Printable Version of Topic

Click here to view this topic in its original format

Colorado Prospector - Gem and mineral prospecting and mining forums _ Prospectors and Rockhounding Field Work _ Mineral learning help

Posted by: jimmyrig Jan 20 2018, 09:00 AM

Was a beautiful day out yesterday so I went out to the hayman burn area to do some poking around and test flight with my drone. I'm still learning my common minerals so thought I'd just take a stab at some. Is this Quartz in Porphyritic Rhyolite?

Posted by: jimmyrig Jan 21 2018, 09:08 AM

Drone Video https://www.youtube.com/watch?v=w8Xs7ji7Fx0

Posted by: Woody Jan 21 2018, 10:44 PM

Hi Jimmyrig,

So... Just learning your minerals huh? Well "Porphyritic Rhyolite" is a mouth full for someone just cutting their teeth on common minerals. Rhyolite is very common and can be found all over the high country. "Porphyritic" on the other hand is a term
used to describe the texture of a host of different rocks. The use of the word Porphyritic describing texture is a direct reference to Crystals contained in the material being described. The size of these crystals are most often very small and can't readily be
addressed unless viewed up-close.

From the picture submitted, I can't honestly tell if that material is Rhyolite or Granite, let alone if it is Porphyritic or not. I'd call it an outcropping of Quartz in Granite.

However, I think I know what you are getting at, and I like it! When a dedicated person first takes on a new hobby they read a lot about it. One of the terms used in describing how to find crystals is Porphyritic. If you are seeing this porphyritic texture,
then you are in the right area. If you are not seeing it, then move on to another area or another material. Once you found it, use it to your advantage! Best case scenario.... you come up to a rocky material face and you can see it has a porphyritic texture.
From as far away as possible, "6 inches or 6 feet, whatever the case may be". Observe this texture and try to work-out which direction the crystals are getting bigger. Is it becoming more, or less porphyritic? Follow the direction of the larger crystals and
hopefully it becomes a seem and then a pocket!

If I seen this outcropping of Quartz in anything, Rhyolite or Granite, I'd go poke at it. See what else it had to say?

Hope this was helpful, Woody.

Posted by: GeoMatt Jan 22 2018, 12:20 AM

Granite.

Posted by: jimmyrig Jan 22 2018, 04:16 PM

I have found it difficult to tell the difference between granite and rhyolite. Only thing I know is that rhyolite seems to be more porous due to surface cooling and granite is a little more dense?

Posted by: dbx Mar 11 2018, 08:09 PM

I have been fairly idle in this endeavor, but hoping to find time again to dive in.
I have bought these Geologic Survey Bulletins from a guy in Lakewood, but
found them online as well. I don't realize their full value yet, but they are interesting.
Bulletin