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Yellow/orange crystal mineral - what could it be?

Posted by Reb Burns  
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Reb Burns March 13, 2018 08:40PM
Hello, I found this lovely mineral on the Mendips Hills in Somerset, UK - in a place called East Harptree Woods. It's known for lead mining and has an old disused Smitham Chimney nearby. The rock doesn't seem typical for the area as I tried to find others but this seemed to be the only one I could find. I've walked there many times and have never before uncovered a mineral like this - but I will be continuing to look on future visits!
I've tried to provide some of the necessary info based on your pinned "Read this first" post... but please be patient as I'm new to this!

I'd say it has a vitreous lustre.

It is an yellow-orange colour. It could be stained with iron? There are marshy bits in the area I found it where the water has an orange colouration, so could there be iron in the ground?

I can mark it with a pen knife (I don't have anything more sophisticated to try it with!).

The entire rock/mineral fits on my hand (as per the uncleaned picture).

It's hard to see what shape the crystals are because they are so crammed together and aren't very long/tall in length. I can't tell if they have four or six sides, for example, but they look a bit like a pyramid shape, but less regular in dimensions. So hard to tell because they all push up against each other!

I tried putting a small chip of the mineral in some white wine vinegar and there was no visible fizz - and it has been in the vinegar overnight without any obvious reduction in size - but it seems a bit soft for quartz as I can mark it with a penknife?



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Owen Lewis March 13, 2018 09:39PM
Good spot with the 'iron staining', I'll guess. As for the crystals themselves, I'm ashamed that I have no knowledge of the local geology but, if no one here can help a local town museum surely will.My first guess would be calcite with quartz as a possibility too. Place a small drop of vinegar on an inconspicuous partof the piece and watch for an effervescence forming slowly. If that happens, then calcite is likely - but it could be one of the other carbonates.
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Doug Daniels March 13, 2018 10:33PM
Are there two specimens? The first photo doesn't look like the next two (maybe just flipped?). The orange-yellow color suggests iron oxides. You can scratch it with a knife blade..does the powder react to vinegar?
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Reb Burns March 13, 2018 11:09PM
It's just one sample. I guess it's different lights that make it look different. Middle is taken indoors. It didn't dissolve or fizz in white wine vinegar... Yet it did mark with a knife.
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Reb Burns March 13, 2018 11:11PM
On the back of the mineral is rock. I can post a photo of that, if it would help ID?

I've contacted Bristol Museum's geology curator who said she'd like me to take it in to show her. Hopefully we might get a definitive ID from her. It's great that she's happy for us to show her.
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Kevin Conroy March 14, 2018 12:54AM
Is it heavy for its size? If so it could be baryte. It would help if you could take a clear photo of the side of the specimen.

When you tried scratching with a knife, did it leave a distinct scratch in a crystal? Try scratching a crystal with a penny. It should be slightly harder than baryte but still leave a mark.
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Owen Lewis March 14, 2018 01:35AM
That's good news from the Bristol museum :-) I have little doubt that the curator will be able to give you an accurate ID having inspected and, perhaps, further simply tested your specimen. So far, you have:

- A likelihood that the orange colour results from iron-staining and is not the colour of the crysals in a pure state.
- No effervescence should mean not calcite.
- That you can scratch with 'any old pen-knife also should mean that the crystals are not quartz.

There's an interesting report of Somerset minerals here https://www.mindat.org/locdetailed-18307.html that might help.

I'm sure that toy will get a satisfactory ID for your specimen and I'd personally be grateful if you report it back here when you do. It's always good to have an authoritative outcome to any and all ID queries.
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Doug Daniels March 14, 2018 01:47AM
Yes, when you get a definite ID, we'd all like to know. See how smart we all were.....LOL
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Thomas Lühr March 14, 2018 02:52AM
Zooming in the last photo, i think to see some quartz crystals. But other crystals (baryte? calcite? ) may also present. So make sure that you do the tests with the same stuff.

Thomas
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Tom Goodland March 14, 2018 06:12AM
Robert, a good find. I'm from Somerset and know the Mendips quite well.
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Peter Nancarrow March 14, 2018 01:13PM
The crystals do like quartz to me, but hardness tests can be deceptive. I wonder if the apparent scratch you get with your knife is breaking through a thin poorly attached surface coating rather than making a true scratch? It could even be a thin overgrowth of later quartz which is breaking away from the older surface underneath. It is also quite common to leave a streak of metal from the knife blade on a crystal face which can look very like a scratch.

Pete N.
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Reb Burns March 14, 2018 08:23PM
I'm so grateful for everyone's replies!

Kevin - I'm not sure if it's heavy for its size. I need to get some other rocks and compare weight. Tomorrow I'll have a go at finding some to compare.

Peter - If it is quartz, would that make it citrine simply because of its colour, or is there more to a mineral's classification than that? I went to a local museum today (Radstock) and a lady there showed me citrine and said it looked very similar - but she was a volunteer, and not a geologist / expert - so was just comparing specimens as I was planning to do. They did look very similar, except for the rock they are attached to (hers was white, mine is striped/streaked).

