Text Book Pages : 44 - 53 So what is a mineral? A naturally occurring, inorganic solid that has a crystal structure and a definite chemical composition. What are the characteristics of all minerals? Now I See Crystal Clear! ► Naturally Occurring ► Inorganic (Was Never Alive) ► Solid ► Crystal Structure ► Chemical Composition Naturally Occurring Must be formed by processes in the natural world. Mineral Quartz forms naturally as molten material cools and hardens deep beneath Earth’s surface. Are man made materials like Plastic, brick, glass, and steel can be called minerals? Remember: A mineral can not form from organic materials (living things). Is coal a mineral? No, it comes from the remains of plants that lived millions of years ago. ►Has definite volume and shape ►A solid in which the atoms are arranged in a pattern that repeats again and again. ► Has flat sides, called faces, that meets at sharp edges and corners. ► Like a snowflake pattern or rock candy. Always contains certain element in definite proportions. ► ►Quartz has one atom of silicon for every two atoms of oxygen. Remember!: Rarely is a mineral identified by a single property. These properties need to be considered together to correctly identify a mineral. Of the almost 4000 known minerals, only about 30 are common. The most common are: quartz, feldspar, mica, and calcite. Fact: Over 60% of the Earth’s crust is made up of the family of minerals known as feldspar! Discuss the following! There are four common minerals, how many can you name? Of the four common minerals, which one makes up over 60% of the Earth’s crust? To be able to identify these and other minerals, we need to look at the properties used to separate and distinguish these minerals. Color is the most easily observed mineral property and the least useful! Mineral azurite is always blue. Mineral malachite is always green. Many minerals have a similar color. Caution: Many minerals have colors due to impurities, or they can change colors in various circumstances. For example, pure quartz is colorless or white, impurities can make the mineral rose, purple or pink! Discuss the following! How many of the characteristics of minerals can you name? Why isn’t color a very good property to identify most minerals? Streak of a mineral is the color of its powder when rubbed on an unglazed white (porcelain) tile. The streak is often not the same color as the mineral. A mineral’s color may vary, but the streak rarely will! Mineral sulfur has yellow color, but produces streak of white color on black tile. For example, Calcite occurs in many different colors, shapes, and varieties. But every single variety of Calcite has a white streak. A streak is useful in distinguishing two minerals with the same color but different streak. ► Minerals, which streak very light colors, can be easily identified on black streak plate. ►The way a mineral reflects light from its surface. Notice the difference between these two minerals? The mineral on the left has a metallic luster, the one on the right, a nonmetallic luster. Glassy luster Pearly luster Silky luster Other terms that might be used include greasy, dull, and earthy. Can you tell which of these has an earthy luster and which has a vitreous luster? Vitreous Earthy The hardness of a mineral is it’s resistance to being scratched. Diamond is the hardest of all minerals, and talc is the softest. Friedrich Mohs devised a hardness scale. In this scale, ten well known minerals are given numbers from one to ten. Lets take a look at the ten minerals used and some of the simple tests. Talc is the softest and has a hardness of 1. A soft pencil lead will scratch talc. Gypsum is a bit harder and has a hardness of 2. A fingernail scratches gypsum. Calcite has a hardness of 3. A copper penny just scratches it. Fluorite has a hardness of 4. It can be scratched by an iron or brass nail. Apatite has a hardness of 5. It can be scratched by a steel knife blade. Feldspar has a hardness of 6. It will scratch a window glass. Quartz, with a hardness of 7, is the hardest of the common minerals. It easily scratches hard glass and steel. Topaz has a hardness of 8. It will scratch quartz. Corundum (left) has a hardness of 9. Diamond with its hardness of 10. Corundum will scratch topaz. It can easily scratch the rest of the minerals. ►The cleavage of a mineral is it’s ability to split easily along flat surfaces. Cleavage can even be observed on tiny mineral grains making it a very useful property! Mica is probably the best example as it splits into thin sheets. It is said to have one perfect cleavage. Feldspar splits readily in two directions, always at or near right angles. Calcite and galena cleave in three directions. They are said to have three good cleavages. ►Not all minerals show cleavage. ►Those that don’t break along cleavage surfaces are said to have fracture. ►Minerals break in irregular way. Discuss the following! Does this mineral show cleavage or fracture? Of the four most common minerals, ►which has the highest hardness? ►Which has the lowest hardness? ►Crystal shape can be a useful property to identify minerals if the minerals have had the time and space to form crystals. Most mineral grains that are found in rocks, lack the room to grow. ►Specific gravity tells you how many times as dense as water the mineral is. ►Pure gold can have a specific gravity as high as 19.3! ►Calcite ►If is calcium carbonate, CaCO3. a drop of weak hydrochloric acid is placed on calcite, the acid bubbles as carbon dioxide is released. Magnetic Malleable Radioactive Flourescence Taste Minerals that can be hammered thin or shaped are said to show these properties. Can you think of a mineral that might be shaped or hammered? Gold would be a perfect example! Some minerals that contain Iron, are magnetic and can be picked up by a magnet. This is the state of glowing while under a ultraviolet light. Some minerals even glow once the light is turned off! Some minerals, such as this uraninite, are radioactive. They give off subatomic particles that will activate a Geiger counter. Halite (rock salt) can be identified by its taste. This practice is not recommended!