Euchroite Cu2(AsO4)(OH)•3H2O is among the most beautiful Slovak secondary minerals. Its name was derived from the Greek word euchros, meaning beautifully coloured. Euchroite was first described in 1823 by J. F. August Breithaupt (1791 – 1873), a professor of mineralogy at the Freiberg Mining Academy, however, this came from the world-renowned deposit at Svätodušná.
Euchroite is a supergene mineral that crystallises as an orthorhombic crystal. It is copper arsenate that forms isometric, tabular, short-prism crystals of mostly emerald green or, less frequently, a blue-green colour. It is normally transparent with a glassy gloss; however, typically some crystals, such as isomeric crystals, have a matt gloss. In Ľubietová, it forms in crystals of more than 1cm long, sometimes forming beautiful aggregates several centimetres in length along the cracks of metamorphosed rocks. Most of the time, it is found with olivenite, strashimirite, malachite and parnauite.
Ľubietová-Svätodušná still holds the world's most significant deposits of this mineral, with the most beautiful and largest crystals and aggregates of the mineral being housed in all of the world's leading museums. Other important deposits of Euchroite include those at Poniky-Farbište in Slovakia and Zapačica in Bulgaria.
The maximum card (CM) bears the image of typical crystals of Euchroite from Ľubierová, rising from black flowing aggregates of Fe-Mn oxides. Part of the crystals on the maximum card is also depicted on the stamp; another double crystal of Euchroite is the theme of the FDC surcharge. On the postage stamp introducing the issue, there is the typical crystal shape of Euchroite from Ľubierová, which was published in 1957 by the important Austro-Hungarian mineralogist, crystallographer and physicist, Wilhelm Josef Grailich (born in Bratislava in 1829) and the leading Austrian chemist, Viktor von Lang (1838 – 1921).
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