More on Silicate Minerals

Mohs scale is much more convenient and inexpensive compared to the Vickers test but they both can be used by mineralogists.

Remember too that hardness depends on the direction of the mineral you are pressing or scratching on. If you test the hardness parallel to the long axis of a crystal, you might get a 5 but if you test it parallel to the short axis, you'd get a 7. This is because of the different ways a compound will bond with another neighboring compound. This is how gemologists base the cutting of gems like diamonds for the best shine and polish.

MORE ON SILICATE MINERALS

Silicates are so common that you'll find them in every part of the world and in most grains of sand. When we think of the term "crystal", we are usually talking about quartz crystal. Quartz and other silicates have some combination of oxygen and silicon in varying forms – usually with one or more elements mixed into a larger molecule. Quartz is SiO2 chemically, while Olivine is SiO4 plus magnesium or iron. While seemingly so similar, they have widely differing properties.

Silicates are ionic salts based on a tetrahedral shape. The silicon atom is in the middle with four oxygen atoms sticking out. This is a very stable tetrahedral molecule, often found in groupings. The bond between silicon and oxygen is extremely strong. There are about 25 common types of silicate molecules in nature. This is because the base silicate molecule can easily polymerize to make others. You can get chains or sheets of these structures at any given point in time.

Olivine is a simple silicate. It has the silicate molecule of SiO4 plus magnesium and iron. The silicate molecule is an ion that is negatively charged. Like any salt, it will form a salt with either magnesium or iron, giving this rock a dark or dense coloration. Olivine does not cleave because there is no polymerization of the tetrahedral shapes.

Take these same tetrahedrons and put them in chains to get amphiboles and pyroxenes. The chains bond to any of the major cations like calcium, magnesium, or iron. Bridges form between the chains in some cases. If there are no bridges, you'll get the pyroxenes, which are single-chain polymers of the silicate molecules. These are also dark in color. Pyroxene forms the dark-colored mineral in gabbro. Amphiboles like hornblende is a