Mineralogy and Geology. 273 
4. On a Twin Composition between Mala- 
cone and Xenotime, Apatite, or Monazite ; 
by Ernst Zscuav, (communicated by G. J. 
Brusu.)—It isan interesting question whether 
crystals of different kinds can form twins 
Xenotime ; the two are homceomorphous, 
: 1 being 138° 45’ in Xenotrme, and but 
little less in Malacone (138° 15’). 
lalacone also occurs at Hitteroe compounded with apatite and mon- 
azite. The Oon 1 in apatite is 136° 47’. 
pit ; 
_ Subsequently Prof. Dana pointed out its homaeomorphism with Datho- 
lite, and suggested that Prosopite might possibly be an altered Datholite. 
Through the kindness of M. E. Zschau of Dresden, | was put 
possession of a number of crystals of Prosopite, which, although pre- 
senting the same crystalline furm, had-such a diversity of physical char- 
acters, that [ was led to give them a chemical examination, to ascertgyn. 
if possible the true composition of the unaltered mineral. segs: Fe 
wing to the small quantity of the mineral at my copmand, the 
results obtained are for the most part incomplete. 
The physical and chemical characters of the crystals were as follows : 
Color light violet and transparent to white and opaque. H. from 4 to so 
equal to 71°44 milligrammes or 50°4 r. centof calcium. re- 
sult of the decomposition by sulphuric acid was completely soluble in 
water and the solution gave no reactions for any other base t lime. 
The crystals which had a less brilliant color and a diminished hardness 
contained more or less of a hydrous silicate of alumina according to 
the extent of alteration they had undergone, the results of complete al- 
teration being pure kaolin as shown by rer. : 
nall of the crystals that contained even traces of water, alumina 
was also found; and although silica was not in each case detected, there 
* Poggendorff’s Ann. xc, 315. 
Szconp Series, Vol. XX, No. 59—Sept,, 1855. 35 
