Minerals and Rocks

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 molten rock called magma. The lower part of the Earth's mantle maintains a solid state as a result of high pressure and reaches a depth of 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. In fact, 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 homogeneous, regular geometric solids 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 14 classes: Native Elements, Sulfides, Sulfates, Halides, Oxides and Hydroxides, Carbonates, Nitrates, Borates, Iodates, Sulfides, Phosphates, Arsenates and Vanadates, Wolframites and Molybdates, Organic Compounds and a large group of Silicates that make up 95% of the Earth's crust.

Pyrite – Brušani, Lika

Without a doubt, one of the most interesting minerals is pyrite – a sulfide mineral with a metallic luster and gold-yellow hue, which at first glance resembles gold. Hence, pyrite was popularly nicknamed “fake gold” and “fool's gold”. Pyrite, however, is harder and less malleable than gold, which explains scenes from Westerns in which gold miners in the American Wild West check to see if it is real gold by biting into it.

The name pyrite is derived from the Greek word pyrites, meaning the stone which strikes fire, because pyrite would create sparks when struck against steel. It is synonymous with iron or sulfur splendent mineral, which correspond to the old mining division of sulfide minerals in terms of luster, but besides luster, minerals from the same group share other common properties. This classification distinguishes splendent (pakovine), glance (sjajnici), blende (blistavci) and fahlore (sinjavci) minerals, and splendent minerals include sulfide minerals with metallic luster, that are opaque, lighter in color and with black streaks, mostly harder, without significant cleavage and brittle. An illustrative example of splendent minerals is pyrite.

Pyrite is iron disulfide, a cubic alteration of FeS2, with the same chemical composition as the mineral marcasite – a rhombic alteration of FeS2 – but differs from it by its internal structure, i.e. the arrangement of iron and sulfur atoms in the structure and consequently, other physical properties. In nature, pyrite is found in the form of crystals with developed surfaces of hexahedrons, octahedrons, or pentagon-dodecahedrons, and is also commonly found in the form of massive, granular, and kidney-shaped aggregates. One interesting thing to note when it comes to pyrite crystals is that their surfaces are very often striated – which is the occurrence of a series of thin, mutually parallel grooves or streaks, usually caused by the way crystals grow.

Pyrite is a quite abundant mineral in the Earth's crust, occurring in various geological formations and is an essential element of various rocks. In igneous rocks it usually forms clear crystals as it is one of the minerals that first begin to crystallize in magma. Pyrite is common as an accessory mineral in sedimentary rocks, where it is also found in the form of regular crystals, as can be seen in dense, chalky sandstones near Brušan in Lika, where pyrite occurs in the form of small pentagonal dodecahedral crystals.

Given the fact that pyrite contains sulfur and iron, we might be led to the conclusion that it is an ore mineral of iron, but this would be wrong, given that the process of extracting iron from pyrite due to the nature of chemical bonds between iron atoms and sulfur atoms is very complex and inefficient, and pyrite is most often used for the production of sulfuric acid.

Nevertheless, occurrence of pyrite may sometimes result in real wealth, so its presence in sedimentary rocks is indicative of the reductive conditions under which oil deposits otherwise form. Having this in mind, it is fair to say that pyrite is an indicator of the so-called black gold, however, the story of the connection between pyrite and gold does not end there. This is supported by the article “Pyrite Is Not Born to Fool” by Dana Hunter, published in the journal Scientific American. Pyrites can contain tiny amounts of gold in which case we are talking about invisible gold, and today's technology is so advanced that it makes it possible to extract gold from pyrite. The largest deposit of invisible gold contained in pyrite in North America is located in the Carlin Trend in Nevada with estimated reserves of more than 107 million ounces of gold, which is equivalent to about 3,317 tons, indeed confirming that “pyrite was not born to fools”.

Loess – loess doll, Vukovar

Loess is a pale yellow to yellowish terrestrial, fine-grained sediment formed by the accumulation of wind-blown particles, hence it is also known as aeolian sediment. It is composed primarily of flint and clay particles and sometimes contains considerable amounts of calcite. The loess and other related deposits stretch in an almost uninterrupted belt from France to China, where they can reach a thickness of up to 400 meters. In Croatia, loess and related sedimentary deposits cover 35% of the total surface area, with the thickest deposits found in Eastern Slavonia, Baranja and Srijem. The slopes of Bilogora and Papuk, and the area surrounding Zagreb contain loess and sedimentary deposits of lesser thickness and surface area. The island of Susak is also known for its loess deposits that stretch over almost the entire island and may grow up to 90 meters thick, and some of them can be found on Korčula, Mljet and some other Adriatic islands, where they mostly cover smaller areas and are less than two meters thick.

Loess is a soft, dusty and porous sediment permeable to water and provides a good basis for the formation of fertile soils for the cultivation of important crops such as grains, sugar beets, grapes and many others.

The first study of loess deposits in Croatia was carried out a hundred years ago by one of the most famous Croatian natural scientists, Dragutin Gorjanović-Kramberger, who described the profile of loess deposits on the right bank of the Danube in the center of Vukovar. It is one of the most beautiful protrusions of loess in Croatia, which at the time of Gorjanović's research was a steep slope that reached a height of 22.5 meters, while today it is almost four meters lower, which is partly due to backfilling and for the purpose of urban design of the promenade. This loess terrace stretches along the Danube bank to the famous Neolithic site of Vučedol.

Given that they are unbroken deposits, they are also a kind of testimony to climate change, which was particularly pronounced during the Pleistocene. The frequent interchange of cold and warmer periods over the last hundred thousand years influenced the formation of various sediments. For example, during the periods of extreme cold, otherwise known as glacials (ice ages), the loess was deposited, whereas during warmer periods, so-called interglacials (interglacial periods), soils were formed, which are evident as darker layers within the light-colored loess. A closer look at Gorjanović's profile reveals three layers of a typical loess: three fossil soils and a less developed humus horizon. Paleontological research has shown that the lower, oldest layer of the loess, contained the Pupilla fauna, which is dominated by different species of land snails of the genus Pupilla, typical for cold and dry steppes, while the middle and upper layer of the loess contained a community of mollusks, characteristic for forest-shrub steppes.

The loess deposits of Vukovar also contain a macroscopically visible accumulation of wind-borne volcanic ash, the so-called tephra, which is quite important due to its comparability with other deposits of the Carpathian Basin, which is rich in tephra.

One of the most interesting facts is that loess deposits can sometimes form loess dolls or dwarfs. They are formed as the result of water impact. Cavities and cracks form in the loess, in which calcium carbonate is discharged around quartz, feldspar and clay grains, leaving concretions in various shapes resembling dolls or dwarfs.

Biserka Radanović-Gužvica
Croatian Natural History
Museum