52 The American Geologist. •^"'^'' ■^'^''-• 
The values e-w show that hussakite has a very strong- 
double refraction and consequently the fine powder gives very 
vivid interference colors in polarized light. 
Relation of Hussakite to Xenotime. 
From the foregoing, it is evident that several points yet re- 
main to be explained, which we now wish to discuss briefly. 
The different results obtained by Gorceix and ourselves 
are very striking, for the material analyzed by Gorceix is from 
the same locality from which our crystals were obtained. Ac- 
cording to Gorceix, the mineral examined by him is without 
a doubt identical with that which we possess. For the sake 
of comparison we wish to give both analyses side by side. 
Gorceix : Reitinger : 
P2O3 35-64 33-51 
RoO., (!)3-7S 60.24 
FcoOs 0.20 
Insol. 0.40 
SO3 6.13 
99.79 100.08 
The 6.13,^ sulphuric acid found by us is evidently divided 
m Gorceix's analysis between PoO., and RoO.j. We cannot, 
with any degree of certaintv. account for this difference; we- 
infer, however, that Gorceix simply overlooked the sulphuric 
acid and decomposed his material with either concentrated 
sulphuric acid or potassium bisulphate and thus had no oc- 
casion to determine the sulphuric acid, which he had no rea- 
son to suppose present. Nevertheless, we would like to em- 
phasize the fact that all undecomposed hussakite crystals 
from Dattas have the composition given by us above. 
Why is it that the crystal form of hussakite is identical 
with that of xenotime. In general, we are accustomed, when 
two substances have the same crystal form, to consider them 
as identical. This question has a simple solution when we 
consider that all analyses of xenotime, aside from those of 
Gorceix and one by Blomstrand,* were made with more or 
less decomposed material. It seems plausible to consider that, 
which is called xenotime, as nothing else than a hussakite from 
*Zeitsch. f Krystnl. etc. .Id. G8. Blom.«trand indicates that the mineral 
was decomposed with sulphuric acid and hence it was impossible for him to 
determine whether sulphuric acid was present in the mineral or not. 
