102 MINERALOGY AND LITHOLOGY. 



63. MiCROCLINE [K' Al Si' O'']. 



Microcline is a feldspar of the same composition as orthoclase, but 

 differs from it in being triclinic. Its position as a species has been fixed 

 by Des Cloizeaux. The difference in the angle of its planes that deter- 

 mines it to be triclinic is very slight, for the angle between the base and 

 the clinopinnacoid varies but 16' from a right angle, — a result obtained 

 as the mean of many measurements. This difference is of course imper- 

 ceptible to an ordinary observer; but the optical distinctions are so 

 marked that its determination becomes very easy. As already explained, 

 a basal section of orthoclase is black between crossed Nicols when the 

 edge of the clino-pinnacoid is parallel to the plane of vibration of the 

 light. If, now, a section of microcline is placed in the same position, it 

 will be brightly colored, and will not be dark until it is revolved 15° from 

 this position. This indicates that a plane of elasticity normal to the 

 plane of the lateral axes cuts the base, making an angle with an axis, — a 

 property of triclinic crystals. 



Microcline is most commonly green, forming what is called Amazon 

 stone. At the Notch very pretty crystals of green orthoclase occur, and 

 also a more massive variety. I have examined this feldspar, and, as might 

 be anticipated, it proves to be microcline. As is the case with microcline 

 in general, it possesses a complicated structure induced by twinning, 

 which takes places in planes parallel and perpendicular to the clino-pinna- 

 coid, and accompanied with this twinning there is also an interlamina- 

 tion of orthoclase in the same planes. Therefore a basal section of our 

 Amazon stone, when examined in polarized light, appears as made up of 

 innumerable laminae running at right angles to one another. Its appear- 

 ance between crossed Nicols is illustrated in Fig. 2 on PI. 8, which is drawn 

 from a section of this Notch microcline. It is placed in the figure in such 

 a position that the edge of the brachy-pinnacoid is parallel to the plane 

 of vibration of the light, as indicated by the lines. Now, in this position 

 the orthoclase will be black like the field to the left, and it is plainly seen 

 in bands of various lengths and breadths, while the larger part, being 

 microcline, is highly colored, and one part of it becomes dark on turn- 

 ing 15'^ to the right, and the rest, on turning the same amount to the left. 



Now in sections of our granites and gneisses, crystals with this struct- 



