Figure 12 Mohs hardness scale ranks minerals according to hardness on a scale from 1 to 10.
Inferring Which of the minerals in the table would scratch a glass plate, which has a hardness of 5.5?
dQuick Lab
Mineral Hardness
Procedure 
Select four minerals.
Try scratching each mineral's surface with your fingernail (hardness: 2.5), the edge of a penny (hardness: 3.5), the tip of an iron nail (hardness: 4.5), and the face of a steel file (hardness: 6.5). Then, scratch each mineral with every other mineral. Record your observations.
Analyze and Conclude
Observing Based on your observations and Mohs hardness scale provided above, estimate the hardness of each mineral.
Classifying Make a list of the minerals, going from the softest to the hardest.
Posing Questions What other information would you need in order to identify each mineral?
Hardness
The atoms of minerals are held together by chemical bonds of different kinds and strengths. As a result, minerals have different hardnesses. Hardness is the resistance of a mineral to scratching. A hard mineral can scratch a softer mineral, but not the other way around. The hardness of minerals is ranked on a scale from 1 to 10, called Mohs hardness scale. Diamond is the hardest mineral, with a hardness of 10. Talc is one of the softest minerals, with a hardness of 1. Figure 12 shows the hardness of some minerals according to Mohs scale.
Geologists use scratch tests systematically to determine the hardness of mineral specimens. First, a geologist may try to scratch a glass plate, which has a hardness of 5.5, with the mineral. Having determined that the mineral is either harder or softer than 5.5, the geologist can continue to perform scratch tests. Everyday materials can also be used to compare the hardness of minerals. For example, a copper penny, which has a hardness of about 3.5, can scratch talc but it cannot scratch quartz. A fingernail, with a hardness of 2.5, and a streak plate, with a hardness of 6.5, can also be used to test hardness.
Fracture and Cleavage
The fracture of a mineral is how the mineral breaks. When a piece of quartz is broken, it has a curved, shellshaped fracture. Other minerals fracture unevenly along curved or jagged surfaces. Fracture is determined by the crystalline structure of the mineral and the bonds between the atoms in the crystals.
Some minerals split evenly when they break apart. Cleavage is a type of fracture in which the mineral tends to split along regular, welldefined planes (flat surfaces) where the bonds are weakest. For example, mica and graphite form sheets. Each sheet of mica contains chemical bonds that are very strong. However, the sheets are held together with weak bonds, so they can easily be peeled apart from each other. In contrast, halite, shown in Figure 13, can be split into blocklike pieces that reflect the cubic structure of halite crystals.
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