FMF - Friends of Minerals Forum, discussion and message board

[MKomishyn]

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.

[MKomishyn]

Sliver is translucent green

[MKomishyn]

These pieces were fractured from a larger piece.

[MKomishyn]

The original specimen

[John Betts]

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

[Anísio Cláudio]

Hi,

Good afternoon. What is the density?

Anísio

[MKomishyn]

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.

[Pete Richards]

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.

[MKomishyn]

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

[MKomishyn]

Better picture of what I thought were faces.

[Pete Richards]

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?

[MKomishyn]

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

[Pete Richards]

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!

[MKomishyn]

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!

[Bob Carnein]

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.