--- title: "Rock forming minerals and significance" subject: "Engineering Geology II" chapter: 1 key: "semester_4/engineering_geology_ii:rock-forming-minerals" source: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/rock-forming-minerals site: BCT Engineering --- # Rock forming minerals and significance Engineering Geology II · Chapter 1: Introduction to Engineering Geology - Asked in: 2078 Poush, 1+2 marks; 2076 Baisakh, 1.5 marks; 2075 Baisakh, 2 marks; 2071 Magh, 1 marks; 2070 Bhadra, 3 marks; 2069 Bhadra, 3 marks (asked 6 times) - Video: [Rock Forming Minerals | GEO GIRL](https://www.youtube.com/watch?v=VKg1H3qTRIM) — GEO GIRL - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/engineering_geology_ii/rock-forming-minerals - Whole chapter: https://www.bctengineer.com/md/semester_4/engineering_geology_ii/chapter-1 ## Question What are rock forming minerals? Describe their engineering significance. ## Short answer Rock-forming minerals like quartz, feldspar, mica, calcite, clay minerals determine rock strength, durability, and behavior. Quartz gives strength but is abrasive; feldspar weathers to clay; mica causes low shear strength in schists; calcite dissolves forming cavities; clay minerals cause swelling and landslides. ## Answer **Rock-forming minerals** are the common minerals that make up most of the rocks of the earth's crust. About a dozen groups form over 90% of rocks: quartz, feldspars, micas, amphiboles, pyroxenes, olivine, calcite, dolomite, clay minerals and garnet. #### Important minerals and engineering significance | Mineral | Properties | Engineering significance | |---|---|---| | Quartz | Hardness 7, chemically stable | Gives strength and durability to aggregates and concrete; abrasive, so it wears drill bits, tools and hydropower turbines; its content in river sediment is a key result of sediment analysis for hydropower | | Feldspar | Hardness 6, weathers to clay | Rocks rich in feldspar (granite) are strong, but weathering produces clay and weakens the rock | | Mica | Perfect cleavage, soft | Mica-rich schist/phyllite has low shear strength and fails along foliation; poor for aggregate | | Calcite | Hardness 3, dissolves in acid water | Limestone is useful for cement, but dissolution forms cavities and solution channels, giving leakage at reservoirs | | Clay minerals | Plastic, swelling (montmorillonite) | Cause swelling, low shear strength and landslides; being nearly impