Till; RKLATIOX OF MINKKALS To Till, I.U.MIATS 225 



In the classification of minerals the acid radicle has 

 the most import ;uil of the two components, and all minerals occur- 

 ring as sulphates, or the salts of sulphuric acid, have been placed in 

 one .uroup. 



Water in minerals. Some minerals, especially those which 

 arc formed at a low temperature, or which separate from a water 

 -olutiou, contain water, or at least when heated in a closed tube 

 they yield water. This water may be combined with the mineral 

 molecule in various ways, as is indicated by the widely different 

 temperatures at which it is driven off. Water which is driven off 

 at low temperatures is considered to be loosely combined with the 

 other elements of the molecule or with the mineral molecule. It 

 may be directly combined with the mineral as water, though there 

 i> nothing to prove that this is actually the condition or fact. 



Copper sulphate crystallizes with five molecules of water, 

 CuSO 4 , 5 H 2 O. When this salt is heated, four of these molecules of 

 water are driven off at 100 C., but to drive off the fifth molecule of 

 water a'nd to completely dehydrate the salt the temperature must 

 1 K ' raised to 200. Zinc sulphate crystallizes with seven molecules of 

 water, ZnSO 4 , 7 H 2 0, six of which are driven off at 100 C., but the 

 seventh will not be driven off until the temperature of 240 C. is 

 reached. Such facts would indicate that that portion of the water 

 contained in salts which may be driven off at low temperatures is 

 bound up in the mineral molecule in some very simple way, and 

 the bond is easily broken. On the other hand, some is bound up 

 with the molecule very closely and may be dislodged, as is the case 

 with serpentine, only at a red heat. The water which may be 

 driven off at low temperatures is termed water of crystallization, 

 and is usually written or indicated in the mineral formula as water ; 

 thus gypsum, CaSO 4 , 2 H 2 O, contains two molecules of water of 

 crystallization, one of which is liberated at 120 C., the other at 

 200 C. When there are several molecules of water of crystalliza- 

 tion present in a mineral, they are not all liberated at the same tem- 

 perature, but they are given off by steps or one at a time. The 

 evolution of water on heating a crystal is discontinuous. The 

 zeolites present an exception to this rule ; in their case the evolu- 

 tion is continuous. Some of this water of crystallization may be 

 so slightly bound to the chemical molecule, that it evaporates into 

 dry air, and the crystal falls down to a powder, or whitens and is 

 changed in appearance. Such compounds are said to be efflores- 

 cent. Sodium carbonate, NaaCOs, 10 H 2 0, in dry air will lose 

 Q 



