TIM: IIKI.ATION ur MINI;I{.\I.S TO TIM. I.U.MI.NTS 



at (! by the formula, a normal sulphate. When only one of 



Na O\ 

 the hydrogen atoms is replaced, TT _ QySO 2 , an acid salt is formed, 



acid sodium sulphate. 



<r\ _ TT 

 jj, magnesium hy- 



droxide; when hydroxides combine with acids to form salts, 

 all of the hydroxyl (OH) may be replaced, and the salt will 

 be normal, in that there will be present no basic water; as 



| /SOii normal magnesium sulphate ; or again, only a part 



of the hydroxyl may be replaced, 5JSv r ySO 4 , basic magnesium 



sulphate. Malachite is a basic copper carbonate, u _ Q _ ( -t u y 



in which one bond of the copper is taken up by hydroxyl and the 

 other by the acid radicle. In each group of minerals, as the sul- 

 phates, there are possible normal anhydrous salts ; normal salts with 

 water of crystallization, or hydrous salts ; basic salts ; and acid salts ; 

 aiu 1 in some of the complex mineral molecules there may be hydrated 

 and acid and basic water present. 



It is not always possible to determine the structural formula 

 of a mineral, or to tell just what the exact relations of each atom 

 are. The empirical or percentage formula is calculated from the 

 results of analysis, as this is simply the reverse of the calculation 

 of the percentage of any element from a given formula. The 

 percentage of each oxide is divided by its molecular weight, which 

 will give the ratios of the various oxides in the formula; thus 

 in case of the sulphate of calcium, the mineral gypsum: 



RATIO 



CaO = 32.5 -^ 56.1 = .579 = 1 

 SO 3 = 46.6 -^ 80.6 = .577 = 1 

 H 2 O = 20.9 -T- 18.0 =1.15 =2 



Dividing the ratios by the least of their number, as there must be 

 whole atoms in the molecule, we obtain for CaO, one ; for SO 3 , also one ; 

 and for H 2 O, two. The percentage formula will be CaO, SO S 2H 2 O, 

 or CaSO 4 , 2 H 2 O. In the mineral and crystalline molecule cer- 

 tain groups of elements have been found able to replace each other, 

 and at the same time the form or angle of the crystals will be but 

 slightly changed. Such groups of elements are very closely related 

 to each other in several ways. Their molecular volumes are very 



