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Unidentified green crystal
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MKomishyn




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PostPosted: Feb 17, 2020 14:14    Post subject: Unidentified green crystal  

Hello, and thank you all in advance for your help.

I have been amateur rock-hounding this past year, and I found a crystal I cannot identify. I read a lot of conflicting information online regarding mineral properties, but even with these conflicts, I could not find a match.
Is there such a thing as green almandine? Mineralogy is one of the most challenging and rewarding fields I've ever dabbled in. I think I'm hooked, but right now, this gem has me up the creek without a paddle.

Here are the properties--

Hardness: between 6+ and 7
easily scratches titanium but not quartz; My 7 moh's hardness pick leaves a tiny line (white streak) on the mineral, but I can't get it to deeply scratch like other minerals.

Refractive Index: between 1.7825 and 1.783 ; I could not produce any birefringence.
I couldn't find anything matching its refractive index except Almandine, which is harder than my mineral. My refractometer seems to be in working order (tested known PET and quartz samples).

Specific gravity: no setup for this; working on it.

Crystal habit: hexagonal???
I *think* this is a hexagonal specimen because it seems to have 60 degree angles, but I don't know the crystal class-- still a little new with this regard.

Heat:
I also heated a sliver of the specimen (bic lighter until it was blackened so about 1000C for 15 seconds). It stayed untouchably hot for about a minute or two (very thin fragment), but it didn't change color.

Magnetic: no
The mineral has no magnetic drag or pick-up (tested with neodynium magnets).

Any advice would be appreciated. I'd really like to figure out what this is.



small_piece1.jpg
 Mineral: Almandine
 Dimensions: 51mm, 38mm, 25mm
 Description:
 Viewed:  2319 Time(s)

small_piece1.jpg


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MKomishyn




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PostPosted: Feb 17, 2020 14:16    Post subject: Re: Unidentified green crystal  

Sliver is translucent green


sliver2.jpg
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sliver2.jpg


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MKomishyn




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PostPosted: Feb 17, 2020 14:17    Post subject: Re: Unidentified green crystal  

These pieces were fractured from a larger piece.


20200217_124209.jpg
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20200217_124209.jpg


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MKomishyn




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PostPosted: Feb 17, 2020 14:19    Post subject: Re: Unidentified green crystal  

The original specimen


20200215_122025.jpg
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20200215_122025.jpg


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John Betts




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PostPosted: Feb 17, 2020 14:24    Post subject: Re: Unidentified green crystal  

Where was it collected?
Knowing the geologic setting (pegmatite? skarn? vein?) will help narrow the possibilities...

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Anísio Cláudio




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PostPosted: Feb 17, 2020 14:42    Post subject: Re: Unidentified green crystal  

Hi,

Good afternoon. What is the density?

Anísio

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MKomishyn




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PostPosted: Feb 17, 2020 15:14    Post subject: Re: Unidentified green crystal  

The specimen was found in an older riverbed that a creek now runs through. It is East of the NC piedmont area and West of the coastal plain.
This area once was part of a volcanic island chain that migrated from North Africa and added onto what is now North Carolina.
I'll often find large quartz veins, feldspar, hematites and other iron ores, and lots of igneous rocks. The granite I find is usally diorite, and we do have large bedrock formations that stick out of the ground around here.
Mica (all flavors) and muscovite is also abundant.

As for specific gravity, I'm going to have to get an accurate scale. Thanks! I'll post back when I find out the SG. It doesn't feel very heavy, though. I'd say it's less than 4.x.
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Pete Richards
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PostPosted: Feb 17, 2020 15:39    Post subject: Re: Unidentified green crystal  

I don't see any signs of crystal faces or prominent cleavages. It could be quartz with a greenish color imparted by epidote or chlorite, or it could a member of the epidote group. It's hard to say, given its lack of crystal form and near lack of transparency. I think an impure quartz is more likely.
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MKomishyn




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PostPosted: Feb 17, 2020 15:41    Post subject: Re: Unidentified green crystal  

Would that throw off the refractive index that much? Sorry, I'm very new to this.
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MKomishyn




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PostPosted: Feb 17, 2020 15:45    Post subject: Re: Unidentified green crystal  

Better picture of what I thought were faces.


big_piece4.jpg
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Pete Richards
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PostPosted: Feb 17, 2020 15:52    Post subject: Re: Unidentified green crystal  

That's a good question and I don't know the answer, though I would doubt it.

I have no experience with measuring refractive indices using refractometers Doesn't that require a flat surface on the mineral?

Quartz has a range of refractive indices depending on orientaiton, from 1.544 to 1.553. This specimen is probably polycrystalline, so you would probably get an average reading. As long as there is not a large bias in your measurement for some reason, your measurements are not consistent with quartz.

