Secondary arsenic minerals are defined as forms of arsenic that arise during hydrothermal alteration or weathering of primary minerals. In the environmental sciences, the weathering of primary arsenic-bearing ore minerals under near subsurface and surface conditions is important and widely studied. Numerous site-specific studies have been completed documenting the importance of the formation of secondary arsenic minerals in attenuating dissolved arsenic during the weathering of primary arsenic minerals.
Mineralogy of arsenic
Secondary arsenic minerals can be grouped into arsenic sulphides, arsenic(III) oxides (arsenites) and arsenic(V) oxides (arsenates) (see Table 1). Such arsenic sulphides, as arsenopyrite, arsenic-bearing pyrite, orpiment and realgar, are the commonest primary arsenic minerals. However, several spectroscopic studies have verified the formation of secondary orpiment and realgar in low-temperature, shallow aquifer sediments—probably related to microbiological reduction activity. The simple arsenic(III) oxides (arsenolite and claudetite) usually form as secondary weathering products of arsenic sulphides but are more commonly found as the oxidation products of the roasting of arsenic-bearing ore minerals or coal. Arsenic(V) minerals (arsenates) comprise a large class with approximately 300 natural species. Mineralogically, arsenates are usually considered as a subclass of the phosphate mineral group because of the similarity in size and charge of the arsenate [AsO4]3- and phosphate [PO4]3- anionic units. Similar to phosphate minerals found in soil and surface environments, arsenate minerals occur in a variety of arsenic-rich soil and oxidized environments. Also, they are commonly found as weathering products of arsenic-bearing ore deposits—where sulphidic minerals are often coated with surface layers of oxidized and hydrated arsenate minerals. Under such environments, scorodite is by far the most common secondary arsenic mineral.
Locations of Secondary Arsenic Minerals
Secondary arsenic minerals are locally common and are found worldwide. Gorgeous mineralogical samples of hydrothermal secondary arsenic minerals are exhibited in many mineralogical collections of large museums. The best known are macroscopic scorodite crystals from Tsumeb (Namibia), from El Cobre mine (Mexico) and from Hemerdon Mine (England), as well as crystals of annabergite from the Laurion Mines (Greece) or rythrite from Bou Azzer (Morocco). Much less alluring, but more environmentally important arsenic minerals are found in mine-waste heaps and other types of anthropogenic deposits. There are many old mines and historic localities of secondary arsenic minerals in Europe, especially in Bohemia, Germany, Great Britain and Poland. Also, there are historic localities of hydrous ferric arsenates and sulphoarsenates (for example, bukovskyite, ka?kite, pharmacolite, scorodite) in Kutná Hora and Jáchymov ore districts in Bohemia (see Figures 1, 2, 3, and 4), the Cornwall district in England and the Stara Góra deposit in Poland.
There is a wealth of hydrous secondary arsenates in Australia and New Zealand, including those from such localities as the Mole River (arsenolite, pharmacolite, scorodite) in New South Wales and the Blackwater, Macraes and Phoenix gold mines (scorodite and other hydrous ferric arsenates). In Canada, interesting material is frequently produced from such places as northern British Columbia and the Yukon, where it is present in many abandoned as well as recent mine-waste heaps and tailing impoundments. Natural geochemical anomalies provide examples of regional, long-term environmental contamination. At Echassières (Allier, France) and the Mokrsko gold deposit (in central Bohemia) (see Figures 5, and 6) soils have developed on rocks enriched in arsenic over single to tens of km2, and that contain secondary pharmacosiderite and arseniosiderite, formed by oxidation of arsenic-bearing sulphide ores.
Arsenic used as a poison is well known in history and popular literature, and secondary arsenic minerals—as its relatively soluble salts—are no exceptions. There is a risk that prolonged handling or repeated contact can produce toxic conditions by absorption. The lethal dose by ingestion can be as low as 20 mg. Therefore, the collector or curator should not lick fingers after secondary arsenic mineral handling. Some of the secondary arsenic minerals are susceptible to dissolution, whereby a wide range in solubility has been noted. For example, arsenic oxides (arsenolite and claudetite) and many calcium arsenates (that is, haidingerite and pharmacolite) are very soluble in water, whereas some iron arsenates (that is, beudantite, pharmacosiderite and scorodite) are relatively insoluble. As a result, once these minerals are formed, they will effectively immobilize arsenic in contaminated sites—and their precipitation controls the amount of arsenic in the water. On the other hand, the precipitation and re-dissolution of secondary arsenic minerals may influence greatly the chemical composition of ground and surface waters. As a consequence, the amounts and types of secondary arsenic minerals need to be determined in any high-arsenic environments.
- Antony J.W., Bideaux R.A., Bladh K.W. and Nichols M.C. 2000. Handbook of Mineralogy Volume III: Arsenates, Phosphates, Vanadates. Mineral Data Publishing, Tucson.
- Drahota P. and Filippi M. 2009. Secondary arsenic minerals in the environment: A review. Environment International 35: 1243-1255
- O'Day P.A. 2006. Chemistry and mineralogy of arsenic. Elements 2: 77–83.