VALLERIITE: AN EXTRA-SOFT MINERAL

Aerial view of the Mission Complex, looking southwest.  Photo courtesy of  http://www.asarco.com/

Visitors to Arizona traveling along I-19 south of Tucson usually are heading to Nogales, Mexico, and/or the “artist” community of Tubac, Arizona.  I suppose many drivers glance to the west about 15-20 miles south along the road and sort of wonder about the seeming large “piles” of rock.  Those travelers with a greater curiosity will discover that these features represent overburden coming from a really large open pit, copper porphyry mine known as the Mission Complex. The current mine is about 2.5 miles long, 1.5 miles wide and 1200 feet deep.  At one time there were several smaller mines owned by different companies; however, the Mission, Pima, Mineral Hill and North and South San Xavier were acquired by ASARCO and now represent a single integrated mining operation.  If travelers are interested in further exploring the world of copper the company offers a Discovery Center and mine tours---see http://www.asarco.com/

Valleriite nodule from Pima Mina, Arizona.   Length ~3.8 cm.

One of the early mines, the Pima, has produced a rather uncommon, or as the Handbook of Mineralogy (www.handbookofmineralogy.org) states, “an inconspicuous mineral” that is an iron copper sulfide magnesium aluminum hydroxide [(Fe⁺⁺,Cu)₄(Mg.Al)₃S₄(OH,O)₆] known as valleriite. More than uncommon or inconspicuous, I would say just pretty weird and certainly tough to identify unless one is familiar with the mineral.  For one thing, valleriite is an extremely soft mineral at about 1.0 (Mohs) and could be confused with graphite, the softest metal-like mineral on most rockhound’s radar.  It is a dark gray to black mineral with a bronze sheen and a black streak.  The luster is metallic, it is opaque and sort of looks like a lump of metal or coal; however, the softness and bronze sheen are the prime identifiers, at least for me. Most commonly valleriite is massive or nodular and appears to be slightly foliated.  Tiny crystals may be present but are indistinguishable. 

Valleriite is an alteration product of chalcopyrite at the Christmas Mine, Arizona, but probably of magnetite in the Pima Mine (Anthony and others, 1995). As best that I understand, the mineralization at the Pima Mine is in a skarn deposit located along the boundary between various Paleozoic carbonates and a Laramide quartz monzonite intrusion. The volatile elements such as aluminum, iron and magnesium were introduced sometime along the line and new minerals formed such as valleriite.  This metasomatic or pyrometasomatic action occurs at relatively high temperature (up to ??500⁰C) but with fairly low pressure. 

  

As a bit of trivia, I-19 is the 4^th shortest primary interstate highway in the lower 48 states. It runs from Exit 0 Nogales, Arizona, (across the international border from Nogales, Mexico) north to south Tucson at Exit 99 and then merges with I-10. So far so good; however, I-19 is signed in kilometers and its actual length is about 67 miles.  But, the speed limit is posted in MPH.  This dual usage is a source of massive confusion for many first time visitors! 

REFERENCES CITED

Anthony, J.W., S.A. Williams, R.A. Bideaux and R.W. Grant, 1995, Mineralogy of Arizona: The University of Arizona press, Tucson.

VIVIANITE: COLOR-CHANGING IRON PHOSPHATE

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].

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!  

REFERENCES CITED

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