12 STRUCTURAL AND FIELD GEOLOGY 
Anorthoclase (triclinic soda-potash felspar) shows much the same 
structure under the microscope as microcline, but usually on an exceed- 
ingly minute scale. It occurs in the form of phenocrysts with rhombic 
outlines in a well-known igneous rock (rhombenporphyr) met with in S. 
Norway, between Christiania and Langesundfjord. 
The potash felspars generally weather readily into kaolin. 
Not infrequently, however, they are transformed into other 
minerals—muscovite (potash mica) often replacing orthoclase. 
The Plagioclase felspars sometimes occur in crystalline 
masses. As rock-formers, however, they usually appear as 
elongate tabular crystals, not infrequently grouped in bundles 
or forming radiating aggregates, or they may be mere 
crystalline granules. They form a series, of which Addz¢e 
(sodium-aluminium silicate) and Axorthite (calcium-aluminium 
silicate) are the two extremes. The intermediate forms are 
regarded as isomorphous mixtures of these two silicates in 
various proportions, as shown in the following table, where 
Ab stands for albite, and An for anorthite :-— 
Albite (soda felspar). 
Oligoclase : mixture of Ab and An—the former predomi- 
nating. 
Andesine: mixture of Ab and An in nearly equal propor- 
tions—Ab slightly predominating. 
Labradorite: mixture of Ab and An—the latter slightly 
predominating. 
Bytownite: mixture of Ab and An—the latter largely 
predominating. 
Anorthite (lime felspar). 
The silica percentage ranges from 43-16 in anorthite to 68-68 in albite. 
Plagioclase felspars fuse with difficulty before the blowpipe. Anorthite 
is decomposed by hydrochloric acid with gelatinisation, while albite 
resists ordinary acids—the intermediate varieties becoming more 
readily affected the nearer they approach in composition to anorthite. 
All are subject more or less readily to alteration, being transformed 
especially into such minerals as kaolin, sericite (a variety of muscovite), 
and epidote. Saussurite is the name given to an altered plagioclase 
which often occurs in gabbro. It is fine-grained to compact, grey, 
ash-grey, or greenish-white, shimmering or dull, and translucent on thin 
edges. 
Well-developed and more or less perfect crystals of 
plagioclase often occur in the drusy cavities of eruptive rocks; 

