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Identifying possible olivine specimen
  
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MKomishyn




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PostPosted: Mar 06, 2020 06:07    Post subject: Identifying possible olivine specimen  

First off, very new to mineralogy. Open to criticisms.

Found this in an area that commonly has a variety of igneous rocks. The area was an Archean volcanic island chain that slammed into what is now North Carolina. I find quartz, feldspar, some magnetite, asbestos, slate, so it's a hodgepodge of at least several large metamorphic events.

I found a piece of what I think is olivine cutting through a piece of mafic rock (possibly basalt; I will look more into this). I thought maybe epidote at first, but it leaves a colorless streak / no streak while *supposedly epidote's streak is whitish-grey, though I could imagine a variety of epidote could be as hard as olivine.

I read that olivine will dissolve in hydrochloric acid but apparently so will epidote. :(

I could take specific gravity, but I don't really want to destroy the specimen to do it. I could carefully measure and estimate volume of both to do some fun SG math if someone thinks that will help (no expert but doesn't seem too crazy).

Any tricks or tips? I could break a small piece off I suppose, but I'd rather not.



olivine_maybe.jpg
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SteveB




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PostPosted: Mar 06, 2020 07:24    Post subject: Re: Identifying possible olivine specimen  

Are you certain you did the streak test using the pure olive mineral and not a quartz piece or edge of the grey (likely basalt) material?

Do you have hardness scribes to get good hardness values? Test various areas so you can get a confident value.

Likewise I'd do a few acid drop tests noting the reaction (if any) at various locations. note aggressive reaction, slow etc.

These are all data points which can be used to definitively exclude an option. many volcanic minerals seem to come in a variety of names depending on the chemical makeup at each location not being identical everywhere. There are a lot of similarly coloured minerals of different names. I would be leaning towards olivine as a broad plain answer however it may be from the serpentine group.

Specific Gravity would be a fun exercise to attempt and accuracy likely poor unless you have a SG balance to use rather than water displacement volume estimate (I've never had much luck getting in sight of the correct ballpark this way so I bought an SG balance and use denatured water instead of tap water). Anyhow, I was thinking you could use a non destructive version of mountain weighing to give you a good estimate of the ratio of green to grey material to use to estimate SG for each from a measurement of the whole.

A method that should work: grab a roll of cling film, saran wrap(?) you know the thin plastic on a roll you use to cover a bowl of food when putting in the fridge. Hope you know what I mean. next grab two colours of Play Doh (children's modelling clay stuff). choose one colour to represent grey of the rock and another for the green. Based on your photos the specimen is simply structured so start by modeling the larger grey half as accurate to size and shape as you can. When happy it matches, cover the end where the green will start with some cling film, now use the second colour play doh to recreate the green layer accurately, cover with cling film and continue alternating colours and the cling film is to keep the playdoh colours separate. When you've reconstructed a model of the specimen you can pull it apart and ball up together all the grey material play doh to one lump and all the green playdoh to another lump. Weighing the two lumps will give you an accurate estimate of the ratio of the two materials in the mineral specimen which you should be able to use to guess the SG of each from an SG measurement of the whole.

No destruction needed to your specimen. Hope that idea makes sense.

Alternatively you can wrap the specimen in cling film or finger of latex glove then dip into plaster to cast its shape and then again use play doh layers in the cast hole to build up your accurate model. I must be nuts to find methods like this fun to do. Its good if you have kids to help you out too. Just remember the scientific method is not about making your data fit an answer its more about collecting accurate data to prove the possibilities it CAN'T be. And remember you have two minerals that appear to make up the majority of the specimen, so gather data on both. From photos its difficult but you can measure hardness and streak and acid test both, inspect the grey under magnification looking for gas bubble voids. All of these bits of information contribute to what the minerals could be as well as what they couldn't be. The grey you should be able to find more pure specimens from the location which will give you more accurate SG measurements etc. to determine if its in basalt range and help you separate the SG of the green from this specimen. Also look at mindat for the location and typical minerals from the area as it could give you similar minerals you can research as possibilities that might fit your data results better.
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Pete Modreski
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PostPosted: Mar 06, 2020 12:30    Post subject: Re: Identifying possible olivine specimen  

M,
Distinguishing between epidote and olivine is going to be a pretty difficult thing for you. I don't think the hardness or streak test is going to be at all possible to interpret--for a start, both of these minerals are hard enough that they don't really leave a recognizable streak on a regular streak plate, and if they did, the shades of color would be so similar. Likewise, the hardness of both is in the same range.

Overall, epidote is a lot more common to find, than olivine. But, North Carolina is noted for having several olivine-rich rock deposits (peridotite).

So, immediately, I don't have any good suggestion for you. Offhand, the rock looks more like epidote to me.

The fact is, probably one "best" guideline would be, is your rock sample taken from an area that is known to have peridotite occurrences? (For example, there is the Webster-Addie area, known for peridotite outcrops.). If it's not from such a place, olivine becomes much less likely.
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