522 MINERALOGY 



BORACITE 



Boracite. Mg 7 Cl 2 Bi 6 03o ; A chloride and borate of magnesium ; 

 MgO = 31.4, Cl = 7.9, B 2 O 3 = 62.5; Pseudo-isometric; Type, 

 Ditesseral Polar; Common forms, a (100), d(110), o(lll); 

 Twinning plane, 111; Cleavage, 111 traces; Fracture, conchoid al ; 

 Brittle ; H. = 7 ; G. = 2.9-3 ; Color, white, gray, pale yellow, or 

 green; Streak, white; Transparent to translucent; Double re- 

 fraction below 265 ; a = 1.662; p = 1.667; -y = 1.673; -y - a = 

 Oil. 



B.B. Fuses easily at 2, intumesces and yields a green flame. 

 With cobalt solution a flesh color (Mg). Yields a chlorine reaction 

 with CuO, soluble in HC1. The massive variety yields water. 



General description. Crystals cubic or tetrahedral in habit ; 

 generally isolated simple crystals, or massive and granular. The 

 faces of the plus tetrahedron are bright and highly polished, while 

 those of the minus form are dull. The polar nature of boracite is 

 brought out on heating, when the plus and minus tetrahedrons show 

 opposite polarity. 



Boracite is dimorphous, the orthorhombic form is stable at ordi- 

 nary temperatures and passes over to the isometric form at 265. 

 The beautifully formed isometric crystals at ordinary temperatures 

 are complex aggregates of orthorhombic twins and twinning lamel- 

 lae, as is shown by their double refraction, all of which disappear 

 when heated to 265, at which temperature the mineral becomes 

 isotropic, and even the polarity disappears. Chemically magne- 

 sium may be replaced by ferrous iron, as in the green boracites. 

 Artificial boracite has been formed in which Zn, Cd, or Ni enter as 

 the base, while the chlorine has been replaced with iodine. 



Boracite occurs in the Stassfurt salt deposits, imbedded in the 

 gypsum, anhydrite, and carnallite. 



Stassfurtite is a massive variety, which is softer than the crys- 

 talline and yields water in the closed tube. 



BORAX 



Borax. Na2B 4 O 7 . 10 H 2 O ; Hydrous sodium pyroborate ; 

 Na-iO = 16.23, B 2 O 3 = 36.65, H 2 O = 47.12 ; Monoclinic ; Type, 

 Digonal Equatorial ; a : b : c - 1.0995 : 1 : .5632 ; p = 73 25' 



