Minerals are Rock Builders
Minerals are naturally occurring solids formed by crystals that join together. Each mineral has a definite chemical composition. There have been more than 4,000 minerals identified to date. Here are a few of them.
The atoms in crystals are arranged in a geometrically regular shape. Below are images of a quartz (SiO2) crystal where oxygen atoms are represented by the red balls and silicon by the blue balls; and salt showing atoms of sodium (Na) and chloride (Cl).
Crystals can take a variety of different shapes, but some lava cools very quickly before crystals can form - obsidian, or volcanic glass.
Obsidian was prized by stone age humans for making incredibly sharp tools.
Cooled Obsidian Lava Flow
Obsidian Spear Point
Minerals can also be formed by other processes, including evaporation and precipitation.
Hematite, and other iron oxides precipitated out of the ocean when they were first oxygenated during the Precambrian Era more than 3.7 billion years ago. Known as Banded Iron Formations (BIF) they were formed all over the world in sediments at the bottom of the oceans. Today, they are an important source of iron ore. Image at left is a BIF in Western Australia.
Carbonates also form by precipitation. Carbonate rocks are mainly composed of calcium carbonate (CaCO3). Calcium carbonate forms by precipitation from water; either straight from the water, or via organisms as they make their shells or skeletons. Dead organisms fall to the bottom of the ocean and are compressed over time into rocks. Limestones and chalk are common carbonate rocks. At right limestone with shells.
Minerals can also form as evaporites, deposited when oceans, seas and lakes dry out. Rock salt and gypsum are common evaporites. Gypsum is used as a fertilizer, in plaster and as a component in cement. Sometimes gypsum crystals can grow to be quite large...Found in the Cave of the Crystals in Mexico, the longest gypsum crystal is more than 12 metres. The conditions in this cave are extremely rare, and dangerous to humans (note the protective suit and breathing gear) with temperatures at a steady 580C and 90-99% humidity.
Organic minerals contain carbon from dead plant and animal matter. 300 million years ago the Earth was warmer and wetter and dense forest spread over low-lying wetlands. Over time the dead trees and plants were buried by sediments, compressed and heated as they sank deeper and deeper, the carbon eventually crystallised as coal.
Today, coal is used as a solid fuel to produce heat and electricity. Above a geologist inspects a coal seam in New South Wales.
TASK 25: Answer the following questions. If large parts of west Australia are rich in iron ore, what does that tell us about the environment 3.7 billion years ago? What did it look like? What mineral forming processes were going on?
There are large coal seams in eastern Australia, particularly in south-east Victoria. Describe the environment 300 million years ago.
With more than 4,000 minerals they can be very difficult to identify without laboratory analysis, but geologists have developed a simple series a tests that can identify some common minerals. Watch the video.
TASK 26: Make a note of the different tests while you watch the video.
Luckily, you don't have to take the exam.
Next, we'll look at how sedimentary rocks help tell the story of Earth's history.