Another rock question: I was always told that high-polished granite IS marble, and vice versa. However, recently I have learned that marble is, indeed, a rock much different than granite. It is even in a different category (when considering the three categories of all rocks). So, what’s the SD on this? Is polished granite used as a cheap or imitation marble? - Jinx
They are two completely different materials. Granite is silicaceous and marble is calcicaceous. Marble is much softer. Granite can be a expensive if not much much more. ( I priced out some slabs yesterday which came out our cost at $400 a square foot!!!)
Granite is igneous and has crystals. Marble is …sedimentary, I think.
IMHO, granite is much more attractive than marble. It’s also harder to work.
Granite is not a cheep subsitute for marble. Granit is superior for durability, becuase it’s harder. Marble is what they use when they have to shape and detail by hand in bulk. Builders use what ever gives the colors and texture they want, when the money doesn’t matter. Montello Wisconsin supplied much red granite for a long time to the whole country. Though acid dissolves both materials, the marble does so much quicker. The granite placed outdoors may look almost untouched 50 years later while the marble is shot.
I think for home decorating use, such as kitchen countertops, granite is used exactly because it is so durable. Marble would be very, very difficult not to damage under normal kitchen usage. You still see it in bathrooms, because the wear & tear on a bathroom is not the same.
Some people may think that marble is more beautiful than granite, but I think that is a matter of taste, the kind of marble or granite, and what the purpose of it is going to be.
The words you’re aiming for, MikeG, are siliceous and calcareous, respectively.
Granite is an igneous rock with a phaneritic texture, which means it formed from rock that was once molten in the subsurface and cooled slowly enough that distinct individual crystals of different minerals can be seen. The mineral quartz (silica) is common in granite, which gives it its toughness.
Marble is a metamorphic rock; it was once limestone (calcium carbonate), but has been recrystallized and sometimes chemically altered under high pressure and temperature. Marble comes in different colors and may include other scattered mineral grains, depending on the amount of “impurities” (like mud or organic material) that was mixed into the original limestone. Marbles are often soft and “absorbent”, but some marbles can be quite hard if the recrystallization process results in calcium silicate mineralization (essentially adding silica to the rock content).
In the ornamental stone world, lots of rocks are called granite or marble when in fact they are something different; the resemblance is only superficial.
(On preview) Soft marbles, like limestone, are pretty readily dissolved by fairly weak (10%) solutions of hydrochloric acid; over the longer term, acid rain does just as well. Granite AFAIK would really only be susceptible to dissolution in hydrofluoric acid, which is dangerous, nasty stuff.
Granite is used as a surface on machinery that can measure product to the standards required today. It is used as the table and beams on precise air cushion glide machines. It’s hard, dense, and is less prone to heat expansion and contraction, than steel. It’s not easily affected by chemicals.
While both granite and marble can be polished to a decorative finish, they are quite distinct rocks, with different origins and composition, as blue sky dreamer outlines above. It’s not gneiss to confuse them; in fact, one can say that it’s a schisty thing to do.
Concrete can be polished to a decorative finish, too. For some reason we paid to have the concrete floors here at work done in such a fashion, and they look good. In some cases they ground down to the aggregate level before polishing, and it makes a nice looking, natural formation.
As it turns out, there’s a nascent industry doing countertops out of concrete, too, and some of the samples I’ve seen are just fantastic. I’m considering them for my kitchen.