One of the “things” that I enjoy
about Shannon Minerals is the great variety of rare or uncommon minerals in his
stock. So, how could I pass up a
specimen of laffittite? My initial
reaction to seeing the box of specimens was---what is it? The red and orange colors observed through
the perky case certainly were attractive!
Then I read the label and noted the specimen also contained realgar [the
red As4S4] and orpiment [the orange As2S3],
both arsenic sulfides. The specimen came
from the Getchell Mine in Nevada, one of the more famous U.S. localities for
colorful arsenic minerals. The mine at
Getchell is a Carlin-type, or sediment-hosted, replacement deposit. The mineralization is associated with a fault
zone along a Cretaceous igneous stock where hydrothermal fluids were able to
deposit tiny (micron gold) flakes, or dissolved gold, into pyrite associated
with Cambrian and Ordovician rocks. The
Carlin-type gold deposits in Nevada are the largest gold producing mines in the
U.S.; however, since the gold is essentially invisible, the deposits are tough
to locate. The realgar and orpiment at
Getchell were deposited by late stage hydrothermal solutions.
Location of Carlin-type gold deposits in Nevada. Map Public Domain courtesy of USGS. |
What about the laffittite in the
specimen? If not for an XRD analysis of
some tiny crystals, I would have missed identifying the mineral in the sample. Even with this specified identification
listed on the sample box, I am still sort of guessing!
Laffittite [AgHgAsS3]
is a silver mercury arsenic sulfide associated with other late stage hydrothermal
arsenic minerals like realgar and orpiment.
The Getchell Mine produces short prismatic crystals of laffittite (at
other localities pseudo cubic and tabular) that are very dark red (almost
appearing black) and opaque. They have a
metallic luster and a listed hardness of ~3.5 (Mohs). Getchell crystals are often submillimeter in
length. My camera is not refined enough to pick up other details of the
crystals.
Arsenic minerals are often quite
colorful and make good display specimens – maybe! Exposure to bright light will alter realgar to
pararealgar, and it will also decay to orpiment. In addition, both minerals are toxic, so hand
washing is a necessity.
Every time I am able to attend the
Tucson Shows I am reminded of an early 1970s tune recorded by Rare Earth (not a kind of magnet but a
band!):
One, two, three, four
I just want to celebrate another day of livin'
I just want to celebrate another day of life
I just want to celebrate another day of livin'
I just want to celebrate another day of life
No comments:
Post a Comment