254 DESCRIPTIONS OF MINERALS, 
Phenacite. A beryllium-silicate, rhombohedral in erys- 
tallization. From the Urals, and Durango in Mexico. 
Eudialyte. A pale rose-red mineral, from West Greenland, occurring 
in rhombohedral crystals, and containing 15°6 per cent. of zircoxia. 
Hucolite is a related species from Norway. 
Pollucite. An iscmetric cesium silicate, white, vitreous in lustre, 
with G.=2-868. Analysis afforded Rammelsberg Silica 48°15, alumina 
16°31, potash 0°47, soda 2°48, cesium oxide 30:00, water 2:59=100, 
giving very nearly the bisilicate formula H,Cs,Al0,, Si,. From Elba. 
Ii. UNISILICATES. 
For the convenience of the student, the general formulas 
of the regular Unisilicates are here re-stated. ‘They are as 
follows : 
If the base is in the protoxide state alone, the formula is 
R, O,8i, in which R stands for Ca, Mg, Fe, Mn, Kz, Nas, or 
Li,, or other mutually replaceable base. In analyses, the 
mineral is resolved into protoxides and silica, in the ratio 
of 2 RO to $10,, in which the oxygen of the silica equals 
that of the basic portion. 
If the base is in the sesquioxide state alone, the formula 
is R, O,.Si,, In which & may stand for Al, Fe, or Mn, etc. 
Here the mineral is resolved, in analyses, to sesquioxides 
and silica in the ratio of 2R0O,to38i O,, in which the oxy- 
gen of the silica again equals that of the basic portion. 
If the basic portion is partly in the protoxide state and 
partly in the sesquioxide, the formula, in its most general 
form, is (R,, B).O,. Si, In this formula the ratio of R, to 
R is not stated. If the ratio is 1:1, the formula becomes 
RB O,, Si,, or its equivalent ($R, $B), Oy Si. Ina case like 
this last, the mineral is resolved, in analyses, into protoxides, 
sesquioxides and silica, in the ratio of 3RO:R OMres Mein Oe. 
in which again the oxygen of the bases equals that of the 
silica. 
If the proportion of R, to R is 1:3, this corresponds to 
4R,: RB, or, its equivalent, R: RB; and hence the formula in 
its general form will be RE O; Si, 
