Chemical Composition of Minerals (L3-2)

What to know

1. Define "solid solution" 2. Name the types of solid solution and their differences (substitutional, interstitial, omission) 3. Know the primary controls on solid solution (4) 4. Define "end member" (comp. that defines one end point...) 5. Label the feldspar ternary diagram, with end-members, mineral formulas, and substitution mechanisms between each 6. Illustrate the relationship between Enstatite-Ferrosilite solid solution and the Diopside-Hedenbergite solid solution (pyroxene diagram) 7. Name end members of garnet (pyrope, grossular,...)

Common solid solutions - Garnet Group

Again, graphical representation of the solid solution between end member garnets

Compositional Variations in Minerals - Solid Solution Example amphibole of solid solution (mineral groups with common end members)

Amphibole Tremolite-Actinolite Ca₂(Mg,Fe)₅Si₈O₂₂(OH)₂ Hornblende (Na,K)₀₋₁Ca₂(Mg,Fe,Fe³⁺Al)₅(Si,Al)₈O₂₂(OH)₂

Mineral Formulae and Structural Sites Diopside Example of Charge Balance

CaMgSi₂O₆ ↓ Let's break it down ↓ Ca²⁺+ Mg²⁺+2xSi⁴⁺+ 6x O₂⁻ = 12+ vs 12- charges

Mineral Formulae

Chemical formulae -are a systematic way of presenting and defining the composition of a mineral Quartz (SiO₂) Sillimanite (Al₂SiO₅) -not always written as a chemist would serves a dual purpose: -represents composition -crystal structure

Mineral Formulae What does a chemical formulae of a mineral indicate

Chemical formulae for minerals -Displays elements that typically reside in a "crystallographic site" or group of sites. -therefore signals possible sites of "substitution" or chemical variation. Example: Muscovite (K <-> Na) KAl₂(AlSi₃)O₁₀(OH)₂ <-> NaAl₂(AlSi₃)O₁₀(OH)₂ Example: Fayalite to Forsterite (Fe²⁺ <-> Mg) Fe₂SiO₄ <-> Mg₂SiO4

Mineral Formulae and Structural Sites Muscovite vs. Biotite comparison

Comparison: Muscovite vs. Biotite 2Al³⁺↔3(Fe²⁺,Mg) KAl₂(AlSi₃)O₁₀(OH)₂ K(Fe,Mg)₃(AlSi₃)O₁₀(OH)₂ can easily observe the "genetic" relationship between these two Micas

Common solid solutions - Feldspar Feldspar ternary

Feldspar ternary is a graphical representation of the solid solution series between the feldspars. Each ion (i.e., K, Na, and Ca) indicate an end member composition

Compositional Variations in Minerals - Solid Solution Example garnet of solid solution (mineral groups with common end members)

Garnet: Pyrope - Almandine - Spessartine - Grossular (Mg,Fe,Mn,Ca)₃Al₂Si₃O₁₂

Common solid solutions Garnet group •Pyralspite garnets •Ugrandite garnets

General formula X₃²⁺ Y₂³₊ (SiO₄)₃ •Chemistry: 2 general groups "Pyralspites" Y = Al³⁺ Pyrope Mg₃Al₂Si₃O₁₂ Almandine Fe²⁺₃Al₂Si₃O₁₂ Spessartine Mn₃Al₂Si₃O₁₂ "Ugrandites" X = Ca2+ Grossular Ca₃Al₂Si₃O₁₂ Uvarovite Ca₃Cr₂Si₃O₁₂ Andradite Ca₃Fe³⁺₂Si₃O₁₂

Compositional Variations in Minerals - Types of Solid Solution Mechanisms Interstitial Solid Solution

Interstitial Solid Solution In some crystal structures, interstices (vacancies, voids) may exist that normally are empty, these may occasionally be occupied.

Compositional Variations in Minerals - Types of Solid Solution Mechanisms Omission Solid Solution

Omission Solid Solution •more highly charged cation replaces two or more lower charged cations -to maintain charge balance, another site(s) left vacant - omitted Omission solid solution occurs when an ion of higher charge substitutes for an ion of lower charge. In order to maintain charge balance, two of the lower charged ions will be replaced, but the higher charged ion will occupy only one site, thus the other site will become vacant, or omitted.

Compositional Variations in Minerals - Solid Solution Example pyroxene of solid solution (mineral groups with common end members)

Pyroxene Enstatite - Ferrosilite (Mg,Fe)₂Si₂O₆ Diopside - Hedenbergite Ca(Mg,Fe)Si₂O₆ Augite Ca(Mg,Fe,Al)(Si,Al)₂O₆

Common solid solutions Pyroxene group •Orthopyroxenes •Calcic clinopyroxenes •Sodic clinopyroxenes (non-quad pyroxenes)

Pyroxene quadrilateral Orthopyroxenes (orthorhombic) Enstatite Mg₂Si₂O₂ Ferrosilite Fe₂Si₂O₆ Calcic Clinopyroxenes (monoclinic) Diopside CaMgSi₂O₆ Hedenbergite CaFeSi₂O₆ Non-quad pyroxene Sodic Clinopyroxenes (monoclinic) Jadeite NaAlSi₂O₆ Aegirine NaFe³⁺Si₂O₆ •What does non-quad mean? Does not plot on the pyroxene quadrilateral •What does monoclinic/orthorhombic mean? We will learn this at a later date

Compositional Variations in Minerals Definitions: Solid Solution

Solid Solution: Fixed mineral composition that defines one limit of a compositional range. A mineral with one crystal structure and a range of compositions, i.e. one or more "sites" that can be occupied by different ions

Compositional Variations in Minerals - Types of Solid Solution Mechanisms Coupled Substitutions

Substitutional Solid Solution - coupled substitutions: if an ion is substituting for another ion of difference valence, requires a second ion of difference charge to balance Coupled substitution occurs if an ion of different charge is substituted. This results in having to make another substitution in order to maintain charge balance. Such coupled substitution is common in the silicate minerals where Al³⁺ substitutes for Si⁴⁺.

