 |
|
Specimen from Taxco with numerous crystals of
ilvaite on massive ?sphalerite. The “tan wormy” mineral on the
right is shown below in a photomicrograph. Botryoidal mineral in lower left is unknown.
Length of specimen ~9.5 cm.
|
In rearranging my collection (again) I came across a
couple of specimens garnered from the “infamous” mineral auction that was held
several months ago here in Colorado Springs.
I have written about several of the minerals purchased at that auction and
have a slew of additional boxes needing closer examination.
Ilvaite is/was a mineral that I am not overly familiar
with, and in fact, really did not recognize the name on the label. Turns out ilvaite is a calcium iron
sorosilicate hydroxide (has the unusual Si2O7—two silicate
tetrahedrons share one oxygen) that seems rather uncommon in the mineral record:
CaFe2+2Fe+3(Si2O7)O(OH). In a few localities, manganese 2+ substitutes
for some of the iron and the mineral is then known as manganilvaite. Ilvaite is usually black in color, opaque, has
a sub-metallic luster, a hardness listed as 5.5-6 (Mohs), with prismatic crystals
that are usually striated (length wise).
It usually occurs in contact metamorphic zones, commonly with copper and/or
zinc ores, and often is associated with magnetite ore bodies. Other than this usual information, it seems
really difficult to find out much more about the mineral. Although my two specimens have fairly large
crystals, perhaps fine-grained aggregates are present in the rock record—just not
very recognizable in hand specimens? In
Colorado, nice crystals (to 5 mm) “were found as float in talus at the head of
North Pole Basin on Treasury Mountain [Gunnison County]” (Eckels and others,
1997).
Taxco de Alarcon (~100 miles south of Mexico City) is an old colonial mining area in the Mexican state of Guerreo and is well known for the production of silver (with byproducts of gold, lead and zinc), and especially collectable specimens of wire silver. Tens of millions of ounces of silver have been extracted from the rocks at “Taxco”.
One particular mine, Mina los Remedios, also produces
nice crystals of ilvaite, and some really great specimens of
rhodochrosite. The specimen I have is very
attractive, covered (both sides) with prismatic crystals that have well-defined
faces and edges and the distinctive chisel-shaped terminations. The crystals have grown on a bed of massive ?sphalerite
and are intermixed with a few tiny quartz crystals, some shiny pyrite and a
couple of “unknowns”.
The second specimen in my collection is a single large crystal from
the Greek island of Serifos, a classic collecting locality in skarn deposits. It has a rich black color and a very nice
termination.
 |
|
Unknown mineral, appears six-sided crystal plate. At least two are present on the rock. Width of photomicrograph is ~1 cm; therefore
the crystal is perhaps 2 mm.
|
 |
|
Photomicrograph, width ~ 1cm, noting tiny terminated quartz crystals, and
pyrite crystals.
|
 |
|
Photomicrograph, width ~1 cm, noting ilvaite
crystals along with an unknown mineral labeled quartz but perhaps is fluorite.
|
 |
|
Photomicrograph, width ~1 cm, with unknown mineral
noted in photograph at top of page. I
don’t have the slightest idea about identity!
|
 |
|
Nicely terminated, single crystal of ilvaite from
the Greek island of Serifos. Length ~2.6
cm.
|
So again, I am happy when a simple inquiry takes me
on roads previously unknown.
REFERENCES
CITED
Eckel, E. B. and others, 1997, Minerals of Colorado:
Denver Museum of Nature and Science and Fulcrum Publishing.