SILICATES, TITANATl.s. KTC. 4X<) 



HiKAla(SiO 4 ) 8 . In paragonito H 2 Na takes the place of one Al, in 



biotite IIK(Mg.Fe) 2 is substituted for two Al, and in phlogopite 

 II KMg, takes tlir place of three Al. In all the species hydrogen 

 appears as one element, either as basic or acid, and therefore all 

 micas yield water at a high temperature. Their formation in nature 

 must be a hydn (thermal reaction under pressure. Mixed with the 

 orthosilicate in some species is a trisilicate (I^SisOg), and when this 

 appears in sufficient amounts the percentage of silica may be that 

 of a metasilicate, as in lepidolite. 



They are all monoclinic and assume a pseudohexagonal symmetry, 

 as the angle of the unit prism is 120, which in combination with 

 the pinacoid yields a basal section of hexagonal outline, parallel to 

 which is the characteristic perfect basal cleavage known as mica- 

 ceous. 



MUSCOVITE 



Muscovite. A potash mica; H 2 KAl3(Si0 4 )3 ; K 2 O = 11.8; 

 Al,() 3 = 38.5; SiO 2 = 45.2; H 2 O = 4.5; Monoclinic; Type, 

 Digonal Equatorial ; a : b : c = .5773 ; 1 : 3.3128 ; p = 89 54' = 

 001,100; 100,110 = 30'; 001 A 101 = 80 12' ; 001 A 011 = 73 12'; 

 001 A 221 =85 36'; Common forms, c(001), m (110), b (010) ; 

 Twinning plane, 110; Cleavage, basal micaceous; LaminaB 

 flexible, elastic, and tough-; H. = 2-2.5; G. = 2.76-3; Color, gray 

 and shades of brown, green, rarely red; Streak, white; Luster, 

 vitreous; Transparent to translucent; a = 1.557; (3 = 1.587; 

 y = 1.590; -y - a = .038; Optically ( - ) ; Axial plane J.010; 

 Bxa-L to the base ; 2 V = 64-72. 



B.B. Fuses quietly at 5.5. Yields some water in the closed 

 tube. Not decomposed with sulphuric acid. 



General description. Crystals are short, stout prisms or tabu- 

 lar combinations of the prism, base, and brachypinacoid, pseudo- 

 hexagonal in section and outline. Cleavage laminae yield per- 

 cussion figures indicating a secondary cleavage or a parting. If a 

 blunt-pointed instrument is placed on a cleavage piece of muscovite 

 and struck a sharp blow, a six-pointed star will be produced at the 

 point of contact, formed by the intersection of three straight cracks 

 intersecting each other at an angle of 60. The cracks are parallel 

 to the six sides of the basal section, and the crack parallel to the 

 brachypinacoid is more developed or larger than the other two, indi- 

 cating the monoclinic nature of the mineral ; for if it were hexag- 



