| Rudnik Zrinski / Zrinski mine |
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Prostor "Rudarskog vrta" na kojem se nalazi i Rudnik Zrinski (150 m jugozapadno od planinarskog doma "Grafičar") izgrađuju vrlo stare stijene od 300 milijuna godina iz razdoblja paleozoika. Nastale su u dubini zemlje, a u svoj sadašnji položaj dospjele su prije 12 milijuna godina i to na način da su se pomicali blokovi tih stijena duž velikih rasjeda u zemljinoj kori i konačno formirali izgled kakav danas postoji.
U okolici Rudnika Zrinskog mogu se zamijetiti 3 vrste stijena: zeleni i glineni škriljavci, te dolomiti.
Od škriljavaca su izgrađene neke poznate građevine na Medvednici, Tomislavov dom, Brestovac, Sljemenska kapelica te mnoge fasade kuća.
Škriljavci su dobili naziv po svom posebnom izgledu listićave slojevite građe koji nalikuje listovima knjige ili lisnatom tijestu. Po svom nastanku te stijene spadaju u grupu pretvorenih ili metamorfnih stijena jer su nastale u dubini zemlje pretvorbom drugih stijena pod velikim pritiscima i temperaturama. Pod utjecajem tlaka i temperature oblikovali su se spljošteni listićavi minerali koji su nalijegali jedan na drugog i dali stijeni takav listićav izgled.Zeleni škriljavci su zelene boje i vrlo tvrdi, dok su glineni škriljavci smeđe boje i lako lomljivi.
Dolomiti su druga vrsta stijena, spadaju u sedimentne stijene, točnije u grupu karbonata i slične su vapnencima koji se nalaze u krškom području, no dolomiti osim kalcija sadrže i magnezij. Dolomiti izgrađuju same hodnike rudnika i njegovu okolicu i nalaze se uklopljen u okolne zelene škriljavce.
Kako je ruda došla do rudarskog vrta?
Možemo kazati kao u poznatoj izreci, «donijela je voda», samo vruća. Radilo se o podzemnoj vrućoj vodi koja je bila bogata mineralnim tvarima, a dolazila je iz velikih dubina. Tu vodu je zagrijala užarena magma duboko u zemljinoj kori. Kako se vruća voda približavala površini polako se hladila, što je bilo dovoljno da se iz nje počnu izdvajati rudni minerali.
Da bi nastalo ovo rudno ležište otopine bogate otopljenim olovom i cinkom te srebrom morale su protjecati duže vrijeme. Samo tako je mogla nastati količina rude dovoljna da zainteresira srednjovjekovnog rudara.
Milijunima godina kasnije ta rudna ležišta postala su interesantna ljudima zbog mogućnosti eksploatacije i korištenja ruda u mnoge svrhe.
Na sličan način nastala su i mnogo veća ležišta olovno cinčane rude poput onog u slovenskoj Mežici ili austrijskom Bleibergu. Ovakva ležišta nazivamo hidrotermalnim od grčke riječi za vodu i toplinu.
Vjerojatno građanima grada Zagreba i okolice nije ni poznata činjenica da se nekada rudarilo na Medvednici i da se dobivalo srebro, tako da nam je Rudnik Zrinski dokaz rudarenja u 16. i 17. stoljeću.
Rudnik srebra
Rudnik Zrinski je zapravo rudnik olovno-cinkove rude, no mi ga nazivamo rudnikom srebra, iako je srebro bilo samo primjesa u toj rudi. Rudari su morali iskopati velike količine rude da bi tek zatim preradom došli do dragocjene rude, srebra.
Olovno-cinkova ruda sadrži nekoliko minerala od kojih je galenit-olovni sjajnik najobilniji, zatim u manjoj količini sfalerit koji sadrži cink, te pirit, dolomit i kvarc.
Glavni mineral u rudniku, galenit zbog svog metalnog sjaja lako je uočljiv na stijeni te mu ime galenit dolazi od grčke riječi, GAL, što znači sjati se.
