Mineral Chemical Formulas

Why This Matters

Mineral chemical formulas aren't just abstract notation—they're the key to understanding why minerals behave the way they do. When you look at a formula like $\text{SiO}_2$ or $\text{CaCO}_3$, you're seeing the atomic blueprint that determines everything from crystal structure to hardness to how a mineral weathers in the environment. Your mineralogy exams will test whether you can connect composition to properties, predict mineral behavior based on chemical makeup, and explain geological processes through the lens of mineral chemistry.

The formulas in this guide illustrate core concepts you'll encounter repeatedly: silicate versus non-silicate structures, solid solution series, oxidation states, and environmental reactivity. Pay attention to patterns—why do iron oxides make good ores? Why do evaporites form the way they do? Don't just memorize $\text{NaCl}$ means halite; know what that simple ionic structure tells you about how and where it forms.

---

Silicate Framework Minerals

Silicates dominate Earth's crust because silicon and oxygen are its most abundant elements. The way silicon-oxygen tetrahedra link together determines a silicate's structure, hardness, and stability.

Quartz

• Formula: $\text{SiO}_2$—a three-dimensional framework of silicon-oxygen tetrahedra with no cleavage planes
• Hardness of 7 on the Mohs scale makes it highly resistant to both physical and chemical weathering
• Ubiquitous across rock types—found in igneous, metamorphic, and sedimentary environments, making it a key indicator mineral

Feldspar

• Formula varies by composition: $\text{KAlSi}_3\text{O}_8$ (orthoclase), $\text{NaAlSi}_3\text{O}_8$ (albite), $\text{CaAl}_2\text{Si}_2\text{O}_8$ (anorthite)
• Comprises ~60% of Earth's crust—the most abundant mineral group, essential for classifying igneous rocks
• Solid solution series between sodium and calcium end-members (plagioclase) demonstrates how cations substitute in crystal structures

Muscovite

• Formula: $\text{KAl}_2(\text{AlSi}_3\text{O}_{10})(\text{F,OH})_2$—a sheet silicate with potassium ions between layers
• Perfect basal cleavage allows splitting into thin, flexible sheets due to weak bonds between silicate layers
• Common in granitic and metamorphic rocks—its presence indicates aluminum-rich, potassium-bearing conditions

---

Silicates of the Mantle

These minerals form under high-temperature, high-pressure conditions and reveal Earth's deep interior composition. Their iron-magnesium content makes them denser and darker than crustal silicates.

Olivine

• Formula: $(\text{Mg,Fe})_2\text{SiO}_4$—an isolated tetrahedra (nesosilicate) structure with no silicate polymerization
• Solid solution between forsterite (Mg) and fayalite (Fe)—the Mg:Fe ratio indicates crystallization temperature
• Dominant in the upper mantle and mafic/ultramafic rocks like basalt and peridotite; weathers rapidly at Earth's surface

---

Carbonate Minerals

Carbonates contain the $\text{CO}_3^{2-}$ anion group, making them chemically reactive and critical to the carbon cycle. Their acid reactivity is a defining identification test.

Calcite

• Formula: $\text{CaCO}_3$—calcium carbonate forming rhombohedral crystals with perfect cleavage in three directions
• Fizzes vigorously with dilute HCl—the classic acid test produces $\text{CO}_2$ gas, distinguishing it from similar-looking minerals
• Primary component of limestone and marble—central to sedimentary processes, metamorphism, and global carbon cycling

---

Iron Oxide Minerals

Iron oxides are economically vital as ore minerals and geologically significant as indicators of oxidation conditions. The oxidation state of iron determines color, magnetic properties, and stability.

Magnetite

• Formula: $\text{Fe}_3\text{O}_4$—contains both $\text{Fe}^{2+}$ and $\text{Fe}^{3+}$ in a spinel structure
• Strongly magnetic—the only common mineral attracted to a hand magnet, making identification straightforward
• Major iron ore found in igneous, metamorphic, and sedimentary settings; records Earth's magnetic field in paleomagnetic studies

Hematite

• Formula: $\text{Fe}_2\text{O}_3$—iron in the fully oxidized $\text{Fe}^{3+}$ state
• Distinctive red-brown streak even when the mineral appears metallic gray or black
• Primary iron ore and pigment source—forms through weathering of iron-bearing minerals or direct precipitation in sedimentary environments

---

Sulfide and Sulfate Minerals

These minerals contain sulfur in different oxidation states, leading to dramatically different properties and environmental behaviors. Sulfides are typically metallic and form in reducing conditions; sulfates form in oxidizing, often evaporitic environments.

Pyrite

• Formula: $\text{FeS}_2$—iron disulfide with sulfur in the $\text{S}^{-}$ state, forming cubic crystals
• Called "fool's gold" due to its brassy metallic luster, but distinguished by hardness (6-6.5) and streak (greenish-black)
• Generates acid mine drainage when weathered—sulfide oxidation produces sulfuric acid, a major environmental concern

Gypsum

• Formula: $\text{CaSO}_4 \cdot 2\text{H}_2\text{O}$—calcium sulfate with structural water molecules
• Very soft (hardness 2) and forms in evaporite sequences, indicating arid paleoenvironments
• Industrial importance—used in plaster, drywall, and agriculture; the hydration state distinguishes it from anhydrite

---

Halide Minerals

Halides form from simple ionic bonds between metals and halogen elements. Their high solubility and cubic crystal systems reflect straightforward ionic bonding.

Halite

• Formula: $\text{NaCl}$—sodium chloride with a cubic crystal structure and perfect cubic cleavage
• Forms in evaporite deposits—indicates marine or lacustrine environments where evaporation exceeded inflow
• Easily identified by taste (salty) and solubility; essential for human consumption and industrial de-icing

---

Quick Reference Table

---

Self-Check Questions

1. Which two minerals are both iron oxides but differ in oxidation state and magnetic properties? How would you distinguish them in hand sample?

2. Compare the silicate structures of quartz, feldspar, and olivine. How does the degree of silica polymerization affect each mineral's weathering resistance?

3. Both pyrite and gypsum contain sulfur—what determines whether sulfur forms a sulfide versus a sulfate mineral, and how does this affect environmental behavior?

4. If you found halite and gypsum in the same rock sequence, what could you infer about the paleoenvironment? Which mineral precipitated first, and why?

5. An FRQ asks you to explain why olivine weathers rapidly at Earth's surface while quartz persists in beach sand. Using their chemical formulas and structures, construct your response.