Intro: There are some people, known as mineral collectors, that hunt for the most unique and beautiful mineral specimens in the world… and Tasmania has two of these world renowned and highly sought-after minerals in our own backyard; crocoite and stichtite.
Minerals are naturally occurring substances that make up rocks. Geologists (specifically mineralogists) study and admire them, however, their beauty is admired by far more than just these specialists. These rare and highly prized minerals, specifically crocoite, are found (almost) exclusively here in Tasmania. Luckily one of my friends at the university, a German experimental petrologist, is also an avid mineral collector. He got us access to visit the Adelaide Mine in the Dundas district, near the town of Zeehan in western Tasmania. The Adelaide Mine, as well as other mines in the Dundas area, are the key localities for the best crocoites samples! In this post I will attempt to explain a bit more about these beautiful minerals, how they formed, and where to find them.
Science Spiel: Red, green and purple (Geology, Mineralogy, Ore Deposit Geology)
Crocoite is a rare lead chromate [PbCrO4] and arguably the most famous mineral from Tasmania. It has beautiful elongated, hollow, prismatic to acicular crystals that are bright orange and red. The name comes from the Greek name for saffron, and it isn’t hard to imagine why! Crocoite has a beautiful vitreous lustre, but this is lost once exposed to UV light (i.e. sunlight).
How crocoite forms is still a bit enigmatic, however, simply put it basically forms when a hydrothermal fluid rich in chrome (Cr), that was likely leached and oxidized from the surrounding chrome-rich ultramafic rocks in the area, encounters a hydrothermal fluid rich in lead (Pb). When these two Cr- and Pb-rich hydrothermal fluids meet, or when the Pb-rich hydrothermal fluid encounters the Cr-rich hostrock, crocoite forms (Bottrill and Baker, 2008). The mineral crocoite occurs with Pb-Zn-Ag sulfide veins in siderite and ankerite faults zones, but commonly the crocoite grows in open-space cavities and vugs within the surrounding rocks in areas that have been largely altered to clays and gossans (Bottrill and Baker, 2008).
Stichtite is a beautiful purple magnesium hydroxycarbonate [Mg6Cr2CO3(OH)16·4H2O] that was first discovered here in Tasmania, near the town of Zeehan, in 1981 (Twelvetrees, 1914). It has a waxy luster and is generally within Cambrian-aged serpentine as small disseminations that have replaced chromite/magnesiochromite (Bottrill, 2001). The bright green mineral serpentine forms when high temperature and pressure rocks from the earth’s mantle rich in the mineral olivine, interact with lower temperature fluids. This interaction happens when the mantle-derived rocks are obducted or thrusted to the earth’s surface, and they chemically react with lower temperature fluids to hydrate and produce the mineral serpentine. The light purple mineral stichtite is intertwined with serpentine and also comes from the chemical reaction of mantle-derived minerals, such as chromite, with alkaline lower temperature fluids. As a geologist, I always think mineral specimens are beautiful natural, however, stichtite in serpentine is very frequently polished to highlight the green and purple contrasting colours.
Where do you find these minerals in Tasmania?
The Dundas district, near Zeehan, is where the majority of the stichtite and crocoite are found. Most of the good sites are private mines, with only some open to the general public. For the stichtite in serpentine samples, some key localities are Serpentitne Hill, Nevada Creek and Stichtite Hill. For crocoite samples, the key mines are Adelaide mine, Dundas Extended mine, Red Lead mine, as well as others (Bottrill, 2001).
For more information, check out this .pdf guide from Mineral Resources Tasmania (http://www.mrt.tas.gov.au/mrtdoc/dominfo/download/UR2001_08/UR2001_08.pdf),
and if you are interested in minerals of Tasmania I recommend this book as well (http://www.mrt.tas.gov.au/portal/a-catalogue-of-the-minerals-of-tasmania).
As mentioned above, most of these are private, but if you are keen and ask nicely you might get access to a mine tour, such as the kind folks at Adelaide mine did for us!
Final Thoughts: I usually try and collect an interesting rock or mineral from the place I travel to, and I personally think this is the best type of souvenir as it is unique and a little natural piece of the place. If you want something from Tasmania, I can’t think of a better item than a beautiful mineral that is uniquely from the westcoast of Tassie! It was quite the adventure going out to the westcoast and mines themselves, and just a superb experience to be able to go into the Adelaide mine and see the large vugs and cavities where the minerals have grown in situ. I don’t think you have to be a geologist or specialist to appreciate these beautiful minerals of Tasmania.
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