Ako samo malo zagrebemo po površini stijene ukazat će nam se mineral galenit. Sive je boje, a na tvrdoj podlozi ostavlja sivi trag. Dosta je težak, ali nije velike tvrdoće jer ga možemo zagrebati čavlom. Ako nakupinu galenitne rude lupimo čekićem ona će se raspasti na mnoštvo manjih kockica.
Galenit je važna ruda srebra i može sadržavati 2% srebra, dok u Rudniku Zrinski to iznosi samo 0.05%.
Tako da dolazimo do onog najbitnijeg, koliko srebra su rudari uspjeli dobiti iz galenita? Svega pola kilograma srebra iz tone galenita, što je danas vrlo malo. No trebamo u obzir uzeti podatak o tome kako se mijenjao odnos vrijednosti zlata i srebra kroz stoljeća. Odnos vrijednosti zlata i srebra u srednjem vijeku bio je 1:13, potkraj 19. stoljeća 1:28, četrdesetih godina prošlog stoljeća 1:77, dok je taj odnos danas skoro 1:100 tako da je zbog pada vrijednosti srebra u odnosu na zlato pala i potražnja za srebrom.
Na osnovu toga su rudari kroz stoljeća shvatili da nemaju puno koristi jer je trebalo uložiti mnogo rada i truda da bi dobili malu kockicu srebra od 7 cm. Možda su i to razlozi što nema puno podataka o rudarenju na Medvednici.
Ostali minerali u rudniku Zrinski
Sfalerit, cinkov sulfid, (ZnS), u uzorcima s rudnika Zrinski mnogo se teže zapaža od galenita. Može biti različite boje od sivo smeđe preko brončane do crne. Sjaj mu je poput voska, a nešto je veće tvrdoće od galenita i manje specifične težine. Ostavlja smeđkasti do bijel trag. Ime je dobio po grčkoj riječi sfaleros što znači varljiv. Naime rudari su ga često zbog njegove raznolike boje zamjenjivali za neke druge minerale.
Pirit (FeS2) se ne javlja u velikoj količini na uzorcima s rudarskog vrta no općenito se radi o jednom od češćih rudnih minerala. Javlja se u obliku malih kockica žute boje i metalnog sjaja. Zbog sjaja i žute boje zovu ga također i macino zlato. Ime je dobio od grčke riječi za vatru pyros.
Dolomit (Ca, Mg) CO3 je naziv za mineral i istoimenu stijenu. Srodan je poznatijem kalcitu (CaCO3) koji gradi vapnence. Svjetlo je sive boje i osrednje tvrdoće. Tvori kristale s oštrim bridovima i dobro se kala. Dobio je ime po svom otkrivaču francuskom kemičaru Dolomieu-a. Po ovom čestom mineralu odnosno stijeni, nazvan je i gorski lanac Dolomiti u Italiji.
Kvarc ili kremen (SiO2) je vrlo čest mineral koji se javlja u brojnim vrstama stijena. Najčešće je sivkast ili bijel. Često se nepravilno lomi u obliku školjke. Visoke je tvrdoće, te ćemo komadićem kremena moći zagrebati površinu svih spomenutih minerala. Kremen izgrađuje bijele kremene žile koje često zamjećujemo na Medvednici.
Rudarenje
Rudarenje u srednjovjekovnom rudniku zasnivalo se na iskustvima njemačkih i čeških rudara, te na iskustvima koja su zapisana u poznatom djelu Georgiusa Agricole "De Re Metallica".
Traženje rude u srednjem vijeku započinjalo je traženjem rudnonosne žile, koja se tražila rašljama. Rašljari bi hodali terenom i tražili rudnu žilu, kada bi rašlje zadrhtale znali bi da su pronašli rudu (snaga rudne žile pomiče rašlje). Tada bi na tom mjestu otpočelo iskapanje rude. Prvo su iskopavana okomita okna. Na površini iznad okna izgradili bi nadstrešnicu s koloturom i užetom da ne kisnu i da mogu košarama i vrećama izvlačiti rudu. Kasnije bi okomita okna povezivali horizontalnim hodnicima.