Doug/Owen - I will re-post again once I have an ID, if I get one! The geology curator at Bristol Museum has asked us to bring it in to show her in early April, so she can have a look at it. I think her offer is very generous and she also said she will show my and my 5-year-old daughter around her geology exhibits! Great to have an expert guide. I'll also check out that link, Owen - thanks.

Tom - Hello from a fellow Somerset dweller. I'm Reb - as in "Rebecca" - rather than Rob ;-)
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Scott Rider March 14, 2018 08:33PM
Reb, this is not citrine. Your specimen is iron-coated or stained quartz. Citrine is colored by impurities of iron, where iron atoms replaces Si in the crystal lattice and the color is a result of the light being absorbed and refracted differently. Yours, if proved to be quartz (which I believe it is), would be yellow/orange quartz. Citrine is actually much rarer, and is usually artificially produced by heating purple amethyst so you'll see a lot of specimens that look like yours labeled citrine, but are really heat treated quartz..



Edited 2 time(s). Last edit at 03/14/2018 08:39PM by Scott Rider.
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Reb Burns March 14, 2018 08:52PM
Yes, I thought citrine is rare after doing some research :-)
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Reb Burns March 14, 2018 08:53PM
Scott - the museum I visited today was selling pieces of "citrine", but I believe (since it costs £2 per "rock") it is probably not true citrine!?
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Wayne Corwin March 14, 2018 09:16PM
The museum is probably selling real citrine, it might be the heat treated amethyst, but it's citrine now.
Citrine isn't really rare, but not found everywhere.
Your specimen "looks" kind of like it, but... if you look close at yours, you'll see the colouring is only on the outside of the crystals, any place that has a fresh break shows the milky or clear insides.
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Scott Rider March 14, 2018 09:23PM
I should have used the word uncommon, not rarer... My apologies! But Wayne is right, if its like most rock shops and museums I've visited, they sell citrine that was treated. The fresh breaks is what caught my attention, which is partly why I think its quartz. It also reveals that the color is on the surface, a dead give-a-way its not citrine. You still have an interesting piece, good find!!
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Reb Burns April 03, 2018 10:28PM
Hello! So, today I met the geology curator at Bristol museum. She was really helpful - although sadly a lot of what she told me went a bit above my head because I do not know anything about geology and some terms were a bit alien to me...however, to briefly summarise, she is not sure what it is. She believes it is not just stained on the outside / superficially, but is that colour throughout. I asked about the whiter ones on some parts of the sample and she says crystals often vary in colour within many samples. She said it really looks like citrine, or possibly baryte, but it is hard to see how many sides the crystals have because they are quite small and overlapping / crowded. We don't think it's sort enough to be calcite but she has asked me to do a proper scratch test using a pin, and then report back to her. So - frustratingly it's not conclusive but it was great to meet her (and she also helped me identify a lovely fossil I'd found on the hillside where we live, so all in all it was an interesting trip!).
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Gregg Little April 04, 2018 03:34AM
Reb;

As you can tell by now identification is sometimes problematic even for those studying geology. One important factor is simply seeing a lot of material and not many people in the earth sciences have that interest or opportunity including academics which often have a very focus view for their particular area of expertise.

The specific gravity test can be effective but you would either find a person to undertake it or build a balance yourself. Its cheap, simple although a bit finicky to build but is a cheap and accurate analytical tool. Check out https://www.gemologyonline.com/HSG.pdf. if you want to go down this road.
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D Mike Reinke April 04, 2018 04:52AM
Reb, can you eventually get to a mineral club, show, or store, and handle some baryte? You'll know immediately how it differs from quartz.
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Duncan Miller April 04, 2018 07:25AM
Hi Rebecca - The link Gregg supplied doesn't work for me, but this one does https://www.gemologyproject.com/wiki/index.php?title=Hydrostatic_Balance. Read the section on "Single-pan scales". Here is a simple way to measure specific gravity (SG) of a large object. Weigh the stone on a kitchen scale. Place a suitable container on the scale, fill it with enough water to cover the stone, and zero the scale. Then immerse the stone suspended on a thin, non-absorbant string (nylon or wire) and read the scale. You can hold the string in your hand. Just make sure the whole stone is immersed, that it doesn't touch the bottom or sides of the container, and it doesn't have adhering air bubbles. (You can bob it up and down a bit to get rid of them.) Divide the weight in air by the weight in water. This gives you the specific gravity of the specimen. Look up the SG here https://www.gemologyproject.com/wiki/index.php?title=Specific_Gravity. If the base is not the same material as the crystals the result may not be very helpful.
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Kevin Hean April 04, 2018 04:16PM
Reb.
Donald B Peck has a good Article on Specific Gravity
https://www.mindat.org/article.php/2716/Determining+the+Specific+Gravity+of+a+Mineral
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Reb Burns April 04, 2018 07:04PM
Thanks all for your replies and advice. I'll look into the weight aspect.
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