SILICATI S. TITAN ATI'S, ETC. 



4 ()f> 



(Baveno twins), composition face c with twinning axis _LC = (Mane- 



l>aeh twin- 1 ; Cleavage, basal perfect, b less so and M imperfect; 



Brittle: 1 ia<tniv. uneven; H. = 6; G. = 2.55; Color, white to 



light yellow, pink, or red and gniy ; Streak, white ; Luster, vitreous ; 



Transparent to opaque ; a = 1.519; f3 = 1.524; -y = 1.526 j-y- a 



= .007; Optically ( ); Axial plane perpendicular to 010; Bx aA ft 



= 5-7 in the obtuse angle p; 2V = 69 43'; 2E = 120. 



B.B. Fuses at 5, about 1150 C., rounds on thin edges. With 

 potassium flux and the blue glass shows potassium flame. Insol- 

 uble in acids. 



General description. Crystals when well formed are generally 

 combinations of c, m, b, y, or x, elongated parallel to the clino- 



Fio. 455. Crystals of Orthoclase. Two arc Carlsbad Twins, Brice, New Mexico. 



axis ; or less often parallel to the vertical axis. At times the forms 

 o and n appear ; other forms are rare. Crystals are short and stout ; 

 when elongated parallel to the clinoaxis they have a square cross 

 section ; again they may be tabular, flattened parallel to the base 

 or clinopinacoid. Orthoclase also occurs massive, granular, flint- 

 or jasper-like. Twinning is very common, after three laws which 

 are important. 



1. The Carlsbad law, where the twinning axis is the vertical axis, 

 with the composition plane parallel to the clinopinacoid. They 

 may be either interpenetrating or contact twins, and if repeated 

 polysynthetically produce the banded appearance between crossed 

 nicols so common to the feldspar on the base or basal cleavage 

 faces parallel to the clinopinacoid. Carlsbad twins are the most 



