Clarke and Diller—Topaz from Stoneham, Me. 383 
ho trustworthy means of ascertaining their true molecular 
weights. The formula of topaz, for example, may either be 
the simple expression given above, or a multiple of that; and 
our only method of even approximately solving the problem is 
to study the reactions of topaz, or, in other words, its modes of 
alteration. Now the simplest formula for pure damourite, and 
one which is commonly recognized, is Al,KH,(SiO,), That 
', damourite is far more readily and probably figured as an 
orthosilicate than in any other way; and this fact tends to sus- 
tain the orthosilicate formula for topaz. 
nasmuch as the formula of damourite contains three SiO, 
sroups, we may now triple the simplest formula for topaz, and 
Write in probable structural fashion the formule of the two 
minerals side by side. The results will be suggestive, even 
standing by themselves; but more so if we consider at the same 
me a number of other allied species. These are xenolite, 
fibrolite, paragonite, and eucryptite. Xenolite, which is opti- 
cally identicai with fibrolite, has been reéstablished as a distinct 
mineral by the analysis of Wiik; fibrolite is unquestionably 
telated to topaz; paragonite is the sodium analogue o 
damourite, and eucryptite is especially noteworthy for its altera- 
bility into hydro-mica. Tripling the formule commonly as- 
sie to fibrolite and eucryptite, the series may be presented 
§ lollows: 
/Si0-=Al SiO =(Al0), /8i0 =(AlF,), 
Al—Si0‘=A] Al—Si0,=Al Al—Si0 = Al 
N\SiO=Al \ SiO =Al \ SiO =Al 
Xenolite. Fibrolite. Topaz. 
/ SiO =KH, 7 SiO =NaH, / Si0,=Li, 
Al—Si0 =Al Al—Si0 =Al Al—Si0,=Al 
\Si0O =Al \ SiO =Al \ SiO =Al 
Damourite Paragonite Kueryptite 
Xenolite, it will be observed, is the normal orthosilicate of 
dluminum, and from it, by a simple process of substitution, the 
others may be directly developed. Ina future paper an effort 
will be made by one of us to show that the formule of all 
orthosilicates containing aluminum may be similarly derived, 
0 such a way as to bring out most clearly and unmistakably 
SSi0 Nsi0= Al 
H, - Si "a 
Al—Si0‘=Al "—H,O = Al—si0.=Al 
Al 
