290 HAROLD L. ALLING 
in his domain are called minerals. Familiarity with this system is 
of great assistance in clarifying the problems of mineralogy. 
3. The feldspars belong to a five-component system, of which 
the end members are the potash, soda, lime, barium, and carnegieite 
feldspars. For most purposes it is only necessary to consider the 
potash, soda, and lime components. Every specimen of feldspar 
found in nature contains a certain amount of each of these three 
components. ; 
4. The plagioclase and the hyalophane series constitute a 
series of solid solutions. The potash-soda and the potash-lime 
series possess only limited solubility, and constitute eutectiferous 
systems. 
5. It is believed that both the potash and the soda feldspars are 
dimorphous, each existing in two isomeric forms: each component 
crystallizing either in monoclinic or triclinic modifications, depending 
upon the temperature and the viscosity of the magma; that ortho- 
clase and albite are high-temperature modifications and that 
microcline and possibly (?) barbierite are relatively low-tempera- 
ture forms. 
6. The complete solubility of albite in anorthite, and vice versa, 
presupposes that their chemical structures are analogous, even 
though albite is usually regarded as a “‘trisilicate”’ and anorthite 
an “orthosilicate.” This presupposes that the feldspars are 
aluminosilicates. Since orthoclase and albite (components) are 
not completely soluble in each other (when perfect equilibrium 
prevails), they probably possess somewhat dissimilar chemical 
structures. 
7. Some feldspars contain nephelite in solid solution but this 
mineral cannot be regarded as isomorphous with the normal feld- 
spars. ‘Therefore the mineralographic term ‘‘solid solution” is 
more comprehensive than the crystallographic term “‘isomorphism.” 
8. All feldspars are solid solutions and mixtures of solid solu- 
tions, and therefore do not possess definite chemical compositions. 
No single chemical formula can be assigned to a single species. 
Inasmuch as labradorite is a mineral, the chemical composition of 
which is not fixed, and, furthermore, the mineral is often found 
zonally grown, the usual definition of a mineral as a homogeneous 
