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Improving Mindat.orgUsage of the term 'specific gravity'

5th Feb 2020 18:45 UTCJarrod Hart

I note that on the mineral data pages, the electron density section refers to 'specific gravity', and gives a number in gm/cc. While I might well take issue with the choice of units (cc is rather outmoded) I will rather point out the specific gravity is by definition unitless, and is a relative value (most often relative to water).
If mineralogists have a different convention, no worries, but as a process engineer I find it rather disquieting :)
Have a lovely day! 
Jarrod

6th Feb 2020 00:50 UTCFrank K. Mazdab Manager

Hi Jarrod,

in part I agree with you.  The "convention" in mineralogy is to use specific gravity (presumably because of the nature of the actual measurement; it's easier to precisely measure small masses than it is to precisely measure small volumes, and a volume measurement per se is not required for specific gravity because the volume of the mineral and the displaced water are equal and so cancel each other out).

Still, most of the other modern sciences use density for this intensive relationship of mass and volume of matter, and even though it's generally presumed that density = specific gravity, that's not strictly correct because the volume of water changes much more markedly than the volume of a mineral with changes, especially in temperature. Mineralogists have been slow to change. Again, the difficulty is in measuring the volume of a small, irregularly-shaped mineral to 3 significant figures, and doing so well is not always a trivial matter.

I'm not sure why you're not a fan of g/cm3 (for density that is... you're absolutely correct that specific gravity should technically be unit-less), unless perhaps as an engineer you may be more accustomed to kg/m3? Density is after all is mass/volume, and the specific units selected for each probably reflect whether one prefers most of their numbers to the left of or to the right of the decimal place; mineralogists prefer their "density" numbers to fall roughly between ~1 and ~20, instead of the more cumbersome between ~1000 and ~20000.

Incidentally, it's not only specific gravity that's a head-scratching holdover from mineralogy days of yore. If you look at chemical analysis, many mineral compositions are reported in oxide weights (again, because that's how they were often originally measured in the old gravimetric analysis days). And even more unsettling, analyses of minerals like topaz with high F contents (and similarly other oxygen-bearing minerals with Cl, sulfide S, etc.) will include an added "correction factor" to subtract extra inadvertently added oxygen equivalent to F, Cl, S.  Crazy... lol.

15th Feb 2020 07:57 UTCJarrod Hart

Thanks for the thoughtful response. Sorry to be so slow to reply, I obviously don't have my notifications set up right.
I had not thought that field mineralogists would routinely use water displacement to measure density, I guess its obvious in retrospect! 
No my issue is not with g/cm3, it was with gm/cc, both parts. I guess cc's are just easier to say and have stuck around in culture (like feet for height) . I still talk about a 50cc motor bike for example. 
Its also true that engineers often use kg/m3 or g/L, it really is context dependent - as you say 1-20 is an easier range to use. 
As for oxide basis (for xrf) , we had an issue this week with exactly that, as we used a lab more used to working with ceramic clays doing analysis for our diatomaceous earth people :) 
Anyway take care! 

16th Feb 2020 21:43 UTCDonald B Peck Expert

Hello Jarod,

If I understand Frank's and your discussion,  your concern is with the notation "cc", not "cm3".  Generally, I believe mineralogists use cm3, not cc.  Probably the cc appeared above because it is easier to type.

I tend to use the term Specific Gravity (or SG) because I then do not have to worry about units. I am aware, as Frank indicated, that with very small samples the temperature of the water can become an issue and a correction is necessary.

Don
 
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