Chapter 63 Non Silicates and Mineral Formula Calculations
Year 3 · Descriptive Mineralogy · G336-L3
Compare non-silicate mineral classes, calculate a simple formula and distinguish observed minerals from normative calculations.
This independent lesson follows the topic framework of the supplied University of Peshawar BS Geology curriculum, session 2004–05 onward. It is newly written study material, not an official university course or qualification.
Learning objectives
- Compare major non-silicate mineral classes.
- Normalize a simple chemical analysis to a mineral formula.
- Distinguish observed minerals from calculated normative minerals.
Important minerals beyond the silicates
Oxides, hydroxides, sulfides, sulfates, carbonates and phosphates play major roles in rocks, ores and weathering. Magnetite and other spinel-structure minerals illustrate oxide groups; hematite is another important iron oxide with a different structure. Sulfides include pyrite and chalcopyrite, while gypsum and barite illustrate sulfates. Calcite and dolomite are familiar carbonates, and apatite represents a phosphate group. Classification by anion or anion group supports comparison, but identification still requires physical, optical or analytical evidence.
Radioactive elements can be concentrated in some minerals, including uranium-bearing species and trace constituents of accessory phases. Radioactivity is a property to assess with appropriate laboratory procedures, not a reason to handle an unknown specimen closely or collect it without guidance. Some minerals also contain toxic elements or produce hazardous dust. Digital reference collections and prepared teaching specimens are appropriate starting materials. A mineral name should not be inferred solely from apparent density or color.
Composition, optics and calculated representations
Chemical formula calculation converts measured element or oxide amounts into atomic proportions and normalizes them to an appropriate basis. The basis depends on the mineral structure and analytical assumptions. Oxidation state, water content and unmeasured elements can affect the result. A total near 100 percent does not prove that a proposed formula or mineral identification is correct. Check charge balance, site occupancy and consistency with optical observations.
In polarized-light microscopy, uniaxial and biaxial behavior describes optical symmetry. Interference figures and optic-axis information require suitable grain orientation and correct instrument use. Optical axial angle should not be guessed from an ordinary photomicrograph. Norm calculations assign an idealized set of minerals to a bulk chemical composition under prescribed rules; these normative minerals can differ from the minerals actually observed. Niggli values are another normalized chemical representation. Neither calculation replaces petrographic description or a statement of the chosen calculation method.
Why mineral class does not complete identification
A sulfide and a sulfate both contain sulfur but have very different chemical arrangements. Pyrite and chalcopyrite belong to sulfides, whereas gypsum and barite contain sulfate groups. Calcite and dolomite are carbonates with different cation arrangements; magnetite and hematite are oxides with different structures and iron relationships. These distinctions explain why chemical class is a useful organizing step but does not replace phase identification. Several dark or metallic-looking grains can belong to different classes.
X-ray diffraction investigates repeated atomic arrangements through a pattern of diffraction peaks. Compare several diagnostic features with a suitable reference rather than matching one strong peak, which could overlap another phase. A mixed sample can contain several minerals, and an identified phase is not automatically a quantitative abundance estimate. Sample preparation, preferred orientation and overlapping reflections can affect the pattern. Combine the result with specimen context and chemical information when an interpretation matters.
Turning oxide masses into atomic proportions
A formula calculation begins by converting each oxide mass to moles using its formula mass, then counting the cations and oxygens contributed. For a simplified magnesium olivine example, approximately 80.6 grams MgO is 2 moles and 60.1 grams SiO2 is 1 mole. Their atomic amounts give Mg2SiO4 after normalization. These rounded masses are illustrative and do not represent an actual analysis. Show each conversion so a unit error cannot disappear inside a final formula.
A more complex analysis needs a suitable normalization basis and assumptions about oxidation state and water. Iron reported as total iron cannot automatically be divided into ferrous and ferric iron without further information. A charge-balanced result is necessary but may not be sufficient: several hypothetical allocations could satisfy charge while conflicting with mineral structure or optical evidence. Check whether the calculation represents one clean grain, an altered region or a mixture before naming the mineral.
Optical geometry and normative chemistry answer different questions
A uniaxial mineral has one optic axis in the optical description, while a biaxial mineral has two. An interference figure depends on grain orientation and the optical setup, so a poorly centered view may not reveal the needed geometry. Orthoscopic observations describe features seen in the ordinary thin-section view; conoscopic observations investigate interference figures. The position of extinction or a visible color alone should not be reported as a measured optic axial angle.
Norm calculations distribute a whole-rock chemical analysis among idealized mineral components according to explicit rules. Niggli values provide a different normalized chemical representation. Both summarize composition rather than directly counting the observed mineral mode. A volcanic glass can contribute to bulk chemistry even though it is not a set of normative crystals in the specimen. Report the chosen algorithm, basis and purpose, and compare its result with actual petrography rather than treating disagreement as an error that must be hidden.
