VIVIANITE FROM LEADVILLE, COLORADO

Vivianite is a hydrated iron phosphate mineral [Fe₃⁺⁺ (PO₄)₂-8H₂0] that is somewhat rare in Colorado except for the Treece and Ibex Mines in the Leadville Mining District. Eckel and others (1997) described vivianite from Leadville as among the “finest specimens of the mineral found in the country.”  Now, collectors of minerals from the Blackbird Mine in Idaho may wish to dispute that statement!  But I have not seen the crystals of vivianite (as long as 3 inches) housed in the Denver Museum of Nature and Science (Eckel and others, 1997) so will not cast my vote in either direction.

I have a few specimens of vivianite in my collection so at the recent Denver Spring Show I was not particularly looking for new samples.  However, in glancing at the tables a specific specimen popped into my brain since it was: 1) colorful crystals of vivianite (my other crystals are very dark, essentially black); 2) collected from Leadville; and 3) cheap at $2. It was further intriguing since my first line of information (MinDat.org on my cell phone) only showed five photos of Leadville vivianite and most were not something that I would write home to mama about.  The price certainly was right for someone who receives a monthly social security check, so it came home with me, and I consider it a good decision!

Vivianite is one of those interesting minerals containing the phosphate radical (PO₄) that I love to collect and “study.”  Phosphates are often grouped together with the arsenates (AsO₄), and vanadates (AsO₄) since these three radicals are about the same atomic size and frequently substitute for each other when combining with metal cations.  In some cases, there is a solid solution series between resulting minerals such as mimetite (lead arsenate)—lead phosphate (pyromorphite)---vanadinite (lead vanadate).  In others, the minerals are individuals and no solution series seem to exist.

Vivianite is a crystal of many colors, and in fact, can change color over its lifetime. Freshly exposed vivianite is generally colorless but with time oxidizes to green to bluish green to blue crystals.  Continued oxidation of the iron from Fe⁺⁺ (ferric) to Fe+++ (ferrous)  will produce crystals so dark blue they appear black.  Many crystals have a vitreous luster although they can grade into pearly or dull specimens.  Colorless crystals are transparent while lighter colored specimens become translucent and massive specimens generally are rather opaque. As with the color, mineral streak ranges from colorless to various shades of blue. Vivianite is quite soft, ~2.0 or less (Mohs).  The best “showy” specimens have prismatic (elongated along the C Axis) or flattened/bladed (along the B Axis) crystals and often form in stellate cluster; however, there are a variety of other morphological forms.

Matrix with indigo blue to purple crystals of vivianite.  Collected Treece or Ibex Mine at Leadville, Colorado.  Width of photo ~5.5 cm.

Photomicrographs from above specimen.  Width ~1.4 cm. 

Photomicrograph of broken and isolated cleavage fragment. Length along C Axis of longest fragment is ~5 mm.  S=sticky substance holding fragment on pine.

Vivianite is thought to occur as: 1) as a secondary mineral in metallic ore deposits; 2) in pegmatites as an alteration product of primary phosphate minerals; or 3) as a mineral associated with the phosphate found in sedimentary deposits. However, Petrov (2008) noted the mineral is not characteristic of the oxidized zone but of “deep unoxidized levels of ore deposits.”  I had assumed the crystals at Leadville were from the oxidized zone but am uncertain.

So, members of the Vivianite Group (shown below) include the iron arsenate vivianite, the zinc arsenate köttigite and the cobalt arsenate erythrite. 

Vivianite crystals (ex. Joe Dorris) collected Bingham District, Utah, a large porphyry copper deposit.  The largest crystal is ~2 cm. in length.

Mixed color spray of köttigite and water-clear selenite gypsum (S) and goethite (G).  Width FOV ~1 cm.  The variation and gradation of color may be due to an increase in iron substituting for zinc 

Mass of erythrite crystals.  Width FOV ~1.3 cm.

Vivianite is the namesake of the Vivianite Group of minerals that have a general formula of X₃(AO₄)₂-8(H₂O) where X is a ⁺⁺ metal (Mg, Mn, Fe, Co, Ni, Cu, Zn) and A is either phosphorus (a phosphate) or arsenic (an arsenate).  Most members of the Group are colorful and have weak ionic bonding resulting in mica-like cleavage.  Close relatives of vivianite include erythrite, a hydrous cobalt arsenate, and köttigite, a hydrous zinc arsenate.

REFERENCES CITED

Eckel, E. B. and others (revised edition), 1997, Minerals of Colorado: Fulcrum Publishing, Golden.

Petrov, A., 2008, A scientific study of the absorption of evil by vivianite: www.mindat.org/article.php/137/