Normal halide, matlockite group
Specific gravity: 7.12
Hardness: 2½ to 3
Colour: Colourless, light yellow to light golden-yellow, greenish
Solubility: Soluble in nitric acid
Melting point: 601oC
Matlockite is an extremely rare secondary lead mineral, occurring in
the oxide zone of some lead-bearing mineral deposits
At Challacollo, Tamarugal Province, Tarapacá, Chile, matlockite occurs with pseudoboleite (Dana).
At Sainte-Lucie Mine, Saint-Léger-de-Peyre, Mende, Lozère, Occitanie, France, matlockite occurs on galena (Dana).
At Berg Aukas Mine, Grootfontein, Otjozondjupa Region, Namibia, descloizite crystals have been found with a two-layer coating. The inner yellowish green coating appears to be a mixture of matlockite and descloizite, and the outer chalk-white coating is matlockite (R&M 96.2.132).
At the type locality, Cromford, Derbyshire Dales District, Derbyshire, England, UK, matlockite is associated with phosgenite, anglesite, cerussite, galena, sphalerite, baryte and fluorite (HOM, Mindat). The deposit is supergene in origin and not primary. In limestone hosted orebodies the stable supergene lead mineral is usually cerussite. Minerals requiring a relatively low pH (high acidity), such as anglesite, normally occur in limestone hosted orebodies only if the main gangue minerals are carbonate free, such as baryte and fluorite, or in cavities completely enclosed by galena. This deposit meets these requirements.
Five variables determine the formation of the assemblage of minerals, namely pH (acidity), concentrations of sulphate, chloride and fluoride ions and the partial pressure of carbon dioxide in equilibrium with the system.
An essential requirement for these minerals to form is that the local environment must have remained oxidising over a long period of time, and also the concentration of carbon dioxide must have been raised above atmospheric levels. In addition, the presence of anglesite requires a relatively low pH (high acidity). The most likely explanation is that there was some pyrite or marcasite present in the vein material or wall rock and that the sulphuric acid released by the oxidation of this lowered the pH while releasing carbon dioxide by reaction with carbonates. The lead ions must have come from the oxidation of galena and the chloride ions possibly came from a late stage hydrothermal brine. The source of the fluoride ion needed for the formation of matlockite is most likely to have been the presence of fluorite in the orebody.
With pH in the range 5 to 5.5 (moderately acid), phosgenite and matlockite could readily form and co-exist; the formation of both of these minerals is favoured by high chloride and fluoride ion concentrations in solution. Matlockite and anglesite can also co-exist over a wide range of pH (acidity) (JRS 16.6-12).
At the Mammoth-Saint Anthony Mine, St. Anthony deposit, Tiger, Mammoth Mining District, Pinal county, Arizona, USA, matlockite is associated with diaboleite, boleite, caledonite and leadhillite (HOM).
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