Jour. Sci.— Third Series, Vol. XXXVIII, No. 223.— July, 1889. 



