a reply to: westcoast
Ah, hexagonal grains. I should mention, as I have thus far failed to, that pyrite can sometimes appear hexagonal in cross-section, too. It has a
nasty habit of twinning and can grow in dodecahedron.
Compared to a common garnet (well, common species, anyway)...
It can be hard to tell the two apart from a picture, but if you see the striations (grooved lines) on the crystal surfaces, it is almost assuredly
pyrite. Garnet will usually appear cracked. This is itself a useful test, as garnet fractures and produces a very irregular surface, but the inside
colours will remain consistent. Weathered pyrite will fracture similarly, but it would leave a blackish-grey dust (iron) or soft reddish powder if
the mineral were completely replaced. This is why I asked if you would break a piece off to expose a fresher surface.
One of the big issues with pyrite grains is that they do weather quickly, producing sulfuric acid and leaving behind oxidized iron (rust indeed
The iron can coat the mineral grains, pervasively (into the rock) or on an external surface, leaving rocks an orange-brown shade which they might not
A glass bottle is about 5-6 on Moh's hardness scale, so that is consistent with both pyrite and garnet, as well as many other minerals. On the
bright side, you ruled out a lot of the softer minerals (like fluorite, for example).
If the matrix (the material that isn't the shiny hexagonal-ish grains) is indeed hard material, then it is very likely feldspar or quartz (quartz
also grows hexagonal crystals, and would also scratch glass...), which would likely indicate granite or sandstone (meta or not). Perhaps an orange
stained arkosic sandstone would be a good fit?
As for licking rocks... In one of my mineralogy exams (well, one in particular, all of them after), we were tasked with identifying evaporites, which
are minerals formed by evaporation of mineral rich waters. Table salt (halite) looks, acts, and feels the same as a salt known as sylvite. The only
way to distinguish them is by the rather... uniquely bitter taste of the sylvite compared to halite. >.> It's rarely a good idea to lick rocks, but
sometimes it helps, unless it is coated in space virus.
Good luck with the sci-fi, by the way. I'm a big fan of the genre.