ROCK-MAKING MINERALS. 325 



HDMITE GROUP. 

 CHONDRODITE, HUMITE, AND CLINOHEMITE. 



The humite group includes: 



Chondrodite: 



[Mg(F.OH)] 2 Mg 3 Si 2 8 . 



Monoclinie. 



Sp. gr. 3.1-3.2. 



Humite: 



[Mg(F.OH)] 2 Mg 5 Si 3 12 . 

 Orthorhombic. 



Sp. gr. 3.1-3.2. 



Clinohumite: 



[Mg(F.OH)] 2 Mg 7 SiA 6 - 



Monoclinie. 



Sp. gr. 3.1-3.2. 



In all the above the hydroxide (OH) replaces a part of the fluorine. 



occurrence. — The huniites occur in masses of magnesian limestones and 

 rocks bearing carbonates ejected by volcanoes. Chondrodite has a some- 

 what widespread occurrence in the marbles of eastern United States. In 

 such cases it is sometimes, at least, a contact mineral. Frequently it is 

 accompanied by spinel. 



Alterations. — The mest frequent alterations of the humites are to serpen- 

 tine (monoclinie; sp. gr. 2.50-2.65) and brucite (rhombohedral ; sp. gr. 

 2.38-2.4). In the equations for the alterations the hydroxide will be ignored. 

 The reactions may be written as follows: 



(1) (MgF) 2 Mg 3 Si 2 8 +3H 2 0=H 4 Mg 3 SiA+Mg(OH) 2 +MgF 2 +k. 



(2) 2[(MgF) 2 Mg 5 Si 3 12 ] +9H 2 0=3H 4 Mg 3 SiA+3Mg(OH) 2 +2MgF 2 +k. 



(3) (MgF) 2 Mg,Si 4 1 6+6H 2 0=2H 4 Mg 3 Si 2 9 +2Mg(OH) 2 +MgF 2 +k. 



The increase in volume of the serpentine and brucite as compared with 

 chondrodite, from which it is derived, is 30.15 per cent; as compared with 

 humite, 35.53 per cent; as compared with clinohumite, 38.39 per cent. 



The alterations of humite to serpentine and brucite involve hydration, 

 expansion of volume, and liberation of heat. They are therefore typical 

 reactions of the zone of katamorphism. 



