The science that explores the formation and classification of rocks is called petrology. An important point to note is that rocks make up the lithosphere – a solid mass of rocks with a thickness of 5 to 70 kilometers, which encompasses the Earth’s crust (the solid surface layer of the planet Earth) and the Earth’s upper mantle (the thickest layer containing the majority of our planet’s material). Just beneath the solid lithosphere lies the central part of the Earth’s mantle – the asthenosphere – a layer of partly molten rock called magma. The lower part of the Earth’s mantle maintains a solid state as a result of the high pressure and reaches a depth of up to 2,900 kilometers. Beneath the mantle, all the way to the center lies the Earth’s core, which is made up mostly of iron. By their origin, rocks are classified as igneous, sedimentary and metamorphic. Igneous rocks make up about 95% of the lithosphere, whereas the other two types of rock are only 5% of the lithosphere.
Rocks are aggregates of one or more minerals – structures formed by natural physical and chemical processes, with a specific chemical composition and crystal structure, as well as stability under certain temperature and pressure conditions. Minerals can occur as crystals – as a homogeneous, regular geometric solid with some degree of symmetry, which is a reflection of their regular internal composition – the structure. The study of the chemical, structural and physical properties of minerals, as well as the study of the processes of mineral formation and their classification, distribution and their utilization is called mineralogy. Today, there are more than 5,000 known minerals which, based on their chemical composition and structural properties, are grouped into the following classes: Native Elements, Sulphides, Sulfosalts, Oxides and Hydroxides, Halides, Carbonates, Nitrates, Borates, Iodates, Sulfates, Phosphates, Arsenates and Vanadates, Wolframites and Molybdates, Organic Compounds and a large group of Silicates that make up 95% of the Earth’s crust.
Barite (težac), Sivac, Petrova Gora, Kordun
Barite is a mineral from the sulfate class. Its name is derived from the Greek word barys meaning heavy, indicating the fact that the density of barite is unusually high. According to that property, the old Croatian name for barite is težac. In nature, barite appears in the form of crystals, with the most common being tabular, or as an aggregate that is massive, granular, fibrous, botryoidal or earthy. It can be transparent, opaque, colorless or consist of various colors, but its streak is always white. Barite is quite soft, with a hardness of 3 to 3.5 on the Mohs hardness scale. Among other physical properties, it is important to note that barite has perfect cleavage in two directions and it is also relatively brittle. Certain barite specimens are characterized by fluorescence, emitting light in incredibly bright colors when the mineral is exposed to ultraviolet (UV) radiation.
Barite is one of the rarer minerals that has a wide variety of crystal forms (habits), ranging from tabular, prismatic, acicular to elongated along all three crystallographic axes of the orthorhombic system in which this mineral crystallizes. Research on the morphology of barite indicates that the diversity of shapes depends on the specific structural features and suggests that the lattice (reticular) density of the crystal lattice of barite is increased in the direction of crystal elongation.
The barite crystals found at the Sivac locality on Petrova Gora are acicular and elongated along the crystallographic axis c. However, in most cases, barite appears there as a massive aggregate in relatively thick barite veins. It is interesting to mention that in the barite vein Kamenita – Sivac, clear traces of tectonic activity are visible in the form of numerous faults, some of which have a vertical displacement of over 20 meters.
The formation of barite can be connected to both magmatic and sedimentary processes. Therefore, it usually occurs in hydrothermal veins, especially those that contain lead and silver minerals, as well as in cracks within igneous rocks. It can also be found in sedimentary rocks such as limestone or clay deposits.
Barite is the primary ore of barium and thanks to its high density, it has a wide range of applications in the production of paper, textiles and rubber. In the past, barite powder was used to produce white pigment. Nowadays, it is used to obtain heavy and dense barium mud, which is used in oil wells. Thanks to its property of absorbing radioactive radiation, barite is used in medicine as protection and as a contrast agent in X-ray imaging of the digestive tract.
Tuff, Donje Jesenje, Hrvatsko zagorje
Tuff or pršinac is a rock formed by the deposition, binding and compaction of volcanic ash. Therefore, it can be said that this is a sedimentary rock of volcanic origin. Volcanic ash refers to particles ejected during an eruption from a volcano, with a size that does not exceed 4 mm. Tuffs vary depending on the type of particles they consist of. Consequently, there are lithiclastic tuffs where rock particles prevail, crystalloclastic tuffs where mineral particles prevail and vitriclastic tuffs where glass particles prevail. In addition, the chemical composition of volcanic ash can vary within the broad spectrum of volcanic rock chemistry, from rhyolitic with a high content of silica to basaltic with low silica content. Therefore, tuffs are described as rhyolitic, andesitic, basaltic, etc.
The term tuff is derived from the Italian word tufo, which means soft, loose and brittle, which describes the main characteristic of this type of rock. However, tuff can sometimes be solid, especially if there is an overflow of fresh lava over the deposited volcanic ash. The formed rocks, as a result, have massive texture.
Deposits of tuff are often lens-shaped and are thickest near the source volcano. Their thickness decreases as the distance increases. Certain deposits of tuff can spread for miles away from the source volcano and reach thicknesses of several hundred meters. This can come as a result of a single explosive eruption, successive waves of one eruption or multiple eruptions separated by long periods.
In Croatia, tuffs can be found in the northwestern part, in the vicinity of Hum na Sutli, Donje Jesenje, Lepoglava, Varaždinske Toplice and Slanje. The aforementioned locations are situated in a fault zone known in geological literature as the “tuff zone.” There is a rich deposit of zeolitic tuff at Donje Jesenje, the reserves of which are estimated at three million tons. The most common mineral in these deposits is clinoptilolite, a mineral from the zeolite group that is characterized by its microporous structure with pores and cavities containing cations and water molecules, which can be easily released and exchanged. Thanks to these properties, zeolites are used as ion exchangers, water purifiers and softeners, catalysts or in the treatment of agricultural soil for regulating moisture and continuous release of potassium. Research is being conducted about their potential application in biomedicine and biochemistry.
Biserka Radanović-Gužvica,
Croatian Natural History Museum
