Sedimentology (Particle size distribution parameters)
Folk and Ward (1957) were the ones who created a measure of sorting using graphical moments of particle size distribution based upon φ values obtained graphically from the cumulative frequency curve at specific percentile levels.

I wasn't aware that negative values were possible based on the equation, but I am not up on my sedimentology..

Assuming your value is correct, it would signify extremely well sorted material:  the Inclusive Graphic Standard Deviation (Folk and Ward, 1957) has a phi range of &lt;0.3 (extremely well sorted) to >3.0 (extremely poorly sorted).



Folk, Robert L., and William C. Ward. "Brazos River bar: a study in the significance of grain size parameters." Journal of Sedimentary Research 27.1 (1957).