MINERALS AND ROCKS (S/S)

MINERALS AND ROCKS
 
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 which are known to us build the solid part of the Earth, called lithosphere, which encompasses the crust and the uppermost mantle and reaches until the depth of 100 kilometres. Beneath the lithosphere there is a 2900 kilometres thick layer or mantle, and further to the centre of the Earth, until the depth of 6370 kilometres, there continues the core.  
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 its origin rocks can be eruptive sedimentary or metamorphic. About 95% of the lithosphere is composed of magmatic rocks, while the other two types share the remaining 5%. If a rock is composed of just one mineral it is called monomineralic while rocks which contain more minerals are called polymineralic rocks.
Minerals (today there are more than 4700 known are classified according to their chemical  composition into elements, sulphides, chalcogenides, oxides and  hydroxides, carbonates and nitrates, borates, sulphates, phosphates and a large group of silicates which participate with more than 95% in the building of Earth crust.
 
 
 
Green schist
 
 
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.
 
 
 
Prehnite – Ca2Al[AlSi3O10](OH)2
 
 
 
It has already been mentioned that the silicates participate in the structure of Earth crust with more than 95%, and thus it can be expected that their variances are the most numbered in nature (thy make two thirds of the total known kinds of minerals). All groups of silicates can most easily be described as minerals whose basis is SiO44- tetrahedron, and ways and characteristics of their connections build various types of structures. Therefore they are sub-divided into nesosilicates, sorosilicates, cyclosilicates, inosilicates, phylosilicates and tectosilicates. One of the representative s of silicates is the mineral prehnite, which is called after Hendrik von Prehn, who discovered it at the Cape of Good Hope in the 18th century.
Researches on the island of Vis, in areas built of volcanic rocks, have shown that there is a great variety in the remains of rocks created by repeated volcano eruptions more than 250 million of years ago. Especially interesting are the forms of rocks which present rounded fragments of lava and products of explosive volcanic activity (volcanic bombs). On the fractures of such rock fragments most often easily noticeable is its zonal structure. Geodes, however, are filled with secondary minerals among which prehnite is the most common.  By preparing microscopic facets and examining these prehnites a “lace” of thin, small fibrous crystals in all colours can be seen.
As concerns  the mineral  prehnite, it is also very important to point out that it is specific  also by the fact that its structure  in not typical for phyllosilicates  and that therefore it is classified  as heterophyllosilicate (with chain layers). It crystallises in rhombic system, its colour is from white to light green and its hardness 6-61/2. It is the product of exchange of basic plagioclases in hydrothermal conditions. It can be found in vesicles, geodes and veins of basic eruptive rocks. It is an indicator of the low grade of metamorphism.
 
 
Dragan Bukovec