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Is this "iolite" ( cordierite) ?

Posted by John Montgomery  
avatar Re: Is this "iolite" ( cordierite) ?
April 23, 2012 12:02PM
Bob Southern Wrote:
-------------------------------------------------------
>
> Cordierite is isotropic so hardness of 7 in all
> directions.
> Good Day
> Bob

Don't think so, Bob.

1. If Cordierite (coloured) was isotropic it could not be pleochroic - which it is, strongly. In fact, Cordierite is of the orthorhombic crystal system, which is anisotropic and biaxial. Yes, the hardness of Cordierite is (at the level of crudeness of measurement on Mohs's scale) omnidirectional.

2. Conversely, Diamond, which *is* isotropic can show a marked difference in hardness according to the direction of testing. The hardness of Diamond cannot be satisfactorily expressed (or assessed) on Mohs's scale but the hardness variation with test direction is about 30 GPa on a rational scale. That is a lot of variance. To put it in familiar terms, 30 GPa is approximately as much extra hardness as Corundum has over Talc!

The only minerals that are isotropic are either amorphous or of the cubic crystal system.
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