362 MINERALOGY 



of phosphorus bead, the silica left behind as the silica skeleton is in 

 the form of tridymite. 



HYDRATED OXIDES 

 BRUCITE 



Brucite. Hydroxide of magnesium, Mg(OH) 2 ; MgO = 

 69, H 2 O = 31 ; Hexagonal ; Type, Dihexagonal Alternating ; 

 c = 1.208 ; r A r' = 97 38' ; 0001 A lOTl = 60 20' ; Common forms, 

 c (0001), r (1011), p (2021) ; Cleavage, basal perfect, laminae sectile ; 

 H. = 2.5 ; G. = 2.4 ; Color, white, greenish or bluish ; Streak, 

 white ; Luster, waxy to pearly ; Translucent to opaque ; = 

 1.579; <o = 1.559; -<o = .020; Optically (+). 



B.B. Whitens and becomes opaque ; when it contains Mn 

 or Fe may change to gray on heating. After ignition reacts alka- 

 line with turmeric paper; with cobalt solution becomes flesh- 

 colored (Mg). Infusible; in the closed tube yields much water. 

 Soluble in acids. 



General description. Crystals are rare ; they occur as tabular 

 combinations of the base and rhombohedrons at Texas, Pennsyl- 

 vania. Brucite is usually foliated or fibrous, variety nemalite. 



Brucite is a secondary mineral formed in the zone of hydra- 

 tion from magnesium silicates and is nearly always associated 

 with serpentine, both being derived from a common origin. 

 By the action of carbon dioxide brucite forms hydromagnesite, 

 3 MgCO 2 . Mg(OH) 2 . 3H 2 O. 



It occurs at the Tilly Foster mine, Brewster, New York ; Rich- 

 mond and Westchester Counties, New York. At Bergen Hill, 

 New Jersey, fibrous. Artificial crystals are produced by precipi- 

 tating MgCl 2 with KOH and heating to 200 under pressure; 

 on cooling, brucite s'parates. It has also been noted as a constit- 

 uent of boiler scale. 



Pyrochroite, Mn(OH) 2 , is isomorphous with brucite; found in 

 simple rhombohedrons and in combinations with the base and scale- 

 nohedrons at Franklin Furnace, New Jersey. Crystals are nearly 

 transparent and light in color when first brought to the surface, 

 but soon darken and become opaque on exposure. Found also in 

 several localities in Sweden. 



