Granaty / Garnets EarthCache

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Granaty

Granaty jsou kremicitanove materialy se sumarnim vzorcem A₃B₂(SiO₄)₃, kde A je dvojmocny kov (Fe²⁺, Mg²⁺, Ca²⁺, Mn²⁺) a B je trojmocny kov (Al³⁺, Fe³⁺, Cr³⁺). Granaty krystaluji v kubicke krystalograficke soustave a patri mezi dulezite strukturni typy. Granaty lze nalezt v prirode nebo je mozne je synteticky pripravit. Mezi nejznamejsi prirodni granaty patri pyrop (A = Mg²⁺, B=Al³⁺), almandin (A = Fe²⁺, B = Al³⁺) a grosular (A = Ca²⁺, B = Al³⁺).

Granaty se nachazi v prirode v hojnem poctu, casto v podobe zrn, ktera jsou "zarostla" do hornin nachazejicich se v miste naleziste.

Pouzivaji se jako brusne materialy (tvrdost 6,5 - 7,5) nebo ve sperkarstvi.

Vznikaji premenou seprentinitickych bazickych hornin v dusledku vulkanicke cinnosti. Naleziste se deli na primarni a sekundarni (kam byly granaty naplaveny - to je prave pripad mista, kam Vas zavedly uvodni souradnice).

Ceske granaty

Jedna se o horecnate granaty (pyropy). Jejich zabarveni je cervene az fialove a jejich rozmery zridka presahnou 1 cm. Nejznamejsi naleziste se nachazi v Ceskem stredohori, kde se granaty dodnes tezi. Dalsi naleziste se nachazi v jiznich Cechach kolem ricky Kremze, kde tvori az centimetrova zrna v hornine hadci (serpentinitu).

Historie

Ceske granaty byly v nasi zemi znamy od nepameti. Jejich organizovany sber a vyvoz do Evropy zacal v rannem stredoveku (6. - 8. stoleti). Za zlaty vek ceskych granatu je povazovano obdobi Rudolfa II., ktery podporoval brusice a uplatnoval predkupni pravo na granaty vyjimecne velikosti. Velka poptavka po ceskych granatech ve druhe polovine 18. stoleti vedla k zakazu vyvozu granatu ze zeme. Byl tak ochranen cesky monopol na tezbu a zpracovani granatu a granat se tak stal mineralogickym symbolem Cech.

"Nejvetsi kousky z ceskych nalezist vsak pochazeji z dob davno minulych a k nejkrasnejsim patri granat o rozmerech 35 x 18 mm, pripominajici holubi vejce, ktery mel ve svych sbirkach Rudolf II. Dnes je tento unikatni kamen ulozen v drazdanskem muzeu (byl ukraden svedskymi vojaky behem 30-tilete valky, stejne jako vetsina Rudolfovych sbirek; pozn. ownera). Vzdy vsak byly vetsi kusy velmi vzacne a pyropy velikosti liskoveho orisku se cenily stejne, jako prave rubiny. Sila nalezist granatu v Ceskem stredohori je predevsim ve stejne a velmi vysoke kvalite kamenu, v mocnosti a rozlehlosti vrstev pyroponosnych sterku i v hojnosti zrn granatu v techto stercich." (zdroj 1)

Lokalita Kremzsky potok

V lokalite na Kremzskem potoce se ovsem granaty nikdy ve velkem netezily, protoze jde o prilis male krystalky, nez aby mely nejakou vyraznou cenu - nejvetsi kousky, ktere se nam tam podarilo najit, mely kolem jednoho milimetru.

A jak se tedy daji najit? Snadno! Drepnete si na brehu potoka, naberte si do dlane pisek a opatrne ho propirejte a hledejte cervena prusvitna zrnka. Broz po prababicce si tim urcite nedoplnite, ale pro dobry pocit z nalezu to snad staci.

Vase ukoly

1. vyfotit sebe nebo GPS na mustku nebo nekde pobliz a fotku prilozit k logu
2. najit v potoce nejaka zrnicka granatu, vyfotit je a fotku prilozit k logu
3. co nejpresneji zmerit nebo aspon odhadnout jejich velikost a napsat to do logu
4. kdyz uz se budete rachat v potoce, podivejte se, co za dalsi horniny a mineraly zde muzete najit a pochlubte se vysledky sveho pruzkumu v logu (uvedte kratky popis - co to je, jak velke, cetnost nalezu) + zamyslete se, jestli je Vas nalez naplaveny nebo mistni
5. zjistete z jake horniny pochazi Kremzske granaty (muzete to zjistit prostym googlenim, nebo muzete zkusit najit granat jeste porad prirostly k puvodni hornine) a odpovezte pomoci systemu GAS (po spravne zadane odpovedi ziskate kod, ktery vlozte do logu, takze NIC neposilejte pres email!)

Garnets

Garnets are silicate materials with cubic crystal system. It's chemical formula is A₃B₂(SiO₄)₃ (A is Fe²⁺, Mg²⁺, Ca²⁺ or Mn²⁺ and B is Al³⁺, Fe³⁺ or Cr³⁺). Garnets are important structural types. The most f amous garnets are pyrope (A = Mg²⁺, B=Al³⁺), almandine (A = Fe²⁺, B = Al^{3 +}) a grosular (A = Ca²⁺, B = Al³⁺).

Garnets were formed by metamorphose of serpentinitic alkalic rocks caused by vulcanic activity. Deposits can be splitted to two categories - primary and secondary (drifted - that's the case of the locality on starting coordinates).

Searching and mining

First of all were some random finds on fields and washing from creek sand. Later people used to dig in river detrits and wash the material to get out sand and other stones. This method was used without great changes till year 1959(!) when heavy diggers came.

Nevertheless the biggest and most beautiful garnets were found many years ago. The most beautiful was 35 x 18 mm big pyrope shaped like pigeon egg. It's owner was emperor Rudolph II. and you can see it now in Dresden museum (because it was stolen during 30-years war by sweden soldiers, same as many other Rudolphs treasures). But every time the biggest stones were very rare.

Well, on Kremzsky creek was nobody ever mining pyrops for bussiness, because crystals you can find there are very small - the biggest we found there has just a little over one milimeter.

And how to find one? That's easy! Sit on the bank of the creek, get some sand from bottom to your palm, carefully wash it and look for red transcluent stones. Its not really valuable, but its enough to feel like succesfull prospector .

Your quests

1. take a picture of you or your GPS on the bridge or somewhere close and add it to your log
2. find some pyrope granules in the creek, take a picture of them and add it to your log
3. measure (or aproximate) diameter of garnets you found and write it down to your log
4. try to determine some other minerals / rock in the creek sand and describe some of your founds in the log (what was that, how big, how much) + think whether it's drifted or local
5. find out what rock (kind) are Kremze garnets from (you can google it out or try to find bigger stone with garnets in the creek) and aswer using system GAS (after correct answer you'll get specific code and write it to your log, so mail me NOTHING, please (except reporting problems, of course)!)

Zdroje / Sources

1. Poklady Ceske zeme III - Cesky granat
2. Sperkarstvi
3. Wikipedia (EN)