FMF - Friends of Minerals Forum, discussion and message board

[Hugh Cronic]

Amateur prospector here looking for an easy answer I'm sure. I cannot find the info anywhere on the net.

My question is

Can you find sapphires in any granite pockets?
there is tons of granite where I live in the creeks. Also I happen to have a mineral claim on
an existing granite Quarry. The T.V. " Prospectors " Shows those people diggin saphhires outta granite pockets.

Can this happen in all granite?
Most granite?

What are the sign that they wouldn't be present?

Any info or help would be greatly appreciated

[John S. White]

A true granite is a rock containing quartz. Corundum, which is what sapphire is, cannot occur in contact with quartz, it is an impossibility, so you will not find it in granite..

[Hugh Cronic]

Thanks for the reply. I keep reading that granite can be a host rock as well as marble.

If marble is present does that mean sapphire could also be?

I appreciate the info from everyone. I love everything about rocks and minerals and study as much as I can.

Its nice to be able to ask people straight questions which in turn saves me a lot of time.

[Byron Anderson]

Corundum is, in most cases, a medium to high pressure, high temperature forming mineral. Most true granitic bodies form at low to medium temperature and pressures. I'm wondering if the cases where it is being mentioned that the corundum is being found in "granite" if the parent rock is actually a gneiss or migmatite (high temperature-high presure regional metamorphic rocks). These rocks are common in the Colorado geologic environment where the Prospector show is being filmed and these rocks could be mistaken by the lay-person as being a granite. Also, it may be that the corundum is forming in low-silica magmas and pegmatites or near contacts with rocks of high alumnimum content to allow their formation within the contact zones. The rock may have a "granitic" appearance but is probably a syenite or similar low-silica igneous or contact metamorphic rock. I know the corundum found in Chaffee County Colorado comes from a schist - diorite contact. I commonly see the term "granite" overused to simply describe an igneous pluton texture. You first need to determine if your "granite" is a low, medium, or high slica (quartz) rock, which will give you the first clue as to whether you have any possibility of having corundum within or nearby. There are several other critical conditions needed to form corundum, even in high aluminum environments, but this will be the quick and dirty way to at least give you a NO or MAYBE answer. I'm trying to keep this explanation simple, so I may have created some holes or inconsistencies here. My apologies if I did and hopefully someone out there will clarify or revise.

[John S. White]

My point is that corundum cannot occur in a rock that contains free silica (quartz) irrespective of the temperature or pressure of formation. If silica is present you will get an aluminum silicate, not corundum.

[Jesse Fisher]

I think, perhaps, there is some confusion with the usage of the term "granite." The term is often misused to refer to all granitic rocks, which is technically incorrect. The term "granitic" refers to any coarse-grained intrusive igneous rock, while "granite" is a specific quartz-rich variety. Corundum (sapphire, ruby, etc.), as John points out, does not occur in rocks that contain quartz. It can occur, however, in some silica-depleted granitic rock types such as syenite. A syenite, while granitic in texture is not a granite. I believe the Montana sapphires occur in a synetic host rock.

[robynahawk]

Hugh - who is finding Sapphire on the Prospectors show? I missed a couple episodes but all I have seen coming out of the claims there have been Aquamarine, Amazonite, Smokey Quartz, Topaz, and some clear Quartz crystals.

Need to catch up ;)

Robyn

[robynahawk]

This might be a helpful link for you - Mindat lists minerals by Locality...your area shows no Corundum (the mineral name for Sapphire) at all.

But you do have some really rare minerals around you!

Check it out: https://www.mindat.org/loc-498.html

Robyn

[Paul Biehler]

Thank you to all of you who have responded to the question of granite and the formation of sapphire. The responses are educational and elucidating.

Paul and Merrill