200 Fretp Museum oF NaturaAL History — GEotoey, VOL. III. 
The allotment of Si Oz to form silicates begins with the bases which occur with 
silica in but one proportion, and is carried on as follows: 
10. To Zr Oz allot Si O2 in proportion of 1 : 1 for zircon. 
ity) Loder ane Al OM in anorthite is allotted equal Si O, to form Ca O.Al, 
O3.2 S1 Or. 
12. To CaO and (Mg, Fe) O in diopside is allotted equal Si O2 to form Ca O. 
(Mg, Fe) O.2 Si Or. 
13. To Naz: O and Fe, O; in acmite is allotted Si O2 to form Naz O.Fe2 O3.4 Si Oo. 
14. To Nay O (or K, O) set aside for metasilicate molecules allot 5i O2 to form 
Nag O.Si O2 or Ky O.Si Or. 
15. To Na: O and Al, O; in sufficient amount to form with Na Cl sodalite, or 
with Naz SO, noselite, is allotted Si O2 to satisfy the formulas : 3 (Na O.Al, Os 2Si 
O.).2 Na Cl, sodalite, 2 (Naz O.Als O3.2 Si Oz). Nae SO, noselite. 
16. To CaO set aside for wollastonite or akermanite is allotted tentatively 
Si O2 to form wollastonite (Ca O.Si1 Oz). 
17. To extra (Mg, Fe) O is allotted Si O2 to form orthosilicate, olivine (2 (Mg, 
Fe) O.Si Oy). 
18. To Al, O; and K,O + Naz O is allotted SiO. to make the polysilicates, 
orthoclase and albite, Ky O.Al, O3.6 Si O2 and Naz O. Aly O3.6 Si Or. 
a) If there is an excess of Si Op: it is added to the orthosilicate of (Mg, Fe) O to 
raise it to the metasilicate (Mg, Fe) O.Si Og. If Si Os is insufficient to convert all 
the olivine into hypersthene it is distributed according to the following equations: 
x + y = molecules of (Mg, Fe) O. 
x 2 = available Si Or. 
y fat 
where x = hypersthene, fe olivine molecules. 
b) Further excess of Si O2 is to be allotted to Ti O2 and Ca O to form titanite. 
These constituents remain as perofskite when there is no excess of Si Ox. 
c) Further excess of Si O2 is reckoned as quartz. 
19. If there is insufficient Si O2 to form polysilicate feldspar out of all the K, O 
and Naz O with Al, Os: 
a) To Ky O.Alz O; is allotted tentatively enough Si O2 to form polysilicate, ortho- 
clase (K» O.Al: O3.6 Si O2) and the remaining Si Oy, is distributed between albite and 
nephelite molecules by means of the equations: 
x + y = molecules of Naz O. 
6x + 2y = available Si Ox. 
where x = albite, and y = nephelite molecules. 
b) If the available Si O2 in case 15, a) is insufficient to form nephelite with the 
Naz O, then enough Si Oz is first allotted to the Naz O to form nephelite and the re- 
maining Si O2 is distributed between orthoclase and leucite molecules by means of the 
equations: 
x + y = molecules of K. O. 
6x + 4y = available Si Ox. 
where x = orthoclase, and y = leucite molecules. 
20. If there is insufficient Si O2 to form leucite and nephelite with olivine it is 
necessary to reduce a sufficient number of molecules to form the subsilicate aker- 
manite, 4Ca O.3 Si Or. 
