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Identity HelpWhat is this mineral?

24th Jan 2020 19:04 UTCTom D.

05818120015798921665041.jpg
Bought this cheap at a natural history museum gift shop, and the label must have fallen off the display because there wasn't one.

Hardness is approx. 5 (it won't scratch my knife blade but does scratch steel scissors).
Vitreous Lustre
White streak
Non-metallic
Translucent
Specific Gravity >3 (approx. 3.5 measured crudely)
Does not react with HCL
Does not fluoresce under a black light

24th Jan 2020 19:45 UTCFrank K. Mazdab Manager

It looks rather like a big chunk of apatite, which is the "poster mineral" for Mohs hardness 5. But that name derives from the Greek "to deceive", so it could be something else too... :-)

Apatite won't fizz in HCl (and so may appear to not react), but apatite is slowly soluble in it. If you could chip off a small sand-grain-sized with your fingernail without marring your sample, that small grain would disappear if left overnight in a cup of dilute acid. Also, since the chunk looks like all one mineral, if you could measure the density more precisely, you might able to assign a number to "dense/heavy"; apatite is ~3.1 to ~3.2 g/cm3.

11th Feb 2020 16:02 UTCTom D.

I did measure the specific gravity and got approx 3.5, although the volume measurement was the best I could do with a small beaker/graduated cylinder at 20 ml.  

So I narrowed it down to apatite (3.1-3.2), kyanite (3.7) or diopside (3.2-3.3).  Leaning more towards apatite.

24th Jan 2020 19:50 UTCDavid Carter

I’m no expert, but my guess would be Aventurine, although the hardness for that is 6.5-7. It’s the sort of item usually found cheap in natural history museum gift shops.

11th Feb 2020 16:27 UTCDonald B Peck Expert

Tom,

Thank you for the clarity and thoroughness of your description.  While your method for determining SG is a little crude, it will put you in the ballpark.  I agree with Frank and think the hardness at about 5 and the SG above 3 make apatite more likely. If you could find a piece of quartz to see if it scratches your specimen, it would help.

On the measurement of SG.  The use of a graduated cylinder/beaker to measure the volume turns out to be a huge source of error.  The ratio of cross sectional area to depth is just too great.  I know the method is taught in middle school science classes where it is fine for the concept of specific gravity, but it leaves a bit to be desired when it comes to accuracy and precision.  (but if it is all one has, it is better than nothing).

Don

11th Feb 2020 17:46 UTCTom D.

Quartz scratches this relatively easily.

11th Feb 2020 17:22 UTCJosé Zendrera Expert

Looks like fluorite to me, there is a possible cleavage plane near the base, specific weight would fit and measured hardness could be not accurate.

11th Feb 2020 19:34 UTCPaul Brandes Manager

To me it's an odd looking apatite, but all the diagnostic results point to that. Gift shops do tend to have "interesting" specimens for sale at times.

12th Feb 2020 12:29 UTCHarold Moritz Expert

Did you try entering your data into the mindat search minerals by properties page?

Apatite does not fluoresce well under a "black light" - that is, a long-wave UV lamp, but will typically fluoresce yellow under medium and short-wave UV lamp. But fluorite will typically fluoresce purple under long-wave, so I think fluorite is unlikely.
 
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