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
Vivianite crystals (ex. Joe Dorris) collected Bingham District, Utah, a large porphyry copper deposit. The largest crystal is ~2 cm. in length.
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.
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.
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!
Petrov, A., 2008, A scientific study of the absorption of evil by vivianite: www.mindat.org/article.php/137/