24: THE MINEEALS CONSTITUTING BOOKS 



onclary chlorite that gives the characteristic color to many of the 

 so-called greenstones (altered basalts, diabases, and diorites). 



Oalcite (Calcium Carbonate). — Composition: CaC03 = Car- 

 bon dioxide, 44% ; lime, 56%. Hardness, 3. 



This is an original constituent of many secondary rocks^ 

 such as limestone and calcareous shales. It is the essential con- 

 stituent of most marbles, of stalactites, travertine, and calc-sinter. 

 The shells of foraminifera, brachiopods, crustaceans, and many 

 lamellibranchs and gasteropods are also of this material. As a 

 secondary constituent, resulting from the decomposition of other 

 minerals, it occurs filling wholly or in part cavities in rocks of 

 all ages. 



The effervescence of the mineral when treated with a dilute 

 acid furnishes the most ready means for its detection. Under 

 the microscope it appears as colorless grains with faint irides- 

 cent polarization, and is best recognized by its cleavage and 

 characteristic twinning lines as shown in the figure on p. 142. 

 Being soluble in carbonated waters, it is liable to complete re- 

 moval, or leaves only its impurities behind as a mark of its decay. 



Aragonite. — Composition: CaC03 = Carbon dioxide, 44%; 

 lime, 56%. 



This mineral has the same chemical composition as calcite, 

 but differs in its crystalline form and specific gravity. It is 

 found with beds of gypsum and veins of ore, and also in stalac- 

 titic and stalagmitic forms. In small quantities it occurs as a 

 secondary product in many trap rocks and basalts, and is the 

 substance of which the shells of many gasteropods and lamelli- 

 branch moUusks are composed. 



The mineral is distinguished from calcite by its crystallization 

 and cleavage. As a rock constituent it is comparatively unim- 

 portant. This form of calcium carbonate, as long ago pointed 

 out by Sorby, is less stable than calcite, and in many instances 

 where the substance has first crystallized in the orthorhombie 

 form aragonite, it is found to have undergone a molecular altera- 

 tion into calcite. 



Dolomite. — Composition: (CaMg)C03= Calcium carbonate^ 

 54.35% ; magnesium carbonate, 45.65%. Hardness, 3.5-4. 



This mineral, like calcite, is wide-spread, and forms extensive 

 masses which are of value as sources of building material. It 

 is distinguishable from calcite by its greater hardness, higher 

 specific gravity, and in being but slightly acted on by dilute 



