From the guidelines, as from January 2013"People do not need to wait for permission to log your EarthCache. Requiring someone to wait is not supported by the EarthCache guidelines. People should send their logging task answers to you, then log your EarthCache. When you review their logging task answers, if there is a problem, you should contact them to resolve it. If there is no problem, then their log simply stands."
Chromite, a brownish black cubic mineral belonging to the spinel group, is the only ore mineral from which metallic chromium and chromium compounds are obtained. It has the chemical formula FeCr2O4, and a theoretical composition of 32.0% FeO and 68.0% Cr2O3.
Chromite forms in deep ultra-mafic magmas and is one of the first minerals to crystallize. It is because of this fact that chromite is found in some concentrated ore bodies. While the magma is slowly cooling inside the Earth's crust, chromite crsytals are forming and because of their density, fall to the bottom and are concentrated there.
Although its primary origin is ultra-mafic rocks such as peridotites, chromite is also found in metamorphic rocks such as serpentinites. Chromite, as is indicated by its early crystallization is resistant to the altering affects of high temperatures and pressures. Thus it is capable of going through the metamorphic processes unscathed, while other minerals around it are being altered to serpentine, biotite and garnets.
Usually magnesium is present in chromite substituting for the iron and in fact a solid solution series exists between chromite and the much rarer mineral magnesiochromite. All chromite specimens in nature contain some magnesium, likewise all natural magnesiochromites contain some iron. Magnesiochromite is grayer in color and in streak and has a slightly lower density than chromite at a specific gravity of 4.2 to 4.4.
Physical characteristics:
- Color is brownish black to a deep dark black.
- Luster is metallic to greasy.
- Transparency: Crystals are opaque.
- Crystal System is isometric; 4/m bar 3 2/m
- Crystal Habits include octahedrons often with dodecahedral faces modifing the edges of the octahedron to the point of rounding the crystal. Well formed crystals are rare and chromite is usually found massive or granular.
- Cleavage is absent.
- Fracture is conchoidal.
- Hardness is 5.5
- Specific Gravity is 4.5 - 4.8 (average for metallic minerals)
- Streak is brown.
- Other characteristics: Weakly magnetic and an octahedral parting is sometimes seen.
- Associated Minerals include olivine, talc, serpentine, uvarovite, pyroxenes, biotite, magnetite and anorthite.
- Notable Occurrences include several mines in North Carolina, Montana, Maryland, Oregon, Texas, California and Wyoming, USA but it also found in Turkey; South Africa; Philippines and Russia.
- Best Field Indicators are crystal habit, streak, associations with ultra-basic minerals and parting.
Uses of chromite
- used to harden steel, to manufacture stainless steel, and to form alloys
- used in plating to produce a hard, beautiful surface and to prevent corrosion.
- used to give glass an emerald green colour. It is responsible for the green colour of emeralds and the red colour of rubies
- wide use as a catalyst
- dichromates such as K2Cr2O7 are oxidising agents and are used in quantitative analysis and also in tanning leather
- lead chromate as chrome yellow is a pigment
- compounds are used in the textile industry as mordants
- used by the aircraft, cycle and automotice for anodising aluminium
- the refractory industry uses chromite for forming bricks and shapes, as it has a high melting point, moderate thermal expansion, and stable crystalline structure
sources of information: here, here and here:
The cache:
The coordinates of this cache will take you to an old mine NE of Alimonde. Here you will be taken to an open pit working that stopped producing chromite around 1945. I have chosen this particular spot because it presents no danger to you the cacher. The working is approximately 2 m deep but access is made by a wide opening without an elevation change so you don’t even have to jump in to see the features.
Here you will observe dunites – a dark coloured rock with a greenish outer skin due to metasomatic alteration processes occasionally with a yellowish streaks as a result of serpentinization.
Observe carefully the rocks and you will see black, opaque circles with a metallic luster that vary from 1 mm to 3-4 mm-wide. These are the chromite grains. If you search within the loose dumped material, you will also discover loose bits of chromite that you can take with you for collection.
To log your “found”, I need you to send me an e-mail telling me the shape of the working that you are in, i.e. O-shaped, Y-shaped, etc. (Please no photos in the logs spoiling the earthcache for others).
Please note that access to the old mine working is through a short, winding dirt road that is no longer than 500 m but in winter it is soaked and a 2x4 might get stuck. A 4x4 will have no trouble whatsoever. However, I invite you to walk to the working. If you get stuck, there are lots of friendly people about who own tractors that will give you a hand in getting out.
As a word of warning, please note that some people have been known to loose their car keys near some caches! Please pay attention to your personal property.
Happy earthcaching!
The most exciting way to learn about the Earth and its processes is to get into the outdoors and experience it first-hand. Visiting an Earthcache is a great outdoor activity the whole family can enjoy. An Earthcache is a special place that people can visit to learn about a unique geoscience feature or aspect of our Earth. Earthcaches include a set of educational notes and the details about where to find the location (latitude and longitude). Visitors to Earthcaches can see how our planet has been shaped by geological processes, how we manage the resources and how scientists gather evidence to learn about the Earth. To find out more click HERE.
CROMITE - FeOCr2O3
Em Português - Quando tiver tempo. Se tiverem dúvidas mandem-me uma mensagem.
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