Have you got access to a petrographic microscope?

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MKomishyn




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PostPosted: Feb 17, 2020 16:17    Post subject: Re: Unidentified green crystal  

I think you are both right in thinking it is colored quartz. I retested my refractometer and something is off. Sorry, but I really appreciate all the help. I'm an amateur basically stumbling around the house looking for the light switch.. Mineralogy is a pretty big house! :)
Thank you all.

Mike
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Pete Richards
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PostPosted: Feb 17, 2020 16:21    Post subject: Re: Unidentified green crystal  

It's fine to be an amateur! You are doing the right things - trying to gather as much data as you can, trying to learn more, and actively thinking about the new information you get and how it relates to your own ideas. All of us were in that position once upon a time.... and sometimes find ourselves in that position even now!
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MKomishyn




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PostPosted: Feb 17, 2020 16:58    Post subject: Re: Unidentified green crystal  

Thanks for the encouragement. I am pretty sure what is happening is that I don't have a flat enough surface and so I am getting the refractive index of the contact fluid. I'm going to try to grind it down later and see if I can get a clean reading. Thanks again, everyone!
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Bob Carnein




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PostPosted: Feb 17, 2020 21:11    Post subject: Re: Unidentified green crystal  

If the refractive indices are at all close to actually being right, it might be an olivine. NC has several olivine localities. Hardness is 6.5 - 7.
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SteveB




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PostPosted: Feb 18, 2020 08:03    Post subject: Re: Unidentified green crystal  

I’m thinking much along similar lines as others, quartz based, olivine related. My understanding with measuring refractive index is usually requiring a pure crystal, ie. good clarity as well as a polished flat surface to place against the device. Inclusions internally will throw off the reading. I'm more knowledgeable with optics where the light beam enters the front of the sample directly with no air gap and travels into the medium and internally reflected back with deflection angle measured. Light “bends” refracts at every transition between mediums. So here you have the specimen, air and a special oil (made to ensure no air gap with little deflection. Its all geometry and depending on the method used it can be read from the front in relation to the light source or behind where a linear light source is expected to reach a detector. Its easier to linear polarize the light and use spectroscopy method to measure reflected deflection.

Ouch, my brain hurts. Anyway what you thought were crystal facets I don't think are related to crystallography, but are more related to the geometric fracturing of rapidly cooling materials. Think mud cracks or basaltic columns. So I would be looking at minerals often associated with volcanic magma flows into water where it cools more rapidly and cracks at the surface of the lava.
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Pete Modreski
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PostPosted: Feb 18, 2020 18:22    Post subject: Re: Unidentified green crystal  

Hi, M,

I've read all the responses to your question, and people's ideas seem to be circulating around a similar group of minerals. When I looked at your photos and read what you said, my first thoughts were, epidote, and olvine.

If your high (>1.7) refractive index measurement was at all correct, it would suggest epidote. But you do really need to have an optically flat, polished surface (like the table facet of a gemstone) to get a reliable refractive index reading. [One reads, " Epidote has a high refractive index, similar to pyrope garnet."

My comment, your photos, including the "sliver", seem overall too dark green to likely be just green-included quartz. Just my impression! But, photos can be deceptive, in trying to recognize a mineral.

Good luck with further progress in confirming what it is!

P.S., just an amusing sideline, about how "one must be careful about what one reads", when I typed in to Google, "refractive index of olivine", the first thing that came up was a picture of a faceted peridot with, in big bold lettering, "Peridot / refractive index 2.63-2.65". That iss totally wrong--those numbers are the specific gravity of peridot, not the refractive index!
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SteveB




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PostPosted: Feb 19, 2020 04:38    Post subject: Re: Unidentified green crystal  

Thinking further I'd be looking into the Serpentinite group for the answer.
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MKomishyn




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PostPosted: Feb 21, 2020 14:16    Post subject: Re: Unidentified green crystal  

I borrowed a presidium gem tester 2, and it reads as jadeite. That sort of fits with all of the other properties, and jadeite is found in NC (not often). I will post back after I get my triple-beam scale so I can measure specific gravity. Though, I'm leaning heavily toward jadeite after reading all of its properties (vitreous to greasy, around 6.5 mohs, splintery fracture, and I think it shows cleavage, though those faces could have been because they were crammed between two other crystals, I suppose).
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alfredo
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PostPosted: Feb 21, 2020 15:09    Post subject: Re: Unidentified green crystal  

Those gem testers are designed for use on polished gems. They don‘t really work well on rough stones.
But there is another test you can easily do yourself - Fusibility. See description in a Mindat article: https://www.mindat.org/article.php/883/Jade
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