CADMIUM, GREENOCKITE AND NI-CAD BATTERIES

Among others things, the Christmas and holiday season is a time of giving and receiving gifts.  In many instances when I think about gifts the first thing that pops into my head---batteries, and more batteries!

I once bought my kids a set of batteries for Christmas with a note on them saying “toys not included”.              Bernard Manning

So, the other day in one of my periods of shallow thought, some would call it daydreaming, I was thinking about Ni-Cad batteries, you know the rechargeable kind.  OK, I know where nickel is mined and produced but what about cadmium?  Is cadmium found naturally, and mined, or is it a by-product of some other mineable metal?  Beats me but I don’t think cadmium is a mineral found au naturel.  However, I do remember that I have an obscure cadmium mineral tucked away in my collection.  

By this time my curiosity was piqued so off I went to try and find answers.

The Ni-Cad batteries were just about the first rechargeable batteries:  they are charged, store the energy, discharged in a load, and then recharged.  The Ni-Cad batteries use a nickel oxide-hydroxide as a positive electrode and metallic cadmium as the negative electrode.  These rechargeable batteries allow for a chemical reaction as they discharge when a load is detected.  At the positive electrode the reaction is 2NiO(OH)+2H₂O+2e^{-  --------->}2Ni(OH)₂+2OH^- and potassium hydroxide is used as the alkaline electrolyte.  At the negative electrode the reaction is Cd+2OH^- -------> Cd(OH)₂+2e^-  So in the total reaction nickel oxide hydroxide and Cd and water form a nickel hydroxide and cadmium hydroxide.  When recharging takes place the total reaction is just the opposite and the nickel and cadmium hydroxides break down into the free cadmium and water and nickel hydroxide.  Currently these one-time ubiquitous Ni-Cad batteries seem to be on their way out as more efficient rechargeable batteries such as nickel metal hydride (NiMH), lithium ion polymer (Li-ion polymer) and lithium ion (Li-ion) are coming in as replacements.  These newer batteries have bounded into use for a couple of reasons: 1) they are less costly to manufacture than Ni-Cad batteries; and 2) cadmium is toxic to living organisms and quite toxic in the environment. 

As for cadmium, my initial thought about the element occurring “naturally” as a mineral was wrong (certainly not the first time).  Webmineral.com noted that cadmium was “found in the heavy non-magnetic fraction of a mechanical concentrate from a gabbro intrusion…from the Ust’-Khannin intrusive, Vilyui River basin, eastern Siberian platform.”  MinDat listed the Goldstrike Mine in Eureka County, Nevada, as producing native cadmium.  However, both of these occurrences are very minor and more of a curiosity than a source of the metal.

So, if native cadmium is extremely rare and almost non-existent, where does cadmium needed in industry “come from?”  Perhaps from minerals containing cadmium?  What are these minerals?

For starters, all cadmium-bearing minerals are quite rare and only greenockite, a sulfide [CdS], is well known (and some of that may be hawleytite).  Niedermayrite [Cu₄Cd(SO₄)₂(OH)₆-4H₂O] is a hydrated copper cadmium sulfate hydroxide; otavite [CdCO₃] is a cadmium carbonate; cadmoindite [CdIn₂S₄] is cadmium indium sulfide while cadmoselite [CdSe] is a cadmium selenide; hawleyite [CdS] is a sulfide crystalizing in the Isometric Crystal System and is a dimorph (same chemical formula) of greenockite that is Hexagonal; the arsenate in the group is keyite [Cu₃Zn₄Cd₂(AsO₄)₆-2H₂O].  There may be others?

So, cadmium is produced as a byproduct during the refining of zinc (primarily), copper and lead ores.  It appears that ores such as native copper, galena, sphalerite and smithsonite have trace amounts of cadmium present.  I also found a note that stated cadmium is produced in countries that refine these ores as opposed to countries that mine the ore.  In some cases these countries are the same as the mines and refineries are together. 

Within those rare minerals listed above I only have a single small specimen of greenockite.  In fact, if an “ordinary rockhound” has a cadmium mineral in their collection it is usually greenockite, or at least labeled as such since it is sometimes confused with hawleyite. 

Greenockite (CdS) is a sulfide mineral that contains the negative anion sulfur (S) plus the positive cation cadmium (Cd).  The most spectacular specimens have six-sided (hexagonal) crystals that are hemimorphic (are terminated in different manners at each end of the C axis) and are red to red-brown in color.  Other specimens are massive in nature and tend to orange to orange-yellow to yellow in color.  The streak is red to red orange to brown.  The crystals are usually vitreous while the crusting form is earthy to resinous.  Greenockite is fairly soft at 3.0 to 3.5 (Mohs) and crystals are more transparent/translucent while encrustations are usually opaque.  Crystals tend to break with a conchoidal fracture.

Photomicrographs of greenockite occurring as honey orange encrustations with a couple of red poorly formed crystals.  Width ~1.1 cm.  Dark metallic mineral is most likely arsenopyrite.

Photomicrographs of greenockite occurring as honey orange encrustations.  Width ~6 mm.

Greenockite is a secondary mineral and can be located in a variety of environments.  For example, at Patterson, New Jersey, vugs in the basalt produce greenockite along with prehnite and zeolites.  In the old zinc mines near Franklin, New Jersey, greenockite is found as earthy coatings on sphalerite.  In some of the Tri-State (Missouri, Kansas, and Oklahoma) lead and zinc mines greenockite is found as an earthy crust on smithsonite and sphalerite.  In addition, greenockite has been found in some slags where smithsonite was smelted.  It is never a common mineral and tough to identify when the habit is encrusting rather than crystalline.  It is a dimorph of  hawleyite (CdS) and quite similar in appearance; however, hawleyite has not been reported from the Bolivia mines so I am sticking with greenockite.

My specimen came from the tin (cassiterite) mines, specifically the Siglo XX Mine (Siglo Veinte Mine), Llallagua, Rafael Bustillo Province, Potosi Department, Bolivia.

An interesting tidbit or two about toxic cadmium:  cigarette smoke contains cadmium as does coal where the element ends up as part of the flue dust.  Many phosphate fertilizers contain cadmium which is then transferred to agricultural soil (and maybe to the water supply?).  Vincent Van Gogh was a big fan of cadmium based pigments for his paintings.  And so, it goes.