CALCIUM CARBONATE AND SILICA :?i: 



CHIEF POINTS OF CHAPTER XXIII. 



Calcium Carbonate is very largely found in the earth's crust. It 

 occurs pure and crystalline in calcite and aragomte and in a less pure 

 condition in chalk, limestone, stalactites, stalagmites, travertine, etc. 



Chalk, Limestone and Coral are composed chiefly of calcium car- 

 bonate, and consist almost entirely of the remains of animal 

 organisms. 



Marble also is a form of calcium carbonate. It is really limestone 

 which by the effects of great pressure and heat in the earth's crust 

 has become much harder and crystalline. 



Silica, Si0 2 , is even more widely found than calcium carbonate. It 

 constitutes, either free or combined with bases to form silicates, 

 about one-half the weight of the earth's crust. 



In the free state it occurs both crystalline and amorphous. Two 

 crystalline varieties are known, one, tridymite, is unimportant. The 

 other, quartz, is very abundant. 



Many sands are made up entirely of grains of quartz. 



Amorphous Silica is found in three forms (1) chalcedony, (2) jasper, 

 (3) opal. Carnelian, agate, and flint are common kinds of 

 chalcedony. 



Rocks formed of Silica. Sinter results from the deposition of the 

 silica dissolved in the water of hot springs and geysers. 



Diatomaceous Earths and Tripoli Powder are rocks composed of 

 silica which was extracted from the water in which they lived by 

 minute plants known as diatoms. 



Radiolarian Earths are rocks composed of silica which was ex- 

 tracted from the water in which they lived by minute animals known 

 as Radiolaria. 



Chemically prepared Silica is a white powder which is insoluble 

 both in water and in most acids. Fused with caustic soda it forms 

 sodim silicate, which is soluble in water and is known as water-<j!>(**. 



QUESTIONS. 



1. What is silica ? How does it occur in nature ? 



2. How do flint and quartz differ from and resemble one another ? 



3. Give a short account of the naturally occurring forms of 

 silica. 



4. What is calcite ? Describe its appearance and give the formula 

 representing its chemical composition. 



5. Many rocks are deposited through the agency of living 

 organisms. Mention some and briefly indicate how they are thus 

 deposited. 



6. What are stalactites and stalagmites ? Of what do they con- 

 sist and how are they formed ? 



7. Describe three naturally occurring forms of calcium carbonate. 



8. What effect has chalk upon water which flows through or over 

 it? 



