The 8 Classes of Minerals Part 2: Carbonates, Sulfates, and Phosphates

Summary

This tutorial covers four mineral classes with oxyanions: carbonates, sulfates, phosphates, and mineraloids. Carbonates, like calcite and aragonite, form sedimentary rocks such as limestone and are essential to the carbon cycle. Sulfates, like gypsum, have historical uses in construction and can form through seawater evaporation. Phosphates, including the apatite group, are vital for biological structures and found in various rock types. Mineraloids, such as opal and obsidian, lack a crystalline structure but are often mistaken for minerals. The silicates, a key mineral class, will be explored separately.

Takeaways

• 🪨 The previous tutorial covered the first four mineral classes: native elements, oxides, halides, and sulfides.
• ⚛️ The next four mineral classes—carbonates, sulfates, phosphates, and silicates—all feature oxyanions or polyatomic anions bonded with oxygen atoms.
• 🔷 Silicates are the most important class of these minerals and will require additional tutorials.
• 🌿 Carbonates contain the carbonate anion (CO3^2-) and are common in biogenic sedimentary rocks and evaporite deposits.
• 🔎 Calcite (calcium carbonate) is the most abundant non-silicate mineral in the Earth's crust and is a key component of limestone.
• 🧱 Sulfates, containing the sulfate anion (SO4^2-), include gypsum and anhydrite, which can precipitate from evaporating seawater.
• 🍀 Phosphates, characterized by the phosphate anion (PO4^3-), include apatite minerals, which are essential for bone and tooth structure.
• 🦴 The apatite group demonstrates solid solution behavior, with fluorine, chlorine, and hydroxyl substituting in its structure.
• 💎 Mineraloids, such as opal and obsidian, are not true minerals because they lack a crystalline structure.
• 📝 The remaining class of minerals, silicates, will be covered in detail in future tutorials due to its complexity and significance.

Q & A

What are the first four classes of minerals mentioned in the tutorial?

The first four classes of minerals are native elements, oxides, halides, and sulfides.

What are oxyanions, and how are they relevant to the remaining classes of minerals?

Oxyanions are polyatomic anions composed of an atom covalently bonded to oxygen atoms. The remaining classes of minerals (silicates, carbonates, sulfates, and phosphates) all contain oxyanions.

Why are silicates not covered in this tutorial?

Silicates are the most important and abundant class of minerals, so they require several tutorials to cover fully. They will be discussed separately.

What are carbonates, and where are they commonly found?

Carbonates are minerals that contain the carbonate anion (CO3 2-). They are commonly found in biogenic sedimentary rocks and evaporite deposits, such as limestone.

What are the two forms of calcium carbonate, and how do they differ?

The two forms of calcium carbonate are calcite and aragonite. Calcite has a rhombohedral form, while aragonite has an orthorhombic form.

How do speleothems form in limestone caves?

Speleothems form when groundwater degasses (releases dissolved CO2) upon entering the cave atmosphere, typically aided by agitation as the water flows over surfaces or drips onto the cave floor.

What is gypsum, and what are its uses?

Gypsum is a common sulfate mineral composed of hydrated calcium sulfate. It has been used for building and construction since ancient times. A fine-grained variety of gypsum, called alabaster, was used in canopic jars in ancient Egypt.

What are phosphates, and what is a common example of a phosphate mineral?

Phosphates contain the phosphate anion (PO4 3-). A common example of a phosphate mineral is apatite, which has the formula Ca5(PO4)3(X)2, where X can be chlorine, fluorine, or hydroxyls.

What is solid solution in mineralogy, as demonstrated by the apatite group?

Solid solution occurs when a combination of one or more elements can fill a specific crystallographic site in a mineral. In the apatite group, there is solid solution between hydroxyapatite, chlorapatite, and fluorapatite.

What are mineraloids, and how do they differ from minerals?

Mineraloids are naturally occurring substances found in rocks that lack a crystalline structure, unlike minerals. Examples include opal (amorphous hydrated silica) and obsidian (volcanic glass).