Introduction to Granite
A brief history
Granite has been with us for a long while. The oldest granites we’ve found on record are from 300 million years ago when earth was covered in the super-continent Pangea, forming deep within the Earth’s crust. That is 100 million years before the Jurassic Period (the time of the dinosaurs).
You may have already guessed but the word ‘granite’ comes from the Latin word granum, which means ‘grain’, which is appropriate as the granular appearance is its most distinctive characteristic. Thousands of specs of crystals, fused together due to the tremendous pressure and heat.
Because granite forms within the crust, and not on the surface -above ground, it is called an ‘intrusive’ rock, which basically meaning ‘inside’, since it is formed inside the crust.
Granite can be found all over the world, in all continents and, in many parts of the United States as well.
From liquid to solid
Although we have granites from 300 million years ago, it doesn’t take that long to form a slab of granite! The process of molten hot magma cooling into crystals takes a few million years. According to calculations done by scientists, the molten magma’s temperature must cool down from 900 to around 550-650 degrees Celsius at which point the magma will have cooled down enough to be crystalized and become ‘rock’, but make no mistake, 550 degrees Celsius is by no means ‘cool’! Keep in mind the melting point of lead is around 300 degrees Celsius, so, once granite has cooled it is still very hot, when it is still deep within the crust. However, the granite that we find when we are mining is a much cooler around a 27 degrees Celsius.
While this is an over-simplification, the longer time the magma has to cool, the larger the crystals will be. And the shorter cooling time, the finer the crystal grains.
A rainbow of colors
If you think about it for a moment, the range of colors in granite is quite amazing, from white, all the way to black, and then red, greens and pinks in between. How can this be?
When people talk of granite, their apt to think of it as just one mineral, but in fact it is a diverse mix of many minerals: quartz, feldspar, mica, potassium, and many others. It is by this mix that different colors can manifest. Different proportions of different minerals equal different colors.
Here’s a short scale of minerals and the corresponding color:
- Quartz – milky white color
- Feldspar – off-white color
- Potassium Feldspar – salmon pink color
- Biotite – black or dark brown color
- Muscovite – metallic gold or yellow color
- Amphibole – black or dark green color
Next time you see a piece of granite, with a vein of gold, flecks of red, or swirls of brown, realize that you’re simply looking at a bowl of ingredients (minerals) being mixed together, pressed together, and then ‘baked’ into a beautiful ‘cake’. And just like with snowflakes, no two slabs of granite are the same.
It is worth mentioning here that there are some rocks that look like granite, but would not be, ‘technically speaking’ a granite, according to the definition of their mineral composition. These include the stunning man-made pure white ‘quartzite’ – pure white rock does not actually occur in nature. Others include ‘blue granite’ which while a naturally occurring rock, and has the same visual patterns of a granite, it is technically not a ‘granite’, but can be a variety of blue-colored rocks such as larvikite or anorthosite.
Hard as a rock
The old adage ‘hard as a rock’ is applicable to granite. In fact, whoever coined that term was probably referring specifically to granite!
How hard is granite? Using the Moh’s scale of mineral hardness (a scale of 1-10), your average granite comes in, ranging from 6-8. To help you appreciate that, a diamond is a 10 on the hardness scale, steel would be a 5.5, and a copper penny would be a 3.
Granite’s hardness is thanks to its primary mineral: quartz – which scores a 7 on the hardness scale, with most granites having between 10-50% quartz composition.
Granite is so hard, so durable that it can damage knives, when used as a counter top, and it is so overwhelmingly heaving that it has been used as a building material in construction for thousands of years. The Romans were very found of granite and used it in building the Pantheon, as well their roads. It is a wonder that they were able to move these rocks, let alone mine, and cut them!
Stay tuned for next week’s article where we continue on the topic of granite but change gears and discuss building applications for granite and it’s pros & cons: residential, commercial and industrial.