Rad u srednjovjekovnom rudniku jednog rudara u danu trajao je 12 sati. tama, vlaga, hladnoća, prašina i slaba prozračnost bila su rudarima svakodnevica. Rudari su obično rudarili u paru i to s vrlo jednostavnim oruđem, čekićem i klinom, a podzemlje su osvjetljavali svjetiljkama uljanicama. Tim jednostavnim oruđem jedan rudar bi dnevno iskopao u dubinu stijene svega 3-4 cm. Smatra se da je dnevno radilo 30 rudara koji bi iskopali 10 tona rude koja je sadržavala 5 kilograma srebra.
Zadaci za logiranje
Kako biste logirali ovaj EarthCache odgovorite na sljedeca pitanja:
1. Koja ruda se iskopavala u Rudniku Zrinski?
2. Koji je bio glavni mineral u rudniku, metalnog sjaja?
3. Koji se još minerali mogu zamijetiti u rudniku?
4. Oko koliko °C se kreće temperatura u rudniku?
5. Što piše na drvenoj tabli iznad ulaza u rudnik?
Odgovore pošaljite na moju e-mail adresu, a NE u Vašem logu! Ne morate čekati na moje potvrđivanje Vaših odgovora. Jednom kada pošaljete Vase odgovore slobodno logirajte pronalazak ovog EarthCachea. Ako Vaši odgovori budu pogrešni ja cu kontaktirati Vas.
Izvori
1. Fossils, Course Notes, EarthCache Academy,
2. Uprava PP Medvednica, (www.pp-medvednica.hr),
3. Wikipedia, (en.wikipedia.org).
Tijekom zimskog razdoblja Rudnik Zrinski je zatvoren za posjetitelje.
| Rudnik Zrinski / Zrinski mine |
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The area of "Miner's garden" where Zrinski mine is located (150 m southwest from climbers lodging "Grafičar") is formed of very old rocks, 300 million years, from Paleozoic period. They were formed deep in the earth, and they reached its current position 12 million years ago in a way that blocks of these rocks were moved along large faults in the earth's crust, and finally formed the appearance which is present today.
In the surrounding of Zrinski mine one can notice 3 types of rocks: green and clay schists and dolomites.
Some famous buildings on Medvednica are made of schists, such as Tomislavov dom, Brestovac, Sljeme chapel and many facades of houses.
Schists were named after their distinct appearance of laminated layered structure which resembles pages in a book or puffed pastry. For their genesis these rocks belong to a group of metamorphic rocks because they were formed in earth's depth through metamorphosis of other rocks under great pressure and temperature. Under the influence of pressure and temperature flattened layered minerals were formed which overlapped each other and gave a rock such a layered appearance. Green schists are green and very hard, while clay schists are brown and brittle.
Dolomites are a different type of rocks, belonging to sediment rocks, or more precisely carbonates similar to limestones, which are located in karst area, but dolomites besides calcium contain magnesium as well. Dolomites form the same mine corridors and its surrounding, and are incorporated in surrounding green schists.
How did the ore get to Miner's garden?
Similar to a well know Croatian saying «it was brought by water» - in this case it was hot underground water rich with minerals, which came from the depths of Medvednica. As the hot water approached the surface, it became cooler, and what was left as a result of cooling down the water rich with minerals are ore – beds.
For this ore deposit to develop, solutions rich with dissolved lead, zinc, and silver had to flow for a longer period of time. That is the only way a sufficient quantity of ore could develop to intrigue a medieval miner.
Millions of years later those ore – beds became an interest topic of human society, since the survival and economic standard largely depended on available mineral raw materials.
Much larger deposits of lead-zinc ore were formed in a similar way, like the ones in Mežica in Slovenia or Bleiberg in Austria . Such deposits are called hydrothermal from Greek word for water and heat.
Inhabitants of Zagreb and its surrounding probably are not aware that in the past there were silver mines on Medvednica, and Zrinski mine is a proof of mining in 16 th and 17 th century.
Silver mine
Zrinski mine is in fact a lead-zinc ore mine, but we call it silver mine although silver was only an alloy in this ore. Miners had to excavate large quantities of ore, and only after extensive treatment they would reach the valuable ore, silver.
Lead-zinc ore contains several minerals of which galena – lead-glance is most abundant, after which we find smaller quantities of sphalerit which contains zinc, and pyrite, dolomite and quartz.
