Rockin the Rockies

Rockin the Rockies
Stowe Mtn

Thursday, May 30, 2013

Crystals in Miarolitic Cavities of Lake George Pegmatites

My rock hounding friend and I have returned to Lake George in search of crystals. We've had a slow start to the 2013 season because of the cold and snow we've had this spring. So far we haven't had much success, but perseverance and observation are key to finding the miarolitic cavities that contain the much sought after crystals. The miarolitic cavities or vugs is where the heated mineral enriched liquid cavities within the Lake George pegmatites produce crystals. As the pegmatites and cavities cooled over 1 billion years ago the crystals formed in the solidifying pockets. As the pockets contiued to cool they crack and end the crystal forming process. Due to weathering and other erosion processes many of these now collapsed crystal pockets are near the earth's surface and ready for harvesting. The difficulty in finding the collapsed pockets stems from hundreds of people digging through the pegmatites and finding the easy to get crystals. Sometimes a portion of the pegmatite is exposed to the surface and creates "float" crystals. The crystals are called float because they are on the surface and detached from their source. Inexperienced rock hounds often pick up these float crystals or keys to the location of the crystal pocket without realizing they've not only missed the mother lode, but also erased evidence for others to find the crystal pocket. One prospector at Lake George uses track hoes to dig down over 20 feet into the earth's crust to discover and follow pegmatites opening up crystal vugs. So far this year I have found 4 collapsed vugs with crystals, all within 1-3 feet of the surface. Unfortunately the pockets were small and many of the crystals were either broken or had quartz overgrowth. The breakage can be a result of either the original pocket rupturing under pressure during crystal formation, or the grinding of crystals against each other over millions of years of frost thaw cycles and other erosion processes. I believe the overgrowth of quartz over the smoky crystals is a result of a later infusion of quartz into the vug. This quartz covers the smoky crystals to varying degrees and if not gemmy, hoods the smoky crystals making them somewhat unsightly and not very desirable.  If the hooding quartz is smoky or clear, the crystals are more desirable and are described as phantom smoky crystals as the original smoky is visible through the later quartz hooding. These hooded crystals are highly prized.
This smoky cluster has a lot of overgrowth and is rather unsightly. Too bad the bluish fluorite (lower left) was not larger

A nicer crystal grouping found on Monday... no quartz overgrowth (crystals 1.5-2 inches)

1 comment:

  1. Hi, I've been following your blog for some time now, and really ejoy reading your posts and admiring the pictures you take of the digging up of the crystals - the more the better! :D

    As a mineral collector myself (from far away, I'm from France), I can understand that poor specimens don't say anything to you. But as a gemstone cutter too, I can tell you that these opaque quartz-covered specimens are really worth keeping and sending/selling to fellow rockhounds that are into lapidary art.
    Getting beautiful crystals from Mother Nature is great, but turning less pretty or less-than-pretty specimens into something pleasant to the eye is very rewarding too!

    Ebay is a nice place to "get rid" of those unsightly specimens and make other rockhounds happy with them, particularly those that don't have access to huge pegmatite zones like you do ;)

    Can't wait for your next post!