ΔMH: A MEASURE OF THE LIKELY NATURE OF GRAIN SURFACES DURING MUTUAL GROWTH

ΔMH (delta Mohs’ hardness) is the numerical difference between the Mohs’ hardness of growth-competing mineral grains; the Mohs’ hardness is treated as an approximate measure of the bond strengths in the individual mineral grains. This provides a way of determining the likely form of grain surfaces during competitive growth of mineral grains in rocks.

From the study of mineral grains in a variety of rocks over approximately 50 years, it has been observed that if the Mohs’ hardness differs by more than 2, the grain with the higher hardness tends to develop crystal surfaces verses the lower hardness grain(s) during mutual growth. As well, the greater the difference the more highly developed the crystal surface will be. Grains with large differences in bond strength in different crystallographic directions (such as graphite) may develop crystal surfaces in certain directions, but not others. Note that in the most common igneous and metamorphic rocks, the minerals typically found therein ( quartz, feldspars, pyroxenes, amphiboles) are within 2 hardness units of one another and, as a result, do not develop recognizable crystal surfaces during mutual growth. However, the following minerals in selected rocks are particularly instructive:

• pyroxenes, garnets, humite, and spinel in tactites;
• garnet in schists (and to an extent) gneisses and pegmatites;
• magnetite and pyroxenes in carbonatites;
• graphite (a significant difference in certain bond strengths) in tactites;
• certain minerals with elongate growth habits (due to slight differences in bond strengths).

Less certain, but suggested, are other factors that may aid in the development of crystal surfaces such as: length of growth time, presence of fluids, and lack of inclusions. If the forgoing is correct, it is necessary to revise the basic terminology of mineralogy and petrology (see http://mineralkleck.com or search ‘mineralkleck’ for a suggested revision).

One use of such information is (in some cases) being able to distinguish between inclusions and phenocrysts (and perhaps certain phorphyroblasts).