GARNIERITE: LATERITIC GREEN NICKEL "MINERAL"

As mentioned in previous Blogs on millerite/polydymite and siegenite, nickel (metal) is produced either from sulfide minerals or from laterite deposits.  These minor nickel sulfides noted above piqued my interest in nickel minerals so I journeyed down to my favorite rock and mineral store, Ackley’s in Colorado Springs.  I actually was looking for a sample of pentlandite, the most common nickel sulfide, and was unable to find such.  I wanted a sample of this sulfide since several years ago I was with my family traveling and camping across eastern and central Canada and so detoured through the city of Sudbury.  Why Sudbury?  Well, I wanted to see the famous nickel mines and smelter, and also Sudbury is situated in the second largest impact crater known on Earth.  The meteor struck what is now part of Ontario, Canada, about 1.85 Ga (Precambrian: Proterozoic).  Many millions of years have elapsed since the 6-10 mile (diameter) fireball impact and the area has been subject to numerous deformational and orogenic events, and weathering.  Therefore, it is difficult for geologists to define the original size of the crater; however, it was much larger than the present size of about 40 mi x 20 mi. Debris from the impact may have been scattered across the globe but is probably unrecognizable or destroyed after such a long period of time.

At one time geologists speculated that the large nickel deposits at Sudbury were part of the meteorite (as in iron-nickel meteorite).  However, that is an erroneous conclusion (although it sometimes still is floated around).  The Sudbury nickel is found in a typical pyrrhotite-pentlandite massive sulfide ore that is of mantle origin associated with an intrusive event.  The impact force of the meteorite caused wholesale melting of the mantle, triggered the huge intrusion, and emplaced the nickel deposit (Stefano, 2014, personal correspondence).  How much nickel?  Canickel Mining Limited (2014) estimated that over US$120 billion worth of nickel, copper and Platinum Group Minerals have been produced from the region over the past 100 years!  

      

Sudbury’s other claim to fame (probably a claim to shame) was the tremendous pollution of the area by the mine and smelter. The landscape reminded me of what I imagined the lunar landscape looked like. Even my children wanted a bottled (out of area) drink.  Virtually nothing was alive except people and all the vegetation seemed dead or dying.  Keller (2014) noted that in the 1960s and 70s “air pollution from the nickel smelter was so bad it literally blackened the earth: acid rain, along with mining operations, stripped the land of vegetation, leaving 100,000 hectares of barren or semi-barren moonscape. The water was exceptionally bad. More than 20,000 lakes in Ontario were acidified to the point of ecosystem damage, 7,000 of them within Sudbury’s immediate vicinity. Lakes within the city and beyond were some of the most acidified, metal-contaminated lakes in the world.”  I remember taking numerous slide photos to show my intro geology classes about the effects of nickel smelting and mining running amuck. The good news is that the community and mining operations took reclamation seriously and today the remediation is quite noticeable---things are looking much better. However, part of this remediation included building the Inco Superstack (~1250 feet) to disperse SO₂ gases and other byproducts of the smelting process away from the city of Sudbury.  The Superstack placed the gases high in the air where they blew right past the city of Sudbury. While the Superstack lowered the ground-level pollution in the city, it has dispersed sulfur and nitrogen gases over a much larger area.    In fact, environmentalists estimate that the Superstack accounts for ~20% of all of the arsenic emitted in North America, ~13% of the lead and ~30% of the nickel.  

Superstack at Sudbury.  Public Domain photo.

So, mining and smelting of nickel is one of those conundrums. Sort of dammed if you mine and dammed if you don’t.  Countries need the nickel in ever increasing quantities; however, environmental damage may be extensive.  I certainly don’t have the answer! 

OK, back to the minerals.  Although pentlandite was unavailable at Ackley’s, the proprietors did have several samples of a nickel “mineral” called garnierite, a substance completely unfamiliar to me.  Well, pack it up and I will do some reading!

As I noted above, laterite deposits are the major source of nickel in today’s world, and garnierite is a lateritic nickel “mineral.”  Laterites usually develop within 20 degrees latitude of the equator in a tropical environment where chemical weathering is most intense.  The most soluble elements of the country rock dissolve away leaving behind the less-soluble minerals that are progressively redistributed throughout a series in a more-to less-weathered profile.  Certain less soluble minerals often form a concentrated deposit within the profile (Samama, 1986).  These concentrated elements/minerals vary in composition depending upon the makeup of the weathered country rock. Often, nickel and cobalt lateritic deposits are formed from weathering of serpentinite, dunite, peridotite and other ultrabasic rocks (low silica, low potassium, dark colored high iron and magnesium content) that are nickel- and cobalt- enriched.  With exxtensive weathering the more soluble calcium, silicon and magnesium are removed from the profile while elements like manganese, copper, nickel, iron, zinc, aluminum, titanium and some rare earth elements are concentrated (Brand and others, 1998).

