Figure 11 Galena has a metallic luster and a density of about 7.5 g/cm3. Pyrite, also known as “fool's gold,” has a metallic luster and a density of 5.0 g/cm3. Sulfur has a resinous-to-greasy luster and a density of 2.0–2.1 g/cm3.
Designing Experiments How could you distinguish a sample of galena from a sample of pyrite?
Luster
The luster of a mineral is the way in which its surface reflects light. Luster refers to the general appearance of a mineral, or how shiny it is. As seen in Figure 11, galena and pyrite have a metallic luster, and sulfur has a resinous-to-greasy luster. Geologists use a variety of terms to describe a mineral's luster. For example, a mineral with a rough, crumbly surface is said to have an earthy luster. Other terms used to describe luster include silky, pearly, and vitreous (glassy.)
Density
The density of a mineral depends on its chemical composition. In general, minerals made up of elements with higher atomic masses have higher densities than minerals made up of atoms with lower atomic masses. For example, galena contains much lead, which has a relatively high atomic mass of 207. Galena's density is about 7.5 grams per cubic centimeter. In contrast, quartz is made up of silicon and oxygen, which have relatively low atomic masses of 28 and 16 respectively. Quartz's density is only about 2.6 grams per cubic centimeter.
Data Analysis
Density of Minerals
The density of minerals varies, depending on what elements the minerals contain. Samples of the minerals in the data table were analyzed for density, silicon and oxygen content, and the presence of metals. Study the data table, and then answer the questions.
Using Tables Which mineral has the lowest density? The highest density?
Using Tables Which minerals have the lowest percentage of silicon and oxygen? Which has the highest?
Mineral Properties
Mineral
Density (g/cm3)
Percent Si and O
Metals Present
Olivine
3.3
65
Mg, Fe
Augite
3.2
65
Mg, Fe, Ca
Hornblende
3.0
67
Mg, Fe, Ca, Al
Muscovite
2.8
71
K, Al, Mg, Fe
Quartz
2.6
100
None
Formulating Hypotheses Olivine and augite are abundant in oceanic crust and in the mantle. Quartz, muscovite, and hornblende are abundant in continental crust. Formulate a hypothesis to explain why continental crust floats higher on the mantle than oceanic crust.
Drawing Conclusions How is a mineral's density related to its silicon and oxygen content?
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