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.
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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.
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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.
So, members of the Vivianite Group (shown below) include the iron arsenate vivianite, the zinc arsenate köttigite and the cobalt arsenate erythrite.
Mass of erythrite crystals. Width FOV ~1.3 cm.
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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/