In my last Posting I continued to note that the
phosphates (PO4), arsenates (AsO4), and vanadates (AsO4)
are often grouped together since these three radicals are about the same atomic
size and frequently substitute for each other when combining with metal
cations. In some case 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 seems to exist.
Erythrite, a hydrous cobalt arsenate, (see last
Posting) is a member of the Vivianite Group of phosphates and arsenates defined
(www.galleries.com) by the formula 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. In the previous post I noted the bright green
annabergite (nickel arsenate) and erythrite (crimson-red cobalt arsenate). I have in my collection another member of the
Vivianite Group, and its namesake, the hydrated iron phosphate vivianite
[Fe3++
(PO4)2-8H20].
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Vivianite crystals (ex. Joe Dorris) collected
Bingham District, Utah, a large porphyry copper deposit. The largest crystal is ~2 cm. in length.
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I have a really nice crystal ~2 cm. in length, black in color, prismatic (elongated along C
axis), flattened (along B axis), very soft (2 or less; Mohs), vitreous luster,
and appearing opaque. A second specimen from Virginia
is a dark black cleavage fragment (mica-like cleavage parallel to C axis) but
with a dull to pearly luster on the cleavage plate.
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Cleavage fragment of vivianite collected Vivianite
Locality, Richmond Virginia (purchased with other minerals at an auction). Note the dull to earthy luster on the
cleavage plan (parallel to C axis).
Length ~4 cm.
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This description ties in with the description of
Vivianite Group members except the bright coloration. What I have are two specimens that are have
turned black from oxidation of the iron from Fe++ (ferric) to Fe+++
(ferrous). Very fresh vivianite is
colorless but with oxidation it goes through a number of color changes
ultimately ending up black, and perhaps morphing into a new mineral metavivianite
[Fe2++Fe+++(PO4)2(OH)2-6H2O]. Petrov (2008), in a
wonderful article on MinDat, stated the vivianite “alteration is accompanied by
a progressive colour change. Absolutely pure fresh end-member vivianite is
colourless! A minor amount of light-induced oxidation rapidly changes the
colour to a brilliant transparent green... Further oxidation changes the colour
to a deeper emerald green and a strong pleochroism sets in, with the colour in
transmitted light becoming a cobalt blue… Eventually the whole crystal turns an
opaque deep blue and finally bluish black.”
Vivianite, a secondary mineral, is also interesting
in that it is found in igneous environments (granitic pegmatites containing
phosphates) and in recent sediments where it replaces organic material. Many references also place vivianite in
oxidized zones of metallic ore deposits; however, Petrov (2008) noted the
mineral is not characteristic of the oxidized zone but of “deep unoxidized
levels of ore deposits.”
What I don’t know is if my specimens are actually
vivianite or have oxidized, or partially oxidized, into metavivianite. I
presume vivianite since that is the noted mineral (MinDat) at Bingham District,
Utah, and Richmond, Virginia.
I learned much from this little exercise and
hopefully will be able to recognize vivianite if there is an opportunity to again
observe!
REFERENCES
CITED
Petrov, A., 2008, A scientific study of the
absorption of evil by vivianite: www.mindat.org/article.php/137/