Compositional Variations in Minerals - Types of Solid Solution Mechanisms Simple ionic substitution

Substitutional Solid Solution- simple ionic substitutions When ions of equal charge and nearly equal size substitute for one another, the solid solution is said to be simple. Generally if the sizes of the ions are nearly the same, the solid solution can occur over the complete range of possible compositions and the solid solution series is said to be complete. If the sizes are similar, but still very different the substitution may only occur over a limited range of compositions and the solid solution series.

Compositional Variations in Minerals - Solid Solution Controls on Solid Solution (temperature and pressure)

Temperature and pressure -greater ability to substitute at higher-T, due to expanded structures (cation sites) -reduced ability at higher-P, due to compressed structures (cation sites) Availability of ions -the substituting ions must be readily available -Meaning if no ions are available for bonding, no substitution will occur

Common solid solutions - Feldspar Feldspar ternary variations in K-feldspar

There is even various types of K-feldspar, but more on that later in the course. Hint: if chemical composition is fixed... how are they different minerals? If you don't know the answer now, you will later!

Lecture 3-2: Chemical composition of minerals

This lecture -Mineral formulae -Compositional variation and Solid Solution Key solid solutions series Olivine, Feldspar, Pyroxene, Garnet

Quantifying solid solutions •Mineral analysis

• Geologist like to represent Compositions by weight percent oxide (wt. % oxides) 60.1 wt. % SiO₂, 10.1 wt. % Na₂O, .... •Recalculation in atoms per formula unit (apfu). •Must charge balance and cation sum must equal what is typically found in a mineral.

Compositional Variations in Minerals - Solid Solution Controls on Solid Solution

• atoms or ionic groups that are substituting for each other -wide range possible if substituting ions are <15% different in size -15-30% difference: limited or rare - • involved in the substitution -must maintain charge neutrality e.g. Fe2+ and Mg2+ -if charges different between substituting ions - coupled substitution: Al³⁺ + Ca²⁺ = Si⁴⁺ + Na⁺

Quantifying solid solutions •Graphical representation (mole %)

•% En = Mg/(Mg+Ca+Fe)=42.39 % •% Wo = Ca/(Mg+Ca+Fe)=50.27 % •% Fs = Fe/(Mg+Ca+Fe)=7.33 % •Sum = 100 % En₄₂Wo₅₀Fs₈

Common solid solutions •How do we characterize minerals to know quantify their solid solution (i.e., amount of substitution)?

•Electron microprobe analysis (EMPA) -Sample bombarded with electron gun. -Emission of X-rays for specific wavelength for each elements. -Determine the element composition of minerals. Remember minerals are rarely 100% pure of any end member. They reside within a compositional continuum. We must have a way to characterize, quantify, and represent this.

Mineral Formulae and Structural Sites Basic Rules of Mineral Formulae

•Typically written to provide structural information, with some basic rules: •cations written first, then anions or anion groups •charges must balance •cations in same structural site are grouped e.g., Fe and Mg in olivine (Fe, Mg)SiO4 •cations in different structural sites listed in order of decreasing coordination number (will learn later) •exceptions....

Compositional Variations in Minerals - Solid Solution Example olivine of solid solution (mineral groups with common end members)

•Use molecular proportions: Mg/(Mg+Fe)=0.42 •Fo₄₂ means an olivine with 42 Mg out of 100 Mg+Fe total sites randomly occupied (and 58 occupied by Fe)

Common solid solutions •Feldspar •What types of substitutions are observed in feldspars?

•You certainly want to recognize that feldspar undergoes both simple and couple substitution. Why is this? Anorthite (Ca²⁺) and albite (Na⁺) do not charge balance, thus Al³⁺ and Si⁴⁺ partake in the substitution •Would coupled substitution for Ca²⁺ occur with K-feldspar?

Compositional Variations in Minerals

•definition of a mineral includes: "...definite (but not necessarily fixed) chemical composition..." -most minerals show variable composition -exceptions: "pure" substances such as quartz (SiO₂) or corundum (Al₂O₃)

Mineral Formulae Structural Sites

•shows the 2 different "sites" in which Al sits in the Muscovite crystal structure •tells us the ratio of "tetrahedral" cations (Si, Al) to O is 4:10 and is therefore a phyllosilicate

Common solid solutions Pyroxene Group •M2, M1, and tetrahedral crystallographic site

•single-chain inosilicate •general formula: [M2][M1]T₂O₆ M2 crystallographic site X = Ca²⁺ and Na⁺ (clinopyroxene), Mg²⁺ and Fe²⁺ (orthopyroxene), Mn²⁺, Li⁺ M1 crystallographic site Y = Mg²⁺, Fe²⁺, Mn²⁺, Al³⁺, Fe³⁺, Cr³⁺, Ti⁴⁺ Tetrahedral site (don't worry if you don't understand what tetrahedral means yet) T = Si⁴⁺, Al³⁺