4O THK CHICAGO ACADEMY OF SCIENCES. 



and through green glass orange yellow. All of the specimens 

 examined show some sodium. The greater the amount of 

 soda the more easily does the mineral fuse. It is unacted upon 

 by acids. Orthoclase decomposes easily when attacked by 

 carbonated or alkaline waters, and frequently the decomposi- 

 tion proceeds until soda and potash are completely washed 

 away, leaving only the aluminum silicate. If the transforma- 

 tion has proceeded but part way, so that about y$ of the potas- 

 sium remains, m'uscovite may be produced. The potassium 

 lost in these transformations passes in the form of a carbonate 

 in solution over to a gypsum or other sulphate, and forms 

 potassium sulphate, whieh readily furnishes the amount of 

 potassium necessary for plant growth. Hence decomposing 

 feldspars produce a fertile soil. 



Hicroperthite. 



Several specimens of a flesh-red feldspar yield such reac- 

 tions for sodium as to suggest that the intergrowth of various 

 lamellae, giving to the specimen the structure of microcline. 

 These lamellae consist of orthoclase and albite leaves such as 

 form the vraiety of feldspar known as microcline perthite. 



Microcline. 



Microcline crystals very closely resemble orthoclase both 

 in form, color and chemical reaction, but microcline consists of 

 numberless thin lamellae which form repetition twins parallel 

 to the clinopinacoid (oio), and show on the lustrous base (ooi) 

 and the brachydome (loi) very fine striations parallel to the 

 edge formed by the meeting of the base and the brachypinac- 

 oid. There are also lamellae parallel to the macropinacoid 

 (ioo) which are especially evident on sections parallel to the 

 base (ooi) in polarized light. These are thin sheets of ortho- 

 clase or albite, and they cause the characteristic latticed text- 

 ure seen under crossed nicols. This mingling of orthoclase 

 and albite is so common as to appear in many orthoclases in 

 granites and gneisses. The cleavage angle between the base 

 and the brachypinacoid is only 30' less than a right angle. In 

 orthoclase it is a right angle. Sections parallel to the base 

 under the microscope show an extinction angle with the edge 

 formed by base and brachypinacoid as high as 15, while ortho- 

 clase has parallel extinction. Sections parallel to the brachy- 

 pinacoid are similar to those of orthoclase. Tschermak 



