56 OUR PHYSICAL WORLD 



this effervescence with dilute acids make it easy to determine 

 calcite. The only mineral with which it is likely to be confused 

 is dolomite, a carbonate of magnesium that is heavier, harder, 

 and effervesces in strong acids or in weak ones only when 

 powdered. 



Calcite is very prevalent, forming great beds of rock. Lime- 

 stone, chalk, and marble are made of calcite. The calcite in 

 limestone is usually in grains, while in chalk it is still finer a 

 dust. Marble is derived from limestone through alteration by 

 heat and pressure, and is crystalline ; the calcite in limestone and 

 chalk is non-crystalline. 



Calcite is a representative of several minerals that are also 

 carbonates. The most important as a rock-forming mineral is 

 dolomite, a carbonate of magnesium. Marble which contains 

 much dolomite instead of calcite is known as dolomitic limestone. 



There is one sulphate of calcium that is a frequent ingredient 

 of rocks and that forms extensive beds in certain localities. This 

 is gypsum. The very clear crystals of this mineral are known as 

 selenite, while the pure white amorphous form is called alabaster. 



The term feldspar is used to designate a group of minerals 

 rather than one. They are of unlike chemical composition, 

 though closely similar. In this respect, therefore, the term 

 feldspar is not co-ordinate with quartz and calcite, for these 

 terms indicate single substances of a definite chemical composi- 

 tion. The feldspars are, however, very similar in appearance 

 and have like physical properties. They are all complex sili- 

 cates of certain basic elements, sodium, calcium, potassium, 

 and aluminium. Orthoclase is a silicate of potash and alumin- 

 ium (KAlSi 3 O8); albite, similarly, a silicate of sodium and 

 aluminium (NaAlSi 3 O 8 ), while anorthite is a silicate of lime and 

 aluminium (CaAl 2 Si 2 Os). 



These feldspars occur rarely in rocks as such, but freely as 

 mixtures, two of them being usually fused together. Orthoclase 

 and albite fuse in making a series of potassium-sodium-aluminium 

 silicates. If the orthoclase feldspar is largely dominant in the 



