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Can anyone help me identify this rock? (pics included)

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I have recently developed an obsession with a rock I picked up along an abandoned railway in Eldersburg, MD. Initially I assumed that it was just some kind of slag (found it by an old coal hopper), but after spending a few weeks looking at it I burn to know what it's made of and/or how it was produced. I've spent hours online researching rocks and minerals, but I'd be greatful for a more educated guess, or any help determining or describing it's basic properties.

Here's what I can tell you about the rock in question:
-the (very) rough dimensions are 9cm x 6cm x 2.5cm
-it seems very heavy for it's size
-hardness is at least 7 (it easily scratched ceramic and a quartz crystal)
-one side is covered in tiny intersecting translucent brownish crystals that produce bright snowflake-shaped flashes when the rock is moved.

Thank you! alt text

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asked Apr 8, 2012 by soo (200 points)

9 Answers

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From quick looks at the sample it seems to have a nonmetallic (maybe submetallic luster) and if I remember the hardness scale quartz is a 7 and glass is considered a 5 and the only things that are harded than that are things like corrundum, beryl, diamond, etc. But it doesn't look like a type of rock to contain those minerals. It could be a possibility that when you scratched the quartz crystal you may have mistaked the striations going horizontally on the crystal for a scratch. If it is heavy then it contains some heavy metals (iron or lead...something like that) which would rule out all of my ideas cause metals are generally weaker in hardness. So my guesses are just to do all the tests you can do like an acid test (drop some HCl on it to see if it is a carbonate), put a magnet up to it, scratch it with different objects like pennies, steel nails, to try and get an idea of a hardness range and then a streak test. With that general stuff you can get a rough idea of what it may be.

answered Apr 8, 2012 by Mpickens4 (320 points)

Ok. To be more clear- I scratched the quartz with the small protruding crystals on the top of the rock, and examined the quartz (and my mystery rock) carefully prior to and after scratching. I'm fairly sure the hardness is at least 7 because I also gouged a ceramic mortar and pestle set trying to determine streak color. If I was to use a diamond file on the rock would the resulting powder be equivalent to a streak color?

Also as far as I can tell it is not magnetic, or attracted to magnetic fields.

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At this point I think I've narrowed it down to rutile or cassiterite, probably rutile based on the shape of the crystals under magnification. If anyone knows of a way to differentiate the 2 without special equipment or materials, let me know, otherwise I guess I'll try to find someone I can take it to locally.
Thanks!

answered Apr 10, 2012 by soo (200 points)
0 votes

Based from the National Audubon Society Field Guide Rutile and Cassiterite are similar minerals with Rutile (TiO2) and Cassiterite (SnO2)both being metal oxides but cassiterite is heavier and lacks the striated, lusturous, prismatic crystals of rutile. That is what the book states. It also states that Cassiterite was mined as a tin ore from placer clastic sedimentary deposits but does not form there. I do not know if they mined that in your area at all but it is a possibility.

answered Apr 10, 2012 by Mpickens4 (320 points)

I know that rutile can be found in Maryland, but I don't know if it was ever mined. Unfortunately, I don't think I can rely to much on location for identification- I found it along side old railroad tracks, so I assume it could have come from almost anywhere on the east coast.
Does anyone know of a way to estimate specific gravity without lab equipment? I don't have enough experience to make a reasonable guess by size and weight.

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the only way I knew was taking the sample and seeing how much water it displaced to get a volume and multiplying it by acceleration of gravity to find its specific gravity....could be wrong though I haven't done it in a couple years

answered Apr 21, 2012 by Mpickens4 (320 points)

Thanks- I was actually able to find a website with a really good explanation of the procedure, it didn't mention acceleration of gravity though. I'm gonna try to borrow a good digital scale from someone.
http://www.prettyrock.com/specific-gravity-calculator.htm

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Its a leaverite

answered Apr 21, 2012 by jacko (1,060 points)

perhaps- I'm not a collector and I didn't pick it up thinking it had any monetary value. I just really like this rock, and that makes me want to know more about it.

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It could be but I dislike that term

answered Apr 21, 2012 by Mpickens4 (320 points)
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The more I learn, the less I know. I'm adding the tourmaline group to my list of possibilities.

answered Apr 21, 2012 by soo (200 points)
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it could possibly be Galena. hope this helps. :)

answered May 12, 2013 by I am a gem (380 points)
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Good for you. I was looking for Cassiterite in Md., and this came up. Do you have a microscope you could set up to look at it at 40-60 power? You might pick up the individual crystal structure, if there is. I don't know anything. I am fooling around with rocks to try and teach myself something about geology. Get a book buy Schmit(sp.?). An older paperback about Md. geology....has a picture of Sidling Hill on the cover...1993, me thinks. I just leant mine out. It is the one book you want to educate yourself about the area, in my opinion. The rock/thing at hand...the little vesicles are from something, lava is light, slag has them, so does other volcanic stuff. The problem with rock identification is that you kinda hafta get a whole picture of everything at the same time to do it...I can not. That is why people concentrate on minerals, I think, not the varied, and complex causations, such, that caused them. Good luck, down near the Monocacy.

answered Nov 28, 2014 by Whatisthis (160 points)

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