You are here:

Geology/Rock cluster identification

Advertisement


Question
mica cluster?
mica cluster?  
I have a "rock cluster" given to me. It is very beautiful, but am unsure exactly what it is. I'm sure it contains mica, however not too sure what the other is and if its of any value? Can you help? Thanks for you're time.
         DeeAnn

Answer
I can't really see it well enough (in the picture) to tell you what is in it but having seen similar clusters many times in the field (mostly in Colorado) I expect that it is pegmatite.

Pegmatites are a very crystalline, intrusive igneous rock composed of interlocking crystals usually larger than 2.5 cm (about 1 inch) in size;such rocks are referred to as pegmatitic.  Most pegmatites are composed of quartz, feldspar and mica; in essence a granite; which is what I believe yours is; although I can't see all of those minerals in the picture. These are very common along the Front Range of Colorado.

Rarer intermediate composition and mafic pegmatites containing amphibole, Ca-plagioclase feldspar, pyroxene, tourmaline and other minerals are known, found in recrystallised zones and apophyses associated with large layered intrusions.  Crystal size is the most striking feature of pegmatites, with crystals usually over 5 cm (2 inches) in size. Individual crystals over 10 metres (over 30 feet) across have been found, and the world's largest crystal was found within a pegmatite.

Similarly, crystal texture and form within pegmatitic rock may be taken to extreme size and perfection. Feldspar within a pegmatite may display exaggerated and perfect twinning, exsolution lamellae, and when affected by hydrous crystallization, macroscale graphic texture is known, with feldspar and quartz intergrown. Perthite feldspar within a pegmatite often shows gigantic perthitic texture visible to the naked eye.

Value: I have, many times, seen these clusters in rock shops so they do have some value. I would suspect that the value is directly related to the size of the crystals and yours appears to have average size crystals. It probably has a retail value of $20 - $50; but the rock shop owner is probably only going to offer you half that or so.  

Geology

All Answers


Answers by Expert:


Ask Experts

Volunteer


Joe Norris

Expertise

I am an economic geologist. An economic geologist does mineral evaluations and appraisals of mineral or mining properties. I can tell you if your deposit has value - remember that a mineral deposit, no matter how good, only has value when mined. Any value assigned to a mineral deposit, in the ground, is only the speculative value that deposit.

Experience

I have been a economic geologist for most of my 35 year career. Although I have done work in perhaps 45 states and numerious countries much of my work has been in Appalachian coal, intermountain west gold and silver, and Arizona uranium.

Organizations
Past President of the Virginia Section of the American Institute of Professional Geologists and a certified geologist in twelve states.

Education/Credentials
BS Degree from Eastern Kentucky University. Work on MS Degree @ Eastern Kentucky University, Colorad School of Mines & Marshall University Numerious short courses on the value of mineral deposits and how to value same. Also several short courses dealing with the different types of geologic processes; sedimentary, igneous, metamorphic along with the mineral associated with each.

©2016 About.com. All rights reserved.