Most of us have picked up a rock at some time and wondered, “Where did this come from?” Observations and classifications by scientists have helped us understand the great variety of Earth materials. This is all about rocks and minerals.
A Mineral is a naturally occurring, solid, inorganic substance and has a crystalline structure.
Minerals are classified mainly on the basis of their physical and chemical properties and composition.
Physical and Chemical properties
The physical properties of a mineral are due to mainly it’s internal arrangement. There are 6 different tests that can be done to find out the minerals from their physical properties. These properties are:
- Color: Color is the most visible property in a mineral. However, it is an unreliable mineral identification for two reasons: – Many different (like sedimentary, igneous) minerals exhibit the same color, and many minerals are found in a variety of colors.
- Streak: Streak is the color of a mineral in it’s powdered from. The color of a streak of a mineral might be different from the mineral’s original color. Streak is a reliable mineral identification.
- Luster: Luster is in which way a mineral reflects light from it’s surface. Luster can be both metallic and non-metallic. Minerals are metallic if they shine like polished metals. Non-metallic has no metallic shine. They are dull.
- Hardness: Hardness of a mineral is it’s resistance to being scratched. The relative hardness of a mineral can be determined by comparing it to the hardness of the ten standard minerals that make-up the Moh’s Scale of Hardness.
- Cleavage: Cleavage is the tendency of the mineral to split along one or more smooth and flat surfaces. Minerals like Mica and Halite exhibit cleavage.
- Fracture: Fracture is the opposite of cleavage. Minerals that do not have fracture will not split into flat and smooth surfaces, instead into uneven and rough surfaces.
Rocks are classified on the basis of their origin. Rocks are classified into three groups. They are:
- Sedimentary Rocks
- Igneous Rocks
- Metamorphic Rocks
Sedimentary Rocks: These rocks form in layers from the accumulation of sediments, organic matter or chemical precipitates.
At first, the layers of sediments deposits and accumulate. Later, pressure and weight squeezes lower layers. And after hundreds of years these lower layers become sedimentary rocks
There are three different types of sedimentary rocks. They are:
- Clastic: Clastic sedimentary rocks form from rock particles/sediments that are pressed and combined. Example of clastic rock is Conglomerate.
- Crystalline: Crystalline sedimentary rocks form from dissolved minerals that precipitate when water evaporates. Example of Crystalline rock is Dolostone.
- Bio-clastic: Bio-clastic sedimentary rocks form from the accumulation of plant/animal matter that undergoes a transformation into rock. Example of Bio-clastic rock is Coal.
Igneous Rock: These rocks forms from the cooling and solidification of molten lava and magma.
When the liquid molten lava or magma cools and solidifies crystals of different mineral form the rock.
There are two types of Igneous rock, which are:
- Extrusive/Volcanic – Extrusive igneous rock forms from fast cooling lava on or near Earth’s surface. Fast cooling does not allow time for crystals to grow and so rocks have small or no crystals, therefore resulting in smooth or fine texture. Example of and Extrusive rock is Obsidian.
- Intrusive/Plutonic – Extrusive igneous frock forms from slow cooling of magma within the Earth. Slow cooling allows time for large crystals to grow and so rocks have large crystals and therefore resulting in coarse or rough texture. Example of an Intrusive rock is Granite.
Metamorphic Rock: These rocks form from other pre-existing rocks(sedimentary/igneous) that have been changed from extreme heat and pressure.
Under high temperature and high pressure, many metamorphic rocks form by the process of re-crystallization.
There are two types of Metamorphic rocks, which are:
- Foliated – Have mineral crystals arranged in parallel layers or bands. Example of foliated rock is Gneiss.
- Non-Foliated - Do not have mineral crystals in bands. Do not break in layers/sheets. Example of Non-Foliated rock is Quartzite.