Rocks and Minerals found in Zagreb - new stamp issue of Croatian Post

October 26th, 2014 receives information that Croatian Post has issued a set of stamps dedicated to the theme "Minerals and Rocks". The stamp release was unveiled on the 24th of October.

Science which examines the origins and classification of rocks is called petrology while the minerals which build rocks are the subject of investigation of mineralogy Rocks are aggregates of one or more kinds of minerals, while minerals are natural parts whose composition can be expressed by chemical formula. Minerals are characterised by regular inner structure and regular position of atoms, which is reflected also on the outer form of a mineral (such regular forms of minerals are called crystals).

By increased pressure and temperature and through chemical changes new minerals and mineral aggregates can generate. The same is with rocks – already existing rocks (eruptive, sedimentary, or metamorphic) are changed through metamorphosis (their structure and texture, but also mineral composition is changed), so that according to their way of origin they are classified as metamorphic rocks.

One of the metamorphic rocks which can be seen in the vicinity of Zagreb (on the Mount Medvednica) is green schist. Such rock is created by metamorphosis from neutral to basic magmatic rocks by metamorphosis of sedimentary rocks. On Medvednica first there were eruptive rocks: basalt, gabbro and diabase, while from the sedimentary rocks there were fine-grained marl rocks. By metamorphosis from them developed today's green schist. This is rock of intensive green colour (the colour originates from the chlorite and epidote minerals and amphiboles).

A look on the microscopic faceted piece shows also minerals albite, quartz and apatite. Green schist builds the main body of the Mount Medvednica and is the oldest rock of the mountain. Some researches show that original rocks from which green schist originates are up to billion years old (which is probably exaggerated), but also the "sure" four hundred million years also tells us about very old rocks. Green schist is also used as building stone (e.g. St. Mary's - Queen of Croats - Chapel on Sljeme) or as coating for facades of many buildings in Zagreb.

With the development of the city of Zagreb on the slopes of Medvednica many quarries of architectural building stone were opened, among whom also the quarries of green schist (Pustedol, north from Gračani, a quarry north from Čučerje, near Markuševac etc.) Today we are trying to remediate this "wounds" in nature by re-purposing.

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