Exercises: Minerals: concept and classification

List the conditions a solid must meet to be considered a mineral and apply this criterion to decide whether glacier ice and fossil amber are minerals.

What is the difference between a mineral and a rock? Give an example of a rock formed by a single mineral and another formed by several different minerals.

Silicates are the most abundant group of minerals in the Earth's crust. Look at the diagram: its basic unit appears on the left, and two linked units on the right. Explain what the $\text{SiO}_4^{4-}$ tetrahedron is, what each sphere represents, and why the ability of tetrahedra to share oxygen atoms gives rise to so many different silicates.

What is the crystalline structure of a mineral and how is it manifested externally in the habit or shape of its crystals?

Distinguish between an isolated crystal and an aggregate or crystalline mass. Why do many quartz samples not show perfect faces?

Mica, feldspar, amphibole, pyroxene, olivine, and quartz are all silicates. State what they have in common and name at least two subgroups of silicates according to the arrangement of their tetrahedra.

What is the difference between the essential minerals and the accessory minerals of a rock? Give an example of each in a granite.

Name one representative mineral of each of these chemical groups: carbonates, sulfates, sulfides, oxides, and halides.

Classify the following minerals according to their chemical group and state the characteristic anion of each group: quartz ($\text{SiO}_2$), calcite ($\text{CaCO}_3$), pyrite ($\text{FeS}_2$), and gypsum ($\text{CaSO}_4 \times 2\ \text{H}_2\text{O}$).

Graphite and diamond are composed exclusively of carbon and yet their properties are opposite. Based on crystal structure, explain how this is possible (polymorphism).

What is an isomorphous series? Explain, using ion substitution in the crystal lattice, why olivine can vary in composition between forsterite ($\text{Mg}_2\text{SiO}_4$) and fayalite ($\text{Fe}_2\text{SiO}_4$).

Micas cleave into extremely thin sheets. Relate this property to their phyllosilicate structure, in which the tetrahedra form sheets, and explain why they flake apart with a fingernail.

Pyroxenes and amphiboles are single-chain and double-chain inosilicates, respectively. Explain how this structural difference translates into different cleavage angles ($90°$ versus $60°$ and $120°$).

Quartz, a tectosilicate, lacks cleavage and breaks with conchoidal fracture. Explain why its three-dimensional network of tetrahedra makes it so resistant and compare it with calcite.

The color of olivine varies from pale green to dark brown according to its iron content. Relate this variation to the isomorphism of the forsterite–fayalite series and explain why color is not an absolute diagnostic criterion.

Garnet, staurolite and andalusite are typical minerals of metamorphic rocks. Explain why they are called "index minerals" and what information their presence in a schist provides.