The ECAL is constructed from crystals of lead tungstate, PbWO. It forms a solid solution series with scheelite (calcium tungstate, CaWO). Powellite is a mineral form of calcium molybdate containing a small amount of tungstate. Zirconium tungstate (()) is a metal oxide with unusual properties. It may also be prepared from the reaction between hydrogen carbonate and sodium tungstate. Similar materials are calcium tungstate (scheelite) and zinc tungstate. Edison had quickly discovered that calcium tungstate screens produced brighter images. For example, 12-Phosphotungstic acid is formed by condensing phosphate ion with tungstate ions. Sodium tungstate and lead have been studied for their effect on earthworms. In soil, tungsten metal oxidizes to the tungstate anion. These metals are imported as their oxyanions, molybdate, and tungstate. Stolzite is a mineral, a lead tungstate; with the formula PbWO. Sodium tungstate is a competitive inhibitor of molybdenum. Scheelite, the mineral calcium tungstate, often contains a small amount of molybdate. Several polymorphs of sodium tungstate are known, three at only one atmosphere pressure.