When Good Rocks Go Bad
Foundation for roads and seawalls. Processed to steel girders and thin wires. Polished into jewelry. Vacationers find river rock, climbing rock, and campfire rings.
Seemingly inert, they grow, shrink and change on their own time scale, molded by pressure, temperature, and erosion. Sand can be compressed into quartz, and then weathered back to sand. Limestone can be pressured into marble, and mud forms shale, then slate.
Many metals are attracted to sulfide (S2-), a sulfur atom in a reduced state ready to match up with metal atoms with a positive charge. Iron atoms bind sulfide atoms to form the mineral pyrite (FeS2), copper forms chalcopyrite (CuFeS), arsenic forms arsenopyrite (FeAsS), zinc forms sphalerite (ZnS).
Sulfide minerals react with water and oxygen: add some hydrogen and oxygen atoms, and sulfide rock becomes sulfuric acid (H2SO4). The copper, iron, lead, zinc components….dissolve.
Solid rock…. dissolves into battery acid and traces of minerals. Sulfide trades out dancing partners of Fe and Zn for H and O.
Good rocks, gone bad.
Drive past red stains on a hillside and you will see dissolved iron seeping down from weathering pyrite. However, water and air rushing over a hill only penetrate a short distance, the chemical reaction can only occur on the fresh face where sulfides, water and oxygen are all in contact. The erosion of the rock face exposing fresh material is a slow process.
Mining accelerates this process.
Fissures and fractures created by blasting expose fresh rock. Boring mine tunnels en route to mineral rich zones opens up “host rock.” Host rock often contains acid-generating iron sulfides. Because it is low in economic minerals, host rock ends up in mountainous piles of rubble waste, exposed to weather. With greater fresh rock surface area, more acid will form and leach out into the environment.
Good rocks, gone bad – exponentially.
A champion example is found at Iron Mountain, near Redding, California. There lie the remnants of underground tunnels from the silver, gold, copper, lead, zinc, and sulfur mining. The last mine closed in 1963 but the rocks lining miles of tunnels continue to react today. Spectacularly. Creating such high acidity that this area is known as the site of the first documented negative pH water.
In June 2014, USGS researchers Charlie Alpers and Kirk Nordstrom led a tour of the Richmond Adit – an underground tunnel with only one entrance. Lit only by helmet headlamps, you splash through long, shallow low pH puddles as acid drips from the ceiling and walls. As you reach the most reactive areas, you are hit with a wall of sauna-like 120o F heat – the frenzy of exothermic reactions. Pyrite is furiously reacting with water and air in the humid underground, transforming into mineral sulfate salts like blue-green melanterite (Fe2+SO4*7H2O) which proceeds to form orange-pillowed copiapite [Fe2+Fe3+4 (SO4)6(OH)2*20H2O]. Drips from the tunnel roof form colored stalactites and stalagmites.
Crystal colors of blue-green, transparent sea-green, black, purple, red-orange-yellow-brown variations are defined by varying amounts of zinc, copper, magnesium, nickel, cobalt pushing out iron atoms: changing out iron for a more interesting partner.
A mineralogists dream, an environmentalists nightmare. The acid is strong enough that “the periodic table of elements” washed into the Sacramento River. Tens of thousands of salmon and trout were killed by the leached metals until a treatment plant was installed in 1994.
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