Period 6 element also dissolves in alkaline solutions of cyanide, which have been used in mining. Gold dissolves in mercury, forming amalgam alloys. Gold is insoluble in nitric acid, which dissolves silver and base metals, a property that has long been used to confirm the presence of gold in items, giving rise to the term "the acid test." Gold has been a valuable and highly sought-after precious metal for coinage, jewelry, and other arts since long before the beginning of recorded history. Gold standards have been a common basis for monetary policies throughout human history, later being supplanted by fiat currency starting in
Acidophiles in acid mine drainage economically and from a conservation perspective. Control of these acidophiles and their harnessing for industrial biotechnology shows their effect need not be entirely negative. The use of acidophilic organisms in mining is a new technique for extracting trace metals through bioleaching, and offers solutions for the phenomenon of acid mine drainage in mining spoils. Upon exposure to oxygen (O) and water (HO), metal sulfides undergo oxidation to produce metal-rich acidic effluent. If the pH is low enough to overcome the natural buffering capacity of the surrounding rocks (‘calcium carbonate equivalent’ or ‘acid neutralising capacity’), the surrounding area may become acidic,