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Formula: PbClF
Normal halide, matlockite group
Crystal System: Tetragonal
Specific gravity: 7.12 measured, 7.16 calculated
Hardness: 2½ to 3
Streak: White
Colour: Colourless, light yellow to light golden-yellow, greenish
Solubility: Soluble in nitric acid
Melting point: 601oC
Environments
Matlockite is an extremely rare secondary lead mineral, occurring in
the oxide zone of some lead-bearing mineral deposits
(HOM Mindat).
Localities
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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