20 SOILS AND MANUEES 



ing from the action of carbonic and organic acids on ferru- 

 ginous minerals. The rusty deposit often seen in the 

 drainage water from moorland consists of limonite. 



Pyrites. Sulphide of iron, FeS 2 , is easily recognised 

 by its peculiar golden lustre. It is often present in slates and 

 shales and sometimes in clays, to which it imparts a greenish 

 tint. In large quantities it is poisonous to vegetation. 



Fluorite. Calcium fluoride, CaF 2 . 



Calcite. Calcium carbonate, CaC0 3 . Arragonite has the 

 same chemical composition, but differs in crystalline form. 

 Limestone and chalk also consist of calcium carbonate, but 

 are of organic origin and are not regarded as minerals. 



Gypsum. Calcium sulphate, CaS04 + 2H 2 0, contains 

 water of crystallisation and is perceptibly soluble in water. 



Apatite. Several varieties are known ; the commonest, 

 if not decomposed, may be represented by the formula 

 3Ca 3 P 2 08 + CaF 2 . This is called fluor apatite to distin- 

 guish it from those in which the fluorine is wholly or partly 

 replaced by chlorine, and which are known as chlor- 

 apatites. When pure the crystals exhibit a blue or green 

 colour, but some are grey, white, or colourless and trans- 

 parent. Apatite enters into the composition of rocks ; it is 

 also found massive, and is fairly widely distributed. 



Quartz. Quartz, a crystalline form of silica, Si0 2 , is one 

 of the commonest of all minerals. The white pebbles found 

 on the seashore and the large white boulders often seen 

 on mountains consist of quartz. This variety is, from 

 its appearance, often called "milk quartz." Tinted 

 varieties, e.g., "rose quartz," "amethyst quartz," 

 " smoky quartz," etc., are also common. The colours are 

 due to the presence of traces of metallic oxides. " White 

 sand " is simply powdered quartz, i.e., it consists of frag- 

 ments of the crystals. Quartz also enters largely into 

 the composition of many common rocks, both igneous and 

 sedimentary. 



