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Name: Allen
Status: Other
Age: N/A
Location: CA 
Country: United States
Date: August 28, 2005

How is sand, SiO2 formed in the earth?
Approximately what depth, temperature and pressure?
I already know that sand is made by breaking large rocks into small. What I really want to know is what are the temperature and pressure deep in the earth that make silicon, a mostly unreactive element, react with oxygen to make Si02.

You have made an assumption that SiO2 (and presumably other numerous silicate minerals) were formed from the elements during the formation of the Earth. That is not the case. In fact, elemental silicon is not found anywhere in the Earth's crust. Neither is elemental O2 found in any appreciable amounts in the Earth's crust.

Chemical analysis of the Moon, Mars, asteroids and comets also indicate that silicon is present as various silicate minerals. Geological models suggest that silicates being less dense than other compounds forming the mantle and core of the Earth rose to the surface of the early periods of the Earth's formation accounting for its abundance in the Earth's crust and its relative scarcity deep within the Earth. Also your statement that silicon is "mostly unreactive" is not accurate. If finely divided into a powder it reacts readily with oxygen and halogens. The latter also react readily with water to form SiO2 and the halogen acid so one does not find silicon halides except in lab conditions.

Vince Calder

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