Chemical Reactions that Alter the Apperance of Minerals
Some minerals react with chemicals in ways that produce an altering appearance, which can lead to results both fascinating and terrible. Hazing, etching, dissolving and unwanted deposits are some of the results that you can expect with various minerals and you can find out more about those in another section, the subject of this page is minerals that can have an appealing altering of appearance due to a chemical reaction.
A rich solution of Copper Sulfate and water is used to turn calcium carbonate crystals, both Calcite and Aragonite, into green and blue. As the crystals soak in the solution over the course of 2 – 4 weeks, the crystals will start to develop a blue coloring which then becomes a rich green, similar to fine Malachite specimens.
Sodium hypochlorite, or common household bleach, will turn several lead minerals into interesting shades, notably, the red color that is given to the skin of Anglesite crystals. The color change varies by time exposed to the bleach, as it will start to increase the red hue from duration of exposure. The color change is only on the surface of the crystal, as the sodium hypochlorite mixes with the lead to form a thin layer of the mineral minimum as an alteration.
Iron Oxyhydroxide Staining– It’s often desirable to remove iron oxyhydroxide staining on minerals to improve their appearance. These stains, often appearing as yellow, orange, or red coatings are composed primarily of iron oxyhydroxides such as limonite and goethite, and less commonly of iron (III) oxide (hematite) or potassium iron (III) sulfate hydroxide (jarosite.) Super Iron Out is likely the most commonly used commercially available reagent for removing iron staining on minerals; the primary active ingredients are sodium hydrosulfite and sodium metabisulfite which are both chelators and reducing agents. Sodium carbonate and citric acid are added as buffering agents. All other ingredients are decomposition products of sodium hydrosulfite and sodium metabisulfite. Check out the MSDS for more info: http://www.summitbrands.com/summit/downloads/msds/usa/MSDS%20Super%20Iron%20Out.pdf
The temperature, pH, and concentration greatly affect the potency and reactivity of Super Iron Out, although the buffering agents do help to slightly stabilize the properties. When testing out a solution of iron out to clean sensitive minerals, it is best to start with a room temperature, fairly weak solution at neutral pH. If more potency is desired, the concentration of Super Iron Out can be raised, the pH can be changed by addition of an acid or base, or the solution can be heated which increases reactivity but also quickly decomposes the active ingredients. If you want to get really technical with any sort of cleaning, I recommend development of a rough ternary diagram with the three axes being temperature, pH, and concentration (in weight/volume or molar) and points graphed being a numeric scale of “cleaning effectiveness,” which is sort of arbitrary, but you will notice the cleaning ability that certain temp-pH-concentration domains will have. It’s not wise to use Super Iron Out on phosphates, sulfides, transition metal sulfates, many carbonates, vanadinite, and evaporites as they will alter or discolor the surface.
Oxalic Acid is perhaps the most popular reagent used to clean iron staining from Arkansas and Brazilian quartzes. Oxalic acid cleans via chelation- it forms many highly stable complexes with transition metals- in our case iron. Oxalic acid can be purchased at hardware stores as wood bleach. Note that it is slightly toxic and needs to be handled somewhat carefully. It’s highly soluble in water and can readily be dissolved to make a fairly concentrated solution. If any acid soluble transition metal containing minerals (such as pyrite, siderite, malachite, cerussite) are present on the specimen you wish to clean it is best to stay clear of using oxalic- you can create a nasty pitting and prominent discoloration of the surface. The discoloration on iron minerals is mustard yellow which is the most common form of residue encountered from poor cleaning with oxalic acid.