Staurolite EarthCache
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A chance to make a rare crystal find.
The underground bedrock formations in this area, between the towns of Little Falls and Royalton, are referred to by geologists as the Little Falls Formation and are comprised primarily of 2 major types of bedrock. The lower bedrock layer is a high grade gneiss which formed during the early to middle archean eon (older than 3,000 million years). The upper bedrock layer is a garnet-staurolite schist which formed during the early proterozoic eon (2,500-1,600 million years old).
The Blanchard Dam was built back in 1925 and was constructed right on top of an existing natural waterfall. The waterfall has eroded away the top soil and sediment layers to expose the upper layer schist bedrock below the dam. This schist outcropping is unique in that, this is 1 of only a few places in the world where you can find staurolite crystals. The staurolite crystals that are found here were formed about 1800 million years ago by the heat and pressure of a regional metamorphic, mountan-building event called the Penokean Orogeny.
Schist Identification:
You can identify the schist by looking for the "sparkly", "crumbly" rock. Right below the dam, it juts out of the river bank in thick slabs at a sharp angle. Schist is a coarse grained metamorphic rock. Shale is the parent rock. It is made up of clay minerals. Shale can metamorphose into slate, phyllite, schist or gneiss depending on the degree of heat and pressure. Schist has a greater degree of metamorphism than phyllite but less than gneiss. It is classed as a medium-grade metamorphic rock and is associated with regional metamorphism due to mountain building. The crystals in schist are large enough to see with the naked eye. They are flattened and elongated. Schist contains more than 50% platy and elongated minerals. Schist is foliated or layered in appearance. Quartz, micas, and amphiboles are primary minerals in schist. Schist comes from a Greek word meaning "to split".
Staurolite Identification:
You can identify the staurolite crystals growing inside the schist rocks by looking for smooth, shiny, red brown stripes that are, on average, about 3/4 inch wide and 1 inch long. There are much smaller and some larger specimens to be found. Staurolite is made of iron aluminum silicate crystals. A scratch test will yield a white streak. The name "staurolite" is derived from the Greek words "stauros" for cross and "lithos" for stone in reference to a special formational property of staurolite called twinning, which happens in a characteristic cross-shape, called penetration twinning. It can actually form two twin types; one that is nearly 90 degrees and one that is nearly 60 degrees. The 60 degree type is more common but the 90 degree type is the most sought after. It was associated with Christianity as a symbol for the cross and as a good luck charm. It has been given the names "Fairy Stone" or Fairy Cross". Some scarcer specimens show both twin forms and can look like a blunt six rayed star.
Finding Staurolite Crystals:
If you have time, and are visiting in the summer months, look for some crystals. Most of the staurolite crystals you find will not be twinned. They will be a single rod that when looked at from the end is diamond shaped. I still think these are worth collecting, if you find some in good shape, and make nice necklaces. You can locate plenty of crystals in the exposed rocks but you would have to spend some time chisseling them out of the rock and take a chance on cracking them. The easlier method is to sift through the sand and mud at the base of the rocks. Over time, the water softens the schist and turns it back into clay but the staurolite stays intact. When the schist crumbles into the water it releases the crystals. I have found 3 nicely twinned crystals and hundreds of rods this way. If you are not lucky enough to find any of the twinned crystals, a collection of them can be seen on display at the Royalton Museum.
Fairy cross legends:
A popular Christian legend is that at the time of Christ’s crucifixion, the angels shed many tears that crystallized as they fell to earth.
Another legend states that the staurolite rocks are the tears of the native Cherokee Indians who wept over the loss of their homeland when they were forced to evacuate in the “Trail of Tears.”
To claim credit for this EarthCache, email me the answers to the following:
1. Below the dam, where the schist is sticking out of the ground at an angle. Estimate the degree of the angle.
2. What do you think could have caused the sedimentary schist to be positioned at such a strange angle?
Sources used:
http://en.wikipedia.org/wiki/Staurolite
http://www.littlefallsmn.com/CrossRocks.php
http://www.galleries.com/Staurolite
http://www.royaltonmn.com/index.asp?Type=B_LIST&SEC=%7B8A2946F8-7986-4BB7-9A30-512EDC1C3185%7D
http://conservancy.umn.edu/bitstream/58515/11/MGS_CAS_C10_partC.pdf
http://books.google.com/books?id=qzg525aWqF4C&printsec=frontcover&dq=minnesota+geology+ojakangas&hl=en&sa=X&ei=nXgMUdvpPK_8yAGupYCICg&ved=0CDAQ6AEwAA#v=onepage&q=minnesota%20geology%20ojakangas&f=false
http://www.rocksandminerals4u.com/schist.html
http://www.gemfoundation.com/FStones2.html
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