The weathering profile of these laterites is multifaceted and ranges in a continuum from the unweathered country rock through deeply weathered, but in place country rock, through magnesium- and nickel-rich silicates into clay layers and finally into various zones of iron oxide.  Most of the concentrated cobalt is found in the iron oxide zones while the enriched nickel is deeper in the profile in the magnesium- and nickel-rich hydrous silicates.  Nickel-enriched lateritic ores are well-known and productive in Western Australia, Cuba, New Caledonia, Greece and Russia (Marsh and Anderson, 2011).  In the United States the Nickel Mountain laterite deposit (Riddle District) in the Klamath Mountains of Oregon was a producing mine until 1998 with an average grade of perhaps 1.4 % nickel.  Around 25 million tons of nickel was produced and at least that much of the metal remains.  In addition, the smelter near the mine closed shortly after the mine was shuttered.  However, as I understand it, there are numerous mining claims in the Riddle District extending from Oregon south into California.

Map showing hypothetical active west coast of U.S.  Oceanic crust subducted under continental crust.  Sketch courtesy of www.sjvgeology.org/geology/tectonics.html

Map showing accretiated terrane (exotic terrane) in western North America.  These silvers of land were once island arcs or pieces of oceanic crust that were “stuck” to the continent during collision with the proto North America.  Map courtesy of USGS at:   http://pubs.usgs.gov/gip/dynamic/Pangaea.html

The Riddle District laterites are the result, then, of intense weathering of ultramafic rocks.  The question then becomes, at least to me, what is the source of these ultramafic rocks?  The geology of this part of Oregon, and in fact much of the western U.S. Coast, is quite complex since for long periods of recent geological history this edge of the continent was active and colliding with oceanic terrane to the west. Some of the oceanic terrane was subducted below the continental crust.  However, other parts were accreted or “stuck to” the continent.  This later terrane is mostly composed of oceanic ultramafic rocks and deep ocean sedimentary rocks. The weathering of these terranes in the Riddle District is something like Miocene in age.  And, the laterite formation at Riddle is an anomaly since the equator was not within 20 degrees. 

Garnierite: chalcedony infused with nickel (chrysoprase?) and small nodules of concentrated nickel.  Nodule from Nickel Mountain, Oregon. Width ~4.3 cm.

Garnierite a mixture of nickel silicates.  Note some darker green nodules that probably contain a higher percentage of nickel.  Nodule from Nickel Mountain, Oregon.  Width ~ 6 cm.

Although some of the nickel at Nickel Mountain was enriched iron oxides most was contained in hydrous magnesium-and nickel-enriched silicate termed garnierite.  At one time garnierite was considered to be a mineral but the official name has been discredited and garnierite now refers to a variety of green nickel ore with a varied composition and seemingly a quite complex chemical history. 

The green color in garnierite varies, seeming with composition and depositional environment.   Pecora and others (1949) believed light colored garnierite was due to the alteration of olivine-rich ultramafic rocks rich in clay minerals but poor in nickel.  The light green to bright green garnierite resulted from the leaching of manganese oxide, magnesium, nickel and iron from the original dark green garnierite, rich in nickel, which was originally deposited by groundwater.  With newer instrumentation available (Extended X-ray Absorption Fine Structure) Roqué-Rosell and others (2011) found that garnierite is actually a solid solution series between sepiolite, a complex, magnesium-rich clay mineral [Mg₄(Si₆O₁₅)(OH₂)-6H₂O)] and falcondoite, the nickel rich analog of sepiolite [Ni₃Mg(Si₆O₁₅)(OH₂)-6H₂O)] with samples analyzed showing between 3 and 77 percent falcondoite. 

In my advanced years I am often confused J  So, I remain somewhat confused about garnierite.  The notation above indicates the garnierite is a mixture of falcondoite and sepiolite.  OK, I certainly can buy that explanation.  However, if I go to my favorite mineral website (MinDat.com), garnierite is listed as synonym of nepoulite, willemseite, and pimelite of which only the latter is found in Oregon; all are complex magnesium-nickel silicates. I have seen other references that indicate some garnierite is chrysoprase, a nickel-enriched chalcedony. The best that I can do is quote the “official” explanation from MinDat: “[Garnierite is the] generic name for a green nickel ore which has formed as a result of lateritic weathering of ultramafic rocks (serpentinite, dunite, peridotite).  [It is] mostly a mixture of various Ni- and Ni-bearing magnesium layer silicates.  [Garnierite] occurs in many nickel laterite deposits in the world.”

