What is the mechanism of formation of natural large hexagonal quartz
crystals.  Is there an aqueous solution from which they crystalize?

"Quartz is one of the most common minerals in the Earth's continental
crust. It has a hexagonal crystal structure made of trigonal
crystallized silica (silicon dioxide, SiO2), with a hardness of 7 on
the Mohs scale. Density is 2.65 g/cm³. The typical shape is a
six-sided prism that ends in six-sided pyramids"
http://en.wikipedia.org/wiki/Quartz

Natural formation:

Quartz occurs in hydrothermal veins and pegmatites. Well-formed
crystals may reach several metres in length and weigh hundreds of
kilograms. These veins may bear precious metals such as gold or
silver, and form the quartz ores sought in mining. Erosion of
pegmatites may reveal expansive pockets of crystals, known as
"cathedrals."

Quartz is a common constituent of granite, sandstone, limestone, and
many other igneous, sedimentary, and metamorphic rocks.

Tridymite and cristobalite are high temperature polymorphs of SiO2
which occur in high silica volcanic rocks. Lechatelierite is an
amorphous silica glass SiO2 which is formed by lightning strikes in
quartz sand.

http://en.wikipedia.org/wiki/Quartz

Quartz and glass are form of silica which does not dissolve in water at normal
condition, but does so at high temperature and pressure:


" study the solubility of amorphous silica at high temperatures and
high pressures ......The experiments were carried out in a bomb made
of Inconel X with an internal capacity of 89 cm3.."

 http://rruff.geo.arizona.edu/doclib/am/vol62/AM62_1052.pdf
http://www.geothermie.de/egec-geothernet/ci_prof/america/mexico/0031.PDF
http://www.geothermie.de/egec-geothernet/ci_prof/australia_ozean/new_zealand/0909.pdf

Crystal size depends on the process rate. When grwon from melt, on the
rate of cooling, when from solution on the rate of solvent removal:
http://www.xray.ncsu.edu/GrowXtal.html

Hedgie

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Clarification of Answer by hedgie-ga on 02 Jun 2006 23:22 PDT

specific examples of geological processes :

   " Quartz that forms from the crystallization of magmatic rocks
crystallizes from a melt that is rich in silica and water .."

http://rockhoundingar.com/quartz/geology.html

Large natural crystals are rare (and expensive)
http://www.earthchildpendants.co.uk/natural_crystals.html

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Request for Answer Clarification by cjohnsonmmd-ga on 05 Jun 2006 08:41 PDT

My main question is whether water is NECESSARY for the crystalization
process, or can these crystals form from a melt of SiO2 and various
other minerals in the absence of water (aqueous solution).  Artificial
quartz crystals are made from an aqueous solution.

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Clarification of Answer by hedgie-ga on 06 Jun 2006 20:22 PDT

Water is not necessary.  Quartz crystal will grow from the melt.
 
 This can be done artificially, above 1600 C
 It also happens in nature, as mentioned in the answer briefly:


"Lechatelierite is an
amorphous silica glass SiO2 which is formed by lightning strikes in
quartz sand"

So once more: Answer to your question:

"What is the mechanism of formation of natural large hexagonal quartz
crystals.  Is there an aqueous solution from which they crystalize? "

No. Water is not necessary.

Hedgie

Perhaps this helps:
http://www.answers.com/topic/quartz
http://rockhoundingar.com/quartz/qtzgems01.html

A few hundreds Celsius, a few hundred bars and a small quartz crystals
is all what you need.

In industry they grow crystals in autoclaves. Quartz plates hang in a
saline solution and the growth start from that

I appreciate the answer.  However, amorphous silica glass is NOT large
hexagonal crystals.  All of these large crystals that I have seen were
formed inside an empty cavity, which must have been filled with
something- water?  I have never seen these large hexagonal crystals
completely encased in solid rock as would be the case if they formed
from molten rock.  If water is not necessary, what was in the empty
space around these crystals?