Physical properties are used to identify the minerals in rocks All rocks contains minerals. These minerals can be identified from various physical properties, which are determined by the minerals’ chemical composition and crystalline structure.

The colour of a mineral is the easiest and simplest property to observe. However, a mineral cannot be identified just by its colour. Colour is often used to narrow down classification to a smaller pool of possible minerals. COLOUR Gold vs. Pyrite Quartz vs. Quartz

STREAK It is the colour of a mineral in powdered form. To observe this, a streak test is performed by scraping the mineral across a streak plate (an unglazed porcelain plate). As the plate is relatively hard, it crushes the minerals and powders it, producing a streak. This is a more valuable test than the colour of the mineral sample as the colour of a mineral may differ, but its streak does not change. By Ra'ike (see also: de:Benutzer:Ra'ike) - Own work, CC BY-SA 3.0, https://commons.wikimedia.org/w/index.php?curid=10650193

Lustre is the way light interacts with the surface of a mineral. Minerals can either have a metallic lustre or nonmetallic lustre. There are many types of non-metallic lustre with some examples shown below. LUSTRE NONMETALLIC LUSTRE - silky (ulexite) NONMETALLIC LUSTRE - adamantine (diamond) METALLIC LUSTRE (galena) NONMETALLIC LUSTRE - resinous (amber) NONMETALLIC LUSTRE - glassy (quartz)

HARDNESS Hardness is the scratchability of a mineral A mineral’s hardness is tested by scratching its surface with a mineral of a known hardness. The Mohs scale is based on ten common minerals. The minerals are arranged in order of increasing hardness. The hardness of a mineral is measured against the scale by finding the hardest material it can scratch. For instance, Topaz has a hardness of 8. This means that corundum and diamond can scratch it but not the minerals from Talc to Quartz. HARDNESS Hardness Mineral 1 Talc 2 Gypsum 3 Calcite 4 Fluorite 5 Apatite 6 Orthoclase feldspar 7 Quartz 8 Topaz 9 Corundum 10 Diamond An unknown mineral was found to be able to scratch talc and apatite but not orthoclase feldspar, what would be a possible hardness for this mineral?

CLEAVAGE AND FRACTURE Because minerals have crystalline structures, the atoms in a mineral are regularly arranged. It is this pattern that affects how minerals break. Bonds are broken when a mineral breaks, and this usually occurs at bonds that are weaker, such as those between layers of crystals.

MUSCOVITE – layer of sheets CLEAVAGE MUSCOVITE – layer of sheets Cleavage shows how minerals break along its lines of weakness in their structure to give smooth surfaces. Because of varying crystal structures, different minerals will cleave off to form sheets or different shapes. This property of mineral is very important for people who cut gemstones and diamonds as it determines how minerals can be cut to make smooth surfaces. FLUORITE – octahedron HALITE - cube

FRACTURE Fracture describes how a mineral breaks differently to the pattern of a break along a cleavage plane. Fracture occurs without any pattern. QUARTZ – fractures in an irregular manner OBSIDIAN – fractures results in smooth curved surfaces

What other physical properties did you find? Share them with the class.