ILVAITE: WHODA THOUGHT

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”.

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!

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.

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.