snip

Grantites that are low in quartz (and other hard minerals) are
softer and more porus. They will absorb water (and stains).
The softer minerals can also be etched by acids (e.g. lemon
juice).


What are the 'soft" minerals?

Granites consist of varying proportions of quartz, plagioclase
feldspar, potassium feldspar (microcline), perthite; some mafic (dark)
minerals like amphiboles (hornblende, reibekite) and less commonly
pyroxenes (augite, enstatite), and micas (biotite); and lesser
proportions of accessroy minerals (epidote, sphene, zircon,
clinozoisite, etc.).

None of these minerals are soft or porous; most have minimum
hardensses of 6-7 . The mafic minerals may be subject to chemical
attack moreso than the quartz and feldspars, but even that should be
minimal.

Glass (basically fused quartz with impurities) and hard minerals can
be etched with strong acids, but have very low permeabilty and are
resistant to "staining" without the presence of acids. I suspect that
the granite counters that get stained are from quarries in
'low-quality" granite, i.e. rocks that have been weathered (i.e. the
feldspars partially altered to (soft) clay minerals, or quarried from
parts of the granitic pluton that underwent late-stage magmatic
("dueteric") alteration (which also causes the feldspars to alter to
clay minerals).

Anyway, I'd still like a granite countertop.

-Zz, M.S., PhD.