clarke] NEPHELITE TYPE. 19 
observed by Doelter,* that natrolite on fusion yields nephelite. The 
connection between the two species is unmistakable. 
Eucryptite and nephelite both alter with great ease into muscovite, 
a potassium salt of which paragonite is the sodium equivalent. Fur- 
thermore, C. and G. Friedel, f by heating finely divided muscovite to 
500° in a solution of alkali, obtained nephelite in crystals. From this 
evidence the formulae of muscovite and paragonite become directly 
related to those of the nephelite series, thus : 
Nephelite Al 3 (Si0 4 ) 3 :N"a 3 
Muscovite Al 3 (Si0 4 ) 3 KH 2 
Paragonite Al 3 (Si0 4 ) 3 NaH 2 
Physically, the two micas have no resemblance to nephelite, being 
different in form, slightly denser, and refractory toward acids. The 
relationship is purely one of chemical type, and is established by the 
fact of alteration from one into another. 
Through muscovite a connection is recognizable between the forego- 
ing species and the two minerals andalusite and topaz, whose simplest 
formulae, tripled, may be written as follows: 
Topaz Al 3 (Si0 4 ) 3 (AlF 2 ) :i 
Andalusite Al 3 (Si0 4 ) 3 (AlG) 3 
Here we encounter the evidently univalent atomic groups 
-A1=0 and -Al/ 
\F 
both of which play an important part in various other minerals. The 
two species, topaz and andalusite, are closely allied crystallograph- 
ically. They have sensibly identical molecular volumes, and both 
undergo alteration into muscovite mica. In topaz, as shown by the 
investigations of Penfiekl and of Jannasch, hydroxyl commonly 
replaces a part of the fluorine; hydroxyl and fluorine being clearly 
isomorphous. The formula given is that of normal topaz, entirely free 
from alteration. 
To sum up, we have now eight definite species represented by the 
fundamental type Al 3 (Si0 4 ) 3 E' 3 , the first substitution from the normal 
orthosilicate of aluminum. Leaving hydronephelite on one side, because 
of its water of crystallization, the remaining seven species fall into three 
subtypes, as follows : 
Nephelite. Muscovite. Topaz. 
,Si0 4 ^Na 3 .SrD 4 „KH 2 /Si0 4 (A1F 2 ) 3 
Al-SiG 4 Al Al-Si0 4 ZAl Al-Si0 4 : Al 
\si0 4 ^ZAl ^SiO.L^Al ^SiO^Al 
symbols which clearly indicate the known chemical relations between 
the several minerals. In five of the eight examples the simplest pos- 
* Allgom. Chem. Miiieralogie, p. 183. t Bull. Soc. Min., XIII, p. 129, 1890. 
