 |
|
QUARTZ WITH AN INCRUSTATION OF PARAREALGAR (ALTERED
REALGAR) FROM THE BLACK HILLS OF SOUTH DAKOTA.
LENGTH OF SPECIMEN ~6.5 CM.
|
I recently returned home from a fall leaf peeping
and collecting trip to the Colorado high country (near Leadville) and continuing
to the Black Hills of western South Dakota---hence my absence from the Blog! I have often written about the latter area as
the Hills are one of my favorite places to visit-especially in the fall. The children are in school, the campgrounds
are un-crowded, and the many old mines are waiting for my return. In addition, the Hills have numerous
rock/mineral shops that require stops to visit new additions as well as the dusty
old bins that time seems to forget. It
is these latter cardboard boxes that require close examination. The bottom line is that I came home with many
decent collected specimens and some of those weird and strange and rare
minerals that I love to acquire for my collection.
Now realgar is not a weird or strange mineral and
has been mined for centuries at various world-wide locations; however, it is a
rare mineral in rocks of the Black Hills.
Realgar is a sulfide mineral, an arsenic sulfide: As4S4
(sometimes written as AsS). In crystal
form the mineral is usually bright ruby-red or orange-red in color and is
sometimes called “ruby sulfur”. Often
transparent with a resinous luster, realgar is quit soft (1.5-2.0 Mohs) and crystals
are easily damaged. Most realgar forms
in low temperature hydrothermal settings but does occur as a product in hot
springs such as the Norris Geyser Basin in Yellowstone National Park, or in
sublimation products via volcanic vapors.
In hydrothermal veins realgar is often found with other sulfides such as
gold, silver, stibnite, lead, and especially orpiment. The latter mineral, As2S3,
is often confused with realgar since it usually forms in the same environments
and is similar in appearance (more on this in a later blog).
Realgar is one of those minerals that is fairly
unstable and over time alters to a new mineral, pararealgar (As4S4
or AsS)—same chemical formula and chemical composition as realgar but a
different crystal form. But since I am
not a mineralogist confusion often enters my mind—both pararealgar and realgar
are Monoclinic and 2/m Prismatic—so at this time I fail to understand the
difference. Regardless of my confusion,
with exposure to light and over time realgar alters to this yellow to
yellow-orange crust called pararealgar---ah, the wonders of modern instrumentation
to determine such actions! But hold on,
if you leave pararealgar in the light for a “long time” it turns into a
yellow-orange powder and sort of disintegrates into what? Dust? I
have also noticed that not every mineralogy reference seems to accept
pararealgar and simply states that realgar occurs either as the red crystals or
the yellow-orange crust or powder, sometimes all on the same specimen.
In reading an article on medieval painting techniques
and natural pigments I remember the author writing about red pigment made with
beautiful realgar that would fade from the canvas and turn into a yellow or
yellow-orange hue. I can assume that is
true although I don’t have the slightest idea for that source---probably some
magazine that I browsed through at Barnes and Noble!
So, in this small rock/mineral shop in the Hills I
stumbled across a couple of dusty specimens marked “realgar, mine near Sheridan
Lake?, Black Hills”. The price was right
so I picked it up knowing that, at least from my limited knowledge, that
realgar was not common in the Black Hills.
I only wish that I had also picked up the second specimen. I immediately consulted the Black Hills “mineral
bible” Mineralogy of the Black Hills
(Roberts and Rapp, 1965) but could not find a reference for realgar, or for orpiment,
occurring in the Hills. I continued to
search my other books (and that knower of all knowledge, the Internet) and the
only reference that I located was in Encyclopedia
of Minerals (Roberts, Rapp and Weber, 1974): “Realgar is found in the United States…sparingly
in the Homestake gold mine, Lead, South Dakota.”
So, the problem is that Sheridan Lake is not really
close to the Homestake Mine! Maybe there
is a mine near the lake that produced the specimen, or perhaps the locality
information is wrong. I may never know
as I cannot locate photos of Homestake specimens for comparison with my small specimen
of massive quartz (with vugs of tiny quartz crystals) with a crust of
pararealgar.
As stated previously, there are many aspects of crystallography
that confuse me; however, I have hope for the future since to be conscious that you are ignorant is a great step to knowledge:) (Benjamin Disraeli)