Hemimorphite - Encyclopedia

    Class : Silicates
    Subclass : Sorosilicates
    Crystal system : Orthorhombic
    Chemistry : Zn₄Si₂O₇(OH)₂ . H₂O
    Rarity : Common

Hemimorphite is a secondary silicate of zinc present in the oxidation zones of lead and zinc sulphide deposits. It is particularly common in arid climate and clay-marl context where it is the main oxidized zinc ore, which can form entire hills. The "calamine" of the former miners is essentially hemimorphite, more rarely smithsonite. The hemimorphite owes its name to the Greek hêmi (half) and morphê (form) because its doubly-terminated crystals have 2 different terminations due to the absence of center of symmetry. Hemimorphite generally exists in concretions, stalactites or in botryoidal masses, evoking smithsonite, also sometimes in rocky or earthy masses, cavernous, brown in color due to iron oxides ; it is a mineral that "rings" and disintegrates into thin scales. The crystals are fairly common, flattened to lamellar, rarely centimetric, elongated and streaked vertically and often grouped in fans. As with wulfenite, crystal shpe depend on acidity and oxidation-reduction conditions. Hemimorphite is colorless to white, sometimes green, pale blue or yellowish brown, when it is colored by iron oxides. Hemimorphite was once an occasional zinc ore nowadays supplanted by sphalerite.

In France, the best specimens are bluish crystals up to 2 cm frequently grouped in rosettes. Inlaid incrustations of a very beautiful blue color come from the Bentillou mine (Seintein, Ariège) but also from Germs-sur-l'Oussouet (Haute-Pyrénées). It is found in millimetric crystals in Argentolles, Mechelen (Gard), Can Pei (Pyrenees-Orientales), Peyrebrune (Tarn) where the sprays can reach half a centimeter, Propières and Longefay (Rhône), Argentolle (Saône-et-Loire), etc ...

Rare twinning on {101} only.

Some Mexican hemimorphite crystals are dyed in blue with phthalocyanine. The dye penetrates preferentially into fractures and surface roughness, in appearance these hemimorphites resemble stilbites covered with Indian cavansite. Photos Mardani Fine Minerals.