The science that explores the creation and classification of rocks is called petrology, and the science that explores the minerals rocks consist of is called mineralogy. Rocks make up the solid part of the Earth called lithosphere, and it encompasses the crustand the uppermantlereaching the depths of 100 kilometres. Below the lithosphere there is 2,900 kilometre thick outer core. The inner core reaches the depth of 6,370 kilometres to the centre of the Earth.
Rocks are aggregates of one or more types of minerals while minerals are the result of natural physical-chemical processes of a certain chemical composition and a crystal structure stable at certain temperature and pressure conditions. Proper inner mineral structure is also reflected in their external shape. Such proper shapes are called crystals. Rocks, according to their creation, are classified as igneous, sedimentary and metamorphic. Around 95% of the lithosphere consists of igneous rocks while the remaining 5% consists of the other two types. Rocks that consist of just a single mineral are called mono-mineral rocks, while rocks that consist of several minerals are called poly-mineral rocks.
Minerals (more than 4,700 are known today) are divided into 14 classes according to their chemical composition and structural characteristics; Native Elements, Sulphides, Sulphates, Halides, Oxides and Hydroxides, Carbonates, Nitrates, Borates, Iodates, Sulphides, Phosphates, Arsenates and Vanadates, Wolframites and Molybdates, Organic Compounds and a large group of Silicates that make up 95% of the Earth's crust.
QUARTZ “čađavac” – MOSLAVAČKA GORA, CROATIA
Quartz (SiO2) is one of the most common minerals in the Earth's crust. Even though in mineral literature quartz has been classified as an oxide based on its chemical composition, it undoubtedly belongs to the silicate class because its regular internal structure matches the tectosilicate structure. Furthermore, we can say that quartz and other minerals from the SiO2group are silicates above silicates. Quartz crystallises in the hexagon system, it often appears in the form of prismatic crystals whose ends have two rhomboidal surfaces and as granular or microcrystal aggregates (agate, chalcedony, carnelian, chrysoprase, onyx), it is transparent to semi-transparent, colourless to multi-coloured, glassy to dimly bright.
Quartz features numerous varieties including low and high temperature, left and right, and multi-coloured either due to impurities or defects in the crystal structure. In terms of various colours, we distinguish the colourless rock crystal, yellow citrine, purple amethyst, brown smoky quartz, black smoky quartz, green jade, rose quartz and grainy white coloured quartz – milky quartz. Transparent quartz with nice colours is often used to make jewellery. “Čadjavac (smoky quartz) from Moslavačka gora is often very beautiful and clear, and, for a while, it was often polished to create ornamental stones. It is a known fact that the City of Zagreb ordered a diadem for Princess Stephanie as her wedding gift.” Mijo Kišpatić wrote in his book entitled “Ores in Croatia”. From this we can conclude that čađavac from Moslavina adorned the head of Princess Stephanie of Belgium who married Rudolf von Habsburg, the crown prince of the Austria-Hungary Monarchy, in 1881.
In the pegmatite veins of the granite that makes up the central body of Moslavačka gora, there are crystals and conglomerate pieces of transparent yellow to brown quartz. According to one of the earliest records dating back to 1866, Mijo Sabljar, as the location where beautiful čađavac crystals (Rauchtopasa) were found, mentions the hills close to the Moslavina villages of Hruškovica, Gornje Jelenje, Mustafina Klada and Gornja Vlahinićka.
AUGITE DIABASE CONGLOMERATE – Island of BRUSNIK, CROATIA
n addition to the Island of Jabuka, Brusnik is the second island in the Adriatic Sea that was mostly built on magmatic rocks. Unlike the high Island of Jabuka, Brusnik is a low island located three kilometres southeast from the Island of Svetac. Nearly the entire Brusnik and cliffs close to its shore are made up of conglomerates while cliffs of massive diabase (“black eruptive stones used to make whetstone or “brus“ giving the island its name”) criss-crossed by cracks are only found in its central section.
On a shore facing Svetac there are near perfectly rounded conglomerates whose heights reach up to half a meter. Using microscopic analysis of Brusnik conglomerates, Mijo Kišpatić, the first Croatian petrographer, established that this is a diabase, an eruptive rock that we can also find on the Island of Jabuka. By establishing the mineral composition of Brusnik conglomerates, Kišpatić concluded that feldspars and augite dominated. In addition, he also found amphiboles and biotite as ancillary minerals created by modifying one part of the augites. New research has expanded the list of ancillary minerals by adding epidote, titanite and prehnite, minerals also created by modifying the original magmatic minerals and the rock was classified as modified augite diabase.
By comparing the rocks of Brusnik and Jabuka, it can be established that there are no significant differences among them and that they were created by the penetration of subvolcanic magma of same or very similar compositions, probably occurring at the same time, but subsequent geological processes caused the differences. Explaining the creation of Brusnik conglomerates, Fran Tućan emphasized the long term effect of sea waves in order to create conglomerates of such high level curvature from broken pieces of such “tough” rocks such as diabase. As early as the middle of the 19thcentury, it was established that the limestone mortar that glued the diabase conglomerates into a conglomerate was very young. A subsequent analysis established that it contains fossil remains of organisms that lived in a very shallow sea while the crushed shells indicates sedimentation in the zone of wave impact. Therefore, we can conclude that Brusnik conglomerates originate from a previously created conglomerate and that they continue to be rounded by the impact of sea waves. Brusnik has been protected as a geological nature monument since 1951.
Dragan Bukovec, B.Sc. GEO.
