Feldspars are silicates in which one fourth to one half of the
Si4+ ions in the tetrahedra have
been replaced by Al3+, thereby
requiring one additional +1 charge from a positive ion for each
Since feldspars are open, three-dimensional cage structures, they
are lighter than pyroxenes and olivine. In the original stratification
of the planet, feldspars floated to the top, and today make up 60%
of the crust of the Earth by weight.
Quartz, which also is light, is responsible for another 12%. Micas
and clays contribute 5%, the heavier pyroxenes 12%, and dense olivine
only 3% of the crust, with the last 8% being iron oxides and miscellaneous
Granite, probably the most familiar of all rocks, is a mixture
of fine crystals of quartz, feldspar, and mica.
It is interesting to reflect that all of this silicate geology
arose because silicon cannot make double bonds with oxygen.
If by some cosmic readjustment, the size of a silicon atom were
to decrease by 25 %, then such a double bond would become possible.
All of the silicates in our planet would boil away in clouds of
SiO2, gas, leaving behind only a
metallic core of half the present radius, covered by a scum of metal
It is easy to see why the "fitness of the worId" was
used by theologians of the past century as a scientific demonstration
of the existence of a Designer in the universe.
The "fitness" of carbon and silicon for the separate
roles they play on our planet is uncanny.