I’ll tell you what---I learn so much every day of my life and for this I give credit and thanks to my early childhood teachers, and to my parents.  Although both paraphrased the following quote, it was a mantra they instilled in their children:  You can teach a student a lesson for a day; but if you can teach him to learn by creating curiosity, he will continue the learning process as long as he lives. ~Clay P. Bedford

REFERENCES CITED

Allen, K., 2014, What Sudbury can teach China about Air Pollution: www.TheStar.com/news, May10, 2014.

Brand, N.W., Butt, C.R.M., and Elias, M., 1998, Nickel Laterites: Classification and Features: AGSO Journal of Australian Geology and Geophysics, v. 17, no. 4.

Canickel Mining Limited (2014), Sudbury Properties—Ontario: www.canickel.com.  

Marsh, E. and E. Anderson, 2011, Ni-Co laterites: A Deposit Model: USGS Open-File Report-1259.

Pecora, W.T., Hobbs, S.W. and Murata, J.K., 1949, Variations in Garnierite from the Nickel Deposit near Riddle, Oregon: Economic Geology v. 44.

Roqué-Rosell, J., Villanova-de-Benavent, C., Proenza, J.A., Tauler, E. and Galí,S., 2011, Distribution and Speciation of Ni in Sepiolite-Falcondoite-type “Garnierite” by EXAFS: Macla, v. 15.

Samama, J., 1986, Ore Fields and Continental Weathering: New York, Van Nostrand Reinhold Co., 326p.

COLORADO PSEUDOMORPH: HEMATITE AFTER SIDERITE

For many years I taught courses in paleontology and one of the basic types of fossilization students learned about was “replacement.”  It was easy to understand as the class used petrified wood as an example.  Here the original wood is replaced by microcrystalline quartz (usually chalcedony) or opal (or other minerals).  The gross structures of the wood such as “bark” and tree rings are often preserved.  Another common example was looking at Cretaceous snails replaced by pyrite and/or marcasite.  These large snails are common in the black shales of the Kiowa Formation exposed in central Kansas and were an interesting target in field trips.  Today these replacement examples are more commonly termed pseudomorphs---false forms.

In the last few years I have muddled around in the world of minerals and have come to realize that there are a substantial number of mineral pseudomorphs, especially substitution pseudomorphs where there is a mineral to mineral replacement.  Some are fairly easy to recognize such as the goethite after pyrite crystals found on the shores of Utah Lake at Pelican Point.  Here the perfect cubes of pyrite crystals are preserved except the original iron sulfide has been replaced by a hydrated iron oxide.  Others pseudomorphs are much more difficult to recognize, at least for me: acanthite (silver sulfide Monoclinic Crystal System) after argentite (silver sulfide Cubic Mineral System).  The cubic crystals of argentite are present but the internal molecular structure changed as solution temperatures decreased.  The problem is that acanthite “looks like” argentite” and it seems to take some nifty instrumentation to recognize each.  For other examples check Blog postings April 16, 2013 cervantite after stibnite; September 2, 2012 calcite after glauberite; and August 23, 2012 goethite after pyrite.

Pseudomorphs that are much more difficult to understand are 

paramorphs.  Aragonite is a calcium carbonate [CaCo₃] polymorph (Orthorhombic Crystal System) that is unstable in the current earth’s atmosphere and alters to calcite over time.  Calcite [CaCo3] is the stable calcium carbonate polymorph and belongs to the Trigonal Crystal System.  Most aragonite seems younger than about 350 Ma.  However, there seems to be some evidence that very old algal mats called stomatolites found in Precambrian (~2.7 Ga) rocks of Australia may contain tiny aragonite crystals that have been protected by microbes (Lepot and others, 2008).

A perimorph is a type of pseudomorph where a mineral (#2) encrusts or encases mineral #1 and then mineral #1 dissolves and mineral #2 forms a cast or shell indicating the original crystal form of #1.  Quartz, since it crystallizes from solution late in the mineralization process, is a common encrusting mineral (#2). Fluorite and calcite are often minerals encrusted (#1) and seem easily removed through dissolution.  I cannot locate perimorphs in my modest collection.

 

Substitution pseudomorph: hematite after siderite.  Park County, Colorado.  Width specimen ~2.2 cm.

The reason behind this entire pseudomorph banner is that Colorado has produced some unique pseudomorphs of hematite (iron oxide Fe₂O₃) after siderite (iron carbonate FeCO₃).  Some of these pseudomorphs also are found with quartz crystals and microcline.  As best that I can tell the specimen in my collection came from Park County and was collected by the well-known mineralogist Clarence Coil.  See Blog posting on March 5, 2014 describing other Coil finds.

REFERENCES CITED

Lepot, K., K. Benzerara, G. E. Brown, Jr, and P. Philippot, 2008, Microbially Influenced Formation of 2,724-million-year-old Stromatolites: Nature Geoscience, v. 1.