138 



FELDSPAR. 



(Mineralogy of New- York, page 334.) 



The recent researches of Erdmann, in regard to this mineral, deserve 

 to be noticed. The following abstract is given by Berzelius, in his 

 Annual Report for 1847. He makes four different species of feldspar, 

 which are easily distinguished from each other : 



1. Orthoclase. (feldspar, with a base of potash.) Sp. gr. from 2*50 

 to 2*60. Before the blowpipe it melts with more or less difficulty, and 

 yields a bubbly or tuberculous pearl. 



2. Albite, (feldspar, with a base of soda,) Sp. gr. 2*59 to 2*65. 

 Before the blowpipe it melts more easily than the preceding, and gives a 

 bubbly, semitransparent pearl. 



3. Oligoclase, (natron spodumen.) Sp. gr. 2*61 to 2*69, and rarely 

 2*70. One of the faces of cleavage presents very fine striae. It melts 

 easily, and gives a pearl free from bubbles, sometimes transparent, 

 sometimes opaline, and sometimes like enamel. These differences 

 seem to be due to the proportions of lime which they contain. 



None of the three above described, are sensibly acted on by muriatic 

 acid. 



4. Labradorite. This sometimes presents a striated surface, like the 

 preceding. Sp. gr. from 2*67 to 2*73. It melts more easily than oli- 

 goclase, and gives rise to a transparent or opaline pearl ; but its most 

 distinctive character is its solubility, when pulverized, in muriatic acid. 



Mr. Alger, in the supplement to his edition of Phillips' Mineralogy, 

 (page 420) gives a figure of a twin crystal of feldspar from Hammond, 

 St. Lawrence county. It has smooth planes, is very perfect, and is one 

 of the simplest of the twin forms. 



To the locality of crystallized albite, noticed in my Mineralogy, I 

 must now add the hemitropic form found by Dr. Emmons in the Coal 

 Hill mine, St. Lawrence county. Rep. on the Geol. 2d Dist. f p. 366. 



