Images
Formula: Zn4(Si2O7)(OH)2.H2O
Sorosilicate (Si2O7 groups)
Crystal System: Orthorhombic
Specific gravity: 3.475 measured, 3.484 calculated
Hardness: 4½ to 5
Streak: White
Colour: colourless, white, pale blue, pale green, gray, brown
Solubility: Slightly soluble in hydrochloric acid
Common impurities: Cu,Fe
Environments:
Hemimorphite is one of the three main supergene zinc minerals,
the others being smithsonite and
hydrozincite.
Hemimorphite is a high temperature secondary mineral found in
the oxidation portion of zinc deposits, associated with
smithsonite,
sphalerite,
cerussite,
anglesite and
galena.
Localities
At Mapimi, Mexico, fluorellestadite
pseudomorphs after hemimorphite have been found
(KL p222).
At the Ojuela Mine, Mapimí, Mapimí Municipality, Durango, Mexico, hemimorphite occurs with
rosasite
(Mindat photos).
In 2020 remarkable intense blue hemimorphite crystals to 8 cm long were found here. The blue colour was due to a
surface coating that was found to contain phthalocyanine blue BN. This pigment does not occur in nature, so the blue
colour of the hemimorphite must be anthropogenic
(R&M 97.2.177).
At Roughton Gill, Caldbeck Fells, Cumbria, England, UK, hemimorphite is relatively common as radiating
sprays of millimetre-sized platy crystals, which are occasionally associated with
chrysocolla, aurichalcite,
rosasite and in one case mottramite
(JRS 11.14).
At the Roughton Gill mine, At Roughton Gill, Caldbeck Fells, Cumbria, England, UK, fine specimens of blue botryoidal
hemimorphite occur. Rosasite is a common associate, but the botryoidal
forms are not usually coated with other minerals beyond a little pyromorphite
(JRS 14.13-14).
At the Golconda mine, Brassington, Derbyshire, England, UK, hemimorphite has been found on
baryte
(RES p107).
At the Millclose mine, Darley Dale, Derbyshire, England, UK, hemimorphite occurs on
baryte and
on calcite
(RES p98).
At the Wapping mine, Matlock Bath, Derbyshire, England, UK, hemimorphite occurs on
fluorite,
sometimes with minor baryte or limonite
(RES p72).
At Joplin, Missouri, USA, hemimorphite pseudomorphs after
calcite have been found
(KL p225).
At the Kabwe mine, Central Province, Zambia, hemimorphite is common though often inconspicuous. It
occurs mainly as intergrowths with cerussite and
goethite, and it has been found associated with
tarbuttite and
smithsonite, and also with
zincolibethenite on a
limonite matrix
(R&M 94.2.125-126).
Alteration
The first stage in the formation of zinc supergene minerals is the oxidation of
sphalerite to zinc sulphate, which is
very soluble and remains in solution as zinc and sulphate ions:
ZnS + 2O2 → Zn2+ + SO42-
Hemimorphite forms only at high pH (very alkaline conditions), when Zn2+ ions in solution react with a
mobile source of silicate ions, such as silicic acid, normally derived from the weathering of silicate minerals.
Hemimorphite
is therefore most abundant in areas where such a source exists, such as the Askrigg block of the Northern Pennine
Orefield, UK, where there are numerous beds of chert
(JRS 18.14).
Back to Minerals