I am trying to parse an old geologic description of stone suitable for use as road metal:

Care must be taken to distinguish the difference between the hardness of a stone and its toughness...Toughness in granites, syenites, and the like is occasioned by the disposition and size of the crystals forming them, these crystals being more interlocked in the tougher varieties of these stones than in the others.... Although we have described syenite as being made of quartz, feldsparand hornblende (excluding the few accessory minerals, for simplicity), we have not stated in what particular quantities these may exist, and this is an important factor in the inquiry. It will be found that the proportions vary considerably and on this the differences in durability and specific gravity are largely dependent. There cannot be much doubt that in the syenites commonly used as road metal, the rate of decomposition of the feldspar is the principal source of the superiority of one kind over another. The most durable syenite, then, is that which has the least feldspar, and, we may add, the least iron in that mineral. Syenites which contain an excess of feldspar, generally have a low specific gravity, unless the hornblende is superabundant. Those hornblendic granites, in which the feldspar has a tendency to occur in rather large crystals here and there, cannot be expected to make such good road-metal as those kinds of syenite which do not exhibit this feature, are fine-grained, have comparatively little feldspar, but much hornblende.

So, the question is how to figure out which rocks meet the criteria in the quote. For example, in New Hampshire some of the available syenites are as follows from sources like minedat:

Hornblende-biotite quartz syenite to syenite (Late Ordovician)
Hornblende (or alkalic amphibole) syenite (Jurassic)
Nepheline-sodalite-hastingsite syenite (Jurassic)
Porphyritic syenite - Hastingsite or hornblende and (or) hedenbergite are present. (Jurassic)

So, the question is how can I tell if a minedat description of a plutonic rock is favorable according to the criteria of the quoted passage?


1 Answer 1


So, the question is how can I tell if a minedat description of a plutonic rock is favorable according to the criteria of the quoted passage?

No. The description doesn't say how much feldspar is in the rock. It could be 5%, or it could be 20%. It also doesn't say anything about the iron content of the feldspar (although with some petrological hand waving you can assume by the presence of other minerals).

The nepheline syenite is going to have no feldspar (or very little), because it's usually either nepheline or feldspar, not both. But, we don't know whether nepheline is any good for the purpose.

I'm also wondering why is hornblende considered superior because it is a mineral that cleaves and breaks very easily.

One thing that is important is that they stress the fine grain size. Usually rocks with fine grain size are tougher than those with coarser.


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