Worked activity
Use a deliberately simple magnesium silicate analysis reported as molar oxide quantities: 2 moles MgO and 1 mole SiO2. The atoms total Mg2, Si1 and O4, giving Mg2SiO4, the magnesium end-member composition of olivine. Check charge: two Mg2+ contribute +4, one Si4+ contributes +4 and four O2− contribute −8. The total is neutral. Now explain why adding iron requires knowing its amount and oxidation state rather than simply appending Fe to the formula. This activity teaches bookkeeping, not a complete method for every mineral.
Practice and interpretation
Prepare a specimen record with mineral class, candidate formula, observed properties, analytical method and remaining uncertainty. For an X-ray diffraction result, record the phase identification separately from any estimate of its abundance. For a chemical analysis, keep raw values, conversion factors and normalization visible in a spreadsheet. A peer should be able to recover your formula from the original values. If the calculated result disagrees with the microscope, investigate alteration, mixed grains or analytical assumptions rather than silently choosing the preferred answer.
Self-check
- Is pyrite an oxide?
- What distinguishes normative from modal mineralogy?
- Why check charge balance?
Check your answers
- No. It is a sulfide; its dominant anionic component is sulfur rather than oxygen.
- Normative mineralogy is calculated under rules; modal mineralogy records observed mineral proportions.
- A proposed mineral formula must be chemically consistent with its stated ions and structure.
Sources and further study
The explanations and teaching examples are original. Use these institutional resources to check terminology and explore the subject further.
- SERC mineral structure visualizations
- SERC phase diagram teaching collection
- Tulane University petrology teaching notes
Chapter 63 multiple choice questions
Choose one best answer for each question. Open each answer panel after making your choice.
An analysis is represented by 2 moles MgO and 1 mole SiO2. Which formula follows from adding the atoms?
- Mg2SiO4
- MgSiO3
- Mg2Si2O6
- MgSi2O5
Answer and explanation
A. Mg2SiO4 Two MgO units contribute Mg2O2 and one SiO2 contributes SiO2. Together they give Mg2SiO4.
For Mg2SiO4 using Mg2+, Si4+ and O2−, what is the total charge?
- Plus two
- Zero
- Minus two
- Plus eight
Answer and explanation
B. Zero Two Mg ions contribute +4 and Si contributes +4. Four oxygen ions contribute −8, giving a neutral total.
A bulk chemical calculation predicts minerals that differ from those seen under the microscope. Which distinction best explains why this is possible?
- Normative minerals are always directly counted in thin section
- Modal mineralogy ignores actual specimens
- Normative minerals follow calculation rules; modal minerals are observed
- Every chemical calculation must reproduce the observed mode exactly
Answer and explanation
C. Normative minerals follow calculation rules; modal minerals are observed Norm calculations represent idealized mineral allocations under specified rules. They need not match the observed mineral proportions.
Which mineral-class pairing is correct?
- Pyrite is an oxide
- Calcite is a sulfide
- Apatite is a carbonate
- Gypsum is a sulfate
Answer and explanation
D. Gypsum is a sulfate The lesson lists gypsum among sulfates, pyrite among sulfides, calcite among carbonates and apatite among phosphates.
A chemical formula appears plausible, but conflicts with optical observations. What is the best response?
- Check mixed grains, alteration and analytical assumptions
- Choose the preferred result and remove the conflicting evidence
- Assume a total near 100 percent proves the identification
- Treat oxidation state and unmeasured components as irrelevant
Answer and explanation
A. Check mixed grains, alteration and analytical assumptions Formula calculation and optical study should constrain each other. Mixed material, alteration and assumptions can explain disagreement.
All 84 chapters
- Minerals Crystal Symmetry and Identification
- Magma Crystallization Igneous Textures and Volcanoes
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- Light and the Polarizing Microscope
- Birefringence Extinction and Optical Figures
- Crystal Symmetry Forms and Rock Textures
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- Melting Magma Migration and Differentiation
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- Weathering Grains and Sediment Transport
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- Carbonates Chemical Deposits and Sedimentary Basins
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- Relative Plate Motion Rifts and Triple Junctions
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- Ore Deposits and Mineral Concentration
- Sedimentary Deposits and Exploration Evidence
- Mineral Estimates and Responsible Evaluation
- Microfossil Groups and Preservation
- Microfossils as Environmental Evidence
- Microfossil Ages and Basin Correlation
- Reading Phase Diagrams and Mineral Mixtures
- Silicate Structures and Rock Forming Minerals
- Non Silicates and Mineral Formula Calculations
- Planning Geological Surveys and Field Records
- Mapping Rocks Landforms and Structures
- Survey Maps Sections and Geological Reports
- Petroleum Generation Migration and Trapping
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- Petroleum Engineering and the Well Life Cycle
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- Integrating Facies Fossils and Basin Architecture
- Facies and Continental Depositional Systems
- Coastal Systems and Carbonate Platforms
- Slope and Deepwater Depositional Systems
- Structure Contours and Thickness Maps
- Fold Profiles and Orientation Analysis
- Balanced Sections and Deformation Fabrics
- Designing a Focused Geological Research Project
- Managing and Analyzing Geological Data
- Writing and Defending a Geological Thesis
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