220 MINERALOGY 



tendency of one form, the less stable, to pass over to the other, the 

 more stable form, under the fixed conditions. Calcium carbonate 

 when quickly precipitated from solution separates as an amorphous 

 solid, and on standing passes over to the more stable crystalline 

 forms, calcite or aragonite, according to the temperature of 

 the solution, as calcite is the more stable form in cold solutions and 

 aragonite is the more stable form in hot solutions. When the change 

 of phase proceeds in one way only, the compound is said to be mono- 

 tropic, but if the change may go back and forth with the change of 

 temperature, or is reversible, the compounds or phases are said to 

 be enantiotropic. Such a case is represented by sulphur. Sulphur 

 when fused and then allowed to solidify at a temperature above 96 

 will form monoclinic crystals, which are the stable phase between 

 96 and 120, the fusing point of sulphur. Below 96 these mono- 

 clinic crystals become brittle and clouded and pass over to ortho- 

 rhombic sulphur, a phase more stable than the monoclinic form at 

 low temperatures. If the temperature is again raised above 96, the 

 reverse of this will take place and in time the orthorhombic sulphur 

 will form monoclinic crystals. The orthorhombic phase is the 

 more stable below 96, and the monoclinic phase is the more stable 

 above 96. There are several other phases of sulphur, but at ordi- 

 nary temperatures these are all less stable than the orthorhombic 

 phase, and it is for this reason that all natural occurring sulphur is 

 of the orthorhombic phase. Graphite and diamond are different 

 phases of the same element, carbon. There are numerous other 

 examples of dimorphism in minerals, as sphalerite and wurtzite; 

 quartz and tridymite ; smaltite and safflorite ; pyrite and marcasite. 

 Pyroxenes and amphiboles are probably dimorphous conditions or 

 phases of the same compound, and even trimorphic cases occur, 

 as in the three minerals rutile, anatase, and brookite, all three of 

 which are different phases of TiO 2 . 



Source of the elements. Minerals are the source of all the ele- 

 ments ; it has been through the chemical study and the analyses of 

 minerals that, with few exceptions, all the elements have been dis- 

 covered. Those elements which occur in nature in the uncombined 

 state as minerals are few in number and are restricted principally 

 to the group known as metals ; as platinum and the platinum group, 

 silver, mercury, gold, copper, lead, bismuth, iron, arsenic, and anti- 

 mony. Some of these, as iron, lead, antimony, arsenic, and bis- 

 muth, are rare as native elements, though quite common enough as 

 constitutents of minerals when combined with other elements. 



