RKE.] 
ORTHOSILICATES OF TETRAD BASES. 
77 
commingled in ratios nearly, but not exactly, 1:1. Hydroxy 1 replaces 
chlorine to some extent, while iron and manganese partly replace 
calcium ; but the ratios shown by the formulae are constant, and the 
structural analogies with the allied species are perfectly clear. 
The remaining zirconium silicates, wohlerite, rosenbuschite, lavenite, 
and hiortdahlite, are difficult to formulate conclusively. Brogger 
regards them as meta compounds, and as such they are classed with 
the pyroxenes 5 but the ratio Si : O is in each case very nearly 1:4. In 
wohlerite, which contains columbium, the latter pentavalent element 
may be regarded as forming the dyad group =CbOF, and as equivalent 
to the similar group =ZrF 2 . On this supposition wohlerite can be 
written 
Si0 4 =Ca Si0 4 =Ca 
/ ^CbOF / \zrF 2 
4Zr 
\ 
SiO 4 =0a 
Si0 4 =CaNa 
'SiO 4 =0a»a 
+ 1 Zr 
SiO 4 =0a 
SiO=OaNa 
SiO=OaNa 
with small amounts of the groups R'"F replacing ZrF 2 . As thus writ- 
ten the calculated composition agrees fairly with Cleve's analysis, thus : 
Cleve.* 
Calculated. 
Si0 3 
30.14 
.42 
16.12 
12.85 
.48 
.66 
1.26 
1.00 
.12 
26.97 
7.50 
.74 
2.98 
30.27 
\ 18. 46 
13.52 
Ti0 2 
Zr0 2 
Cb0 5 
Fe,0 3 
CeO 
FeO 
J 28. 26 
7.83 
MnO 
Mg . . 
CaO 
Na 2 ; 
HoO 
F . . 
2.87 
Less 
101. 24 
1.24 
101.21 
1.21 
100. 00 
100. 00 
* Zeit. Kryst. Min., XVI, p. 347. 
The other "zircon pyroxenes" can be interpreted in a somewhat simi- 
lar manner, but the results are less satisfactory. The analyses are 
complicated by the presence of small amounts of titanium, which may 
be either acid or basic, and more data are needed before formulation 
can become useful. 
The typical silicate of thorium, thorite, or orangite is another unsatis- 
factory species on account of its wide variations in composition. It is 
commonly supposed that the mineral, as it exists in nature, has been 
