96 THE CONSTITUTION OF THE SILICATES [bull. 125. 
But beryl alters into mica, a fact which is favorable to the first of 
these formula?, and all of its commoner alterations seem to take place 
by replacement of glucinuin. In tbe trisilicate formula the alumina 
should be equally replaceable, and so far the evidence is adverse to it. 
Furthermore, Traube* has effected the synthesis of beryl by precipi- 
tating a mixture of glucinum and aluminum sulphates with a solution 
of sodium metasilicate, and then crystallizing by fusion of the precipi- 
tate with boron trioxide. Since the starting point was a metasilicate, 
there is a fair presumption that the product was a metasilicate also. 
Beryl can be written as a pseudometasilicate, but there are no data to 
justify doing so. 
Two other minerals containing glucinum, leucophauite, and meliphan- 
ite, appear to be essentially metasilicates with the subjoined formulae: 
Leucophauite Na0aGlFSi 2 O 6 
Meliphanite . NaCa 2 Gl 2 FSi : Ao 
These are capable of several interpretations. The simplest regards 
meliphanite as a mixed meta- and orthosilicate, when both species can 
be written 
Si0 3 — Gl— F 
Si0 3 — Gl— F Ca( 
Ca< ;Si0 4 =:Gl 
\Si0 3 — Na Ca< 
\SiO3— Na 
or else with the glucinum as the linking element, and with the calcium 
united to fluorine. This involves no change of type, but only an 
exchange of position between Ca and Gl. Brogger,t to whom the 
empirical formula? are due, also represents meliphanite as a basic 
metasilicate 
Si<3 3 — Na 
Ca< 
7° 
Ca< 
>Si0 3 
Gl/ 
\SiO3— Gl— F 
which is also justifiable. Between the several alternatives there is no 
ground for deciding. The stability of meliphanite toward ordinary 
acids would seem to favor the formula first given, for a basic calcium 
salt would probably be more easily decomposable. 
Danburite, CaB 2 Si 2 8 , is sometimes regarded as the equivalent of 
barsowite, 0aAl 2 Si 2 O 8 , a doubtful isomer of anorthite. But barsowite 
* Neues Jahrbuch., 1894,. I, p. 275. 
t Zeit. Kryst. Min., XVI, pp. 289-291. 