Galena – the name comes from Greece word "gal" – to shine. It is a gray mineral which contains lead, and in which silver is integrated.
If we scratch the surface of the rock a bit we shall see mineral galena. It is gray, and on hard surface it leaves a gray mark. It is quite heavy but not very hard, since we can scratch it with a nail. If we hit a cluster of galena with a hammer it will crumble into many smaller cubes.
Galena is an important ore and can contain up to 2% of silver, while in Zrinski mine the content is only 0.05%.
So we come to the most important question, how much silver did the miners get out of galena? Just one half of a kilogram of silver from one tone of galena, which is today considered extremely little. But we have to take into account that the value ratio of gold and silver changed through centuries. Value ratio of gold and silver in medieval ages was 1:13 , by the end of 19th century 1:28 , during 1940's 1:77, while today it is almost 1:100, which probably is the reason the demand for silver decreased.
Based on this, during centuries miners realized they don't have much use because a lot of hard work and effort had to be put in to receive a small 7 cm silver cube. Maybe those are the reasons there is not much data on mining on Medvednica.
Other minerals in Zrinski mine
Sphalerit , zinc sulfide, (ZnS), can be much more difficult to observe in samples from Zrinski mine than galena. It varies in color from gray-brown to bronze or black. It has a wax-like glow, and is somewhat harder than galena but with a smaller specific weight. It leaves brownish to white trail. Its name comes from Greek word “sfaleros”, which means deceptive. Miners often misplaced it for other materials because of its variable color.
Pyrite (FeS2) does not occur in large quantities in samples from Miners' garden, but generally it is one of the more common mineral ores. It takes a form of a small yellow cube with metal glow. Because of the glow and the color people in these parts call it "kitten's gold". It got its name from Greek word “pyros” – fire.
Dolomite (Ca, Mg) CO3 is a name of a mineral and a rock. It is related to a more known calcite (CaCO3) which builds limestone. It is light gray and of medium hardness. It forms crystals with sharp edges, and splits well. It got its name after its finder, a French chemist Dolomieu. Mountain massive Dolomite in Italy was named after this common mineral and rock.
Quartz (SiO2) is a very common mineral that occurs in many rock types. Most often it is grayish or white. It often brakes irregularly in shell shape. It is of high hardness, and with a piece of quartz we can scratch the surface of all above mentioned minerals. Quartz builds white quartz veins which can often be noticed on Medvednica.
Mining
Mining in a medieval mine was based on experiences of German and Check miners, and on experiences which were collected in a famous book by Georgius Agricola "De Re Metallica".
In medieval times ore seeking begun with looking for an ore vein by dowsing-rod. Dowsers would walk over a certain terrain looking for ore vein, when the rod trembled they knew they found ore (the force of an ore vein moved the rod). At that spot excavation of ore would begin. Vertical shafts were excavated first. On the surface above the shaft they would construct eaves with a pulley, and with a rope and wooden or wicker baskets they would haul the ore to the surface. Later on vertical shafts would be connected with horizontal corridors.
Work day in medieval mine would last 10 to 12 hours. Darkness and high moisture, coldness, dust and inadequate ventilation of the mines was everyday life for our and other European miners. Miners usually worked in pairs with very simple tools, a hammer and a wedge, and the underground was illuminated with oil-lamps. With these simple tools one miner would daily pick only 3-4 cm of rock in depth. It is presumed that 30 miners worked daily, and they could excavate about 10 tons of ore that contained 5 kilograms of silver.
Logging Tasks
1. What ore is excavated in Zrinski mine?
2. What was the main mineral in the mine, has metal glow?
3. What other minerals can be observed in the mine?
4. Approximately how many °C is the temperature in the mine?
5. What is written on a wooden board above the entrance in the mine?
Send your answers to my e-mail address and NOT in a log. You don't have to wait for me to approve your answers. Once you send your answers feel free to log the cache as found. If your answers are wrong, I will contact you.
Sources
1. Fossils, Course Notes, EarthCache Academy,
2. Administration of PP Medvednica, (www.pp-medvednica.hr),
3. Wikipedia, (en.wikipedia.org).
During the winter period Zrinski mine is closed for visitors.