THE MINERALOGRAPHY OF THE FELDSPARS 207 
On the other hand Vogt speaks of the “probable occurrence of 
an inversion point in the potassic feldspar from an alpha to a beta 
modification, viz., from orthoclase to microcline.’”’ Barbier? has 
expressed the opinion that potash feldspar exists in two distinct 
forms which he regards as dimorphous, and which he says is a 
case of polymorphism. Assuming that there are two dimorphous 
forms it is not known whether the difference is due to “‘polymer- 
ism” or “‘isomerism.”’ ‘The term “‘polymerism” is applied where two 
compounds possess the same chemical composition but differ in their 
molecular weights.* For example, butyric acid, C,HsO., and alde- 
hyde, C,H,O, differ by polymerism. On the other hand “isomerism”’ 
is used when two or more compounds possessing the same chemical 
composition differ in physical or chemical properties but have the 
same molecular weight, such as levulose and dextrose. If the 
difference between orthoclase and microcline is due to polymerism 
then the following formulae might express the relation: 
(KAISi,Os)1 and (KAISi,Os) (n--m) 
where and m are whole numbers. If the difference is due to 
isomerism then the kinship of the two minerals may be repre- 
- sented, as suggested by Clarke,‘ as follows: 
O=Si—O—K O=Si-O—-K 
O O 
Si—O = 
OF 0-2 Al 4 5 
Osi La 
0 
t As cited by C. H. Warren, Proc. Amer. Acad. Arts. Sci., LI, No. 3 (1915), p. 144. 
2 Ph. Barbier, ‘‘ Researches sur la composition chemique de feldspaths potassique,”’ 
Bull. Soc. frang. minéral., XX XI (1908), 152-67. 
3 Since the examination of crystals by X-ray spectra has shown that our conception 
of molecules in solid matter has no real basis, it is well to bear in mind that such 
expressions as here used are subject to subsequent revision. 
4F. W. Clarke, U.S. Geol. Surv. Bull. 588, p. 12. 
