[traveler (OP)]

First, let me start by saying I love this forum. I recently came across it while doing some research on Jamaica and have been addicted ever since. There are a lot of really educated people on here with quite a sence of humor. The knowitallogists and smartassists keep learning fun and accurate.

 My question is; Where would you place quartz in the rock cycle. It would be easy to say "All of them" because they all can contain SIO2
sedimentary - contains quartz and can also be held together with quartz.
Metamorphic - almost always contains quartz.
Igneous - Here's where it gets confusing to me. Deep earth rock is usually basic and contains little silica, but olivine is common. Geodes are found in basic rock and I have read that they were made after the lava deposit by water and minerals passing up or down through the cavities. By definition, I would consider this sedimentary deposits.
 Am I looking at this wrong?

[JimBob]

I wouldn't put quartz in any cycle, much less a rock cycle, as it breaks things if it gets in the gears.

BUT it really shouldn't be in the rock cycle at all. Quartz is a mineral. Minerals exist in all types of rocks. Feldspars,(a group of minerals) are in igneous, metamorphic and sedimentary rocks, also. Rocks are made of minerals.

It is just the basics of terminology. Just like saying that water is an element. This is incorrect - water is a molecule.

[Bass]

Silica (SiO2) is the most abundant "molecule" in the crust.  So its present in almost all rocks, whether igneous, sedimentary or metamorphic. 
Quartz is also extremely resistant to erosion- which makes it almost ubiquitous in sedimentary rocks (with minor exceptions).

Igneous - Here's where it gets confusing to me. Deep earth rock is usually basic and contains little silica, but olivine is common. Geodes are found in basic rock and I have read that they were made after the lava deposit by water and minerals passing up or down through the cavities. By definition, I would consider this sedimentary deposits.

Not quite sure what you mean by basic rocks containing "little silica"- most basic rocks are in the 50% silica range.  But they do contain little to no quartz.  The silica is more easily taken into solution as the other minerals break down, allowing quartz to precipitate in cavities and fractures.
I don't consider geodes in volcanic rocks to be sedimentary.  Despite several days of intense training, the resident sedimentologist/oil jockey still believes all rocks are sedimentary- especially when he forgets his memory pills. []

[traveler (OP)]

OK, I see where I'm going wrong. I was confusing SI and SI02 (and SIO4 for that matter). It all depends on the conditions present which way it will combine with oxygen and form quartz, feldspar or other minerals.
   

[JimBob]

That is correct.

And thanks Bass, I was half asleep, as I am now when that answer was written - yours answers the question better than mine.

[traveler (OP)]

Now for the big question and how I got into this subject.
 How can you tell WHEN a crystal formed and how long it took? Can a 3 cm crystal form in a year, 100 years or 1000s of years. I belong to another forum for rockhounds and we frequent a amethyst prospect in Georgia, USA. From the geologic record we can see when the brecciation happened to form the cavities and I was just wondering about the actual crystal formation.
The links below are the forum and site information.
http://dirtyrockhounds.proboards.com/index.cgi [nofollow]
http://www.wncrocks.com/resources/collecting%20site%20diamondhill%20geology%20new.html [nofollow]

Tom

[Bass]

I have a dim memory of a few studies long ago (1950-1960's) that suggested hydrothermal quartz growth rates were probably around 2 to 8 cm per century.  I'm sure there are more recent studies, especially with the amount of silica now used in microprocessors.

[Bored chemist]

The quartz crystals used in things like digital watches are a few mm long and are grown artificially. I don't know what the timescale is but it certainly can't be very long or the process wouldn't be comercially viable.
BTW there's not a lot of silica in a microprocessor. Silicon yes, but not much silica.,

[JimBob]

There are some theories I heard in school that in a magma crystal were in a high temperature mush and when injected into the country rock would grow rather fast - days.

But then, I took geology from James Hutton & Charles Lyell so what do I know?

[traveler (OP)]

Thanks for all the info. It seems that unless someone was there to witness the formation, it is impossible to tell. A few days under perfect conditions and up to a few hundred years for less than optimal conditions. A lot of the amethyst from the area has "ghosting". A visible sing that it stopped growing for a period and then resumed again.