clarke] GARNET-BI0T1TE TYPE. 23 
Al 2 (Si0 4 ) 3 Na 4 H(AlC0 3 ), in winch a little soda is replaced by lime, and 
the univalent group — A1=C0 3 may be partly substituted by— Al = Si0 3 . 
Mierosommite, according to the published analyses, varies in composi- 
tion widely, containing always potassium, and having a notable propor- 
tion of chlorine and sulphuric acid among its constituents. If, how- 
ever, we assume m it the univalent radicles — A1=C1 2 and — Al=S0 4 ,its 
composition reduces easily to the form Al 2 (Si0 4 ) 3 (NaK) 3 Ca(Al(S0 4 .Cl 2 )), 
like cancriuite, both species having then the composition of the general 
type A^SiOj^R'c;. The theory as proposed, then, assumes univalent 
complex radicles for cancriuite and mierosommite, and bivalent radicles 
for the sodalite group, thus: 
In cancriuite group. In sodalite (jroup. 
-A1=C1 2 =A1-C1 
-A1 = S0 4 =A1-SG 4 -Na 
-A1=C0 3 = A1-S 3 -Na 
-Al = SiG 3 
and the typical structures are as follows : 
Cancriuite. Sodalite. 
/ Si0 4 =Na 2 (AlC0 3 ) / Si °^>Al l lci 
Al - Si0 4 HNa 2 H Al - Si0 4 =Na 2 
\si0 4 EiAl \si0 4 "Al 
Concerning these formulae and the peculiar uni- and bivalent radi- 
cles assumed in them, there are arguments both for and against. 
The assumption of a group — Al = Si0 3 , equivalent to and replacing 
— Al = 0O 3 , is clearly suggested by the experiments of Lemberg,* who, 
by the action of sodium silicate solution upon elaeolite, obtained a com- 
pound which he designates as a cancriuite containing !S"a 2 Si0 3 in place 
of Na 2 CO> By similar reactions with sodium carbonate he produced a 
substance having the composition of true cancriuite. Hence, whatever 
the ultimate molecular structure of cancriuite may be, we are amply 
justified in assuming in it the replaceability of 0O 3 by Si0 3 . 
These experiments fairly represent a large number of like kind which 
are due to Lemberg, and which may be found recorded in his various 
papers. Some of these will be cited later, but a reference to the work 
of his colaborer Thuguttt is in place at this point. Starting from a 
hydrated nephelite, artificially prepared from kaolin, Thugutt suc- 
ceeded in producing a large series of compounds analogous to sodalite, 
in which the original silicate had taken up, at moderately high degrees 
of heat and pressure, various other salts of scdium, such as the chlorate, 
selenate, formate, oxalate, and so on. These compounds, however, are 
all hydrated, and so differ from the natural minerals of the sodalite 
group, and they ai;e regarded by Thugutt as formed by molecular 
* Zeitsch. Deutsch. Geoloy. Gesell., 1885, i>. 962. t Mineralcbeiuische Studien, Dorpat, 1891. 
