254 DESCRIPTIONS OF MINERALS. 



Phenacite. A beryllium-silicate, rhombohedral in crys- 

 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 zirconia,. 

 Eucolite is a related species from Norway. 



Pollucile. An isometric caesium silicate, white, vitreous in lustre, 

 with G.=2868- Analvsis afforded Rammelsberg Silica 48 15, alumina 

 16-31, potash 47, soda 2 -48, ca?sium oxide 30 00, water 2 59=100, 

 giving very nearly the bisilicate formula H Q Cs a AlO 20 Si 5 . From Elba* 



II. UXTSILICATES. 



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 

 Ra 4 Si, in which E stands for Ca, Mg, Fe, Mn, K 2 , Ka 2 , or 

 Li 2 , or other mutually replaceable base. In analyses, the 

 mineral is resolved into protoxides and silica, in the ratio 

 of 2 RO to Si0 2 , 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 Ro 12 Si 3 , in which R may stand for Al, Fe, or Mn, etc. 

 Here the mineral is resolved, in analyses, into sesquioxides 

 and silica in the ratio of 2R0 3 to 3 Si 2 , 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 (E 3 . R) 2 12 Si 3 . In this formula the ratio of E 3 to 

 R is not stated. If the ratio is 1 : 1, the formula becomes 

 E 3 R 12 Si 3 , or its equivalent (JE 3 JR) 2 12 Si,. In a case like 

 this last, the mineral is resolved, in analyses, into protoxides, 

 sesquioxides and silica, in the ratio of 3RO : R 3 : 3 Si 2 , 

 in which again the oxygen of the bases equals that of the 

 silica. 



If the proportion of E 3 to R is 1 : 3, this corresponds to 

 £R 3 : R, or, its equivalent, E : R ; and hence the formula in 

 its general form will be RR 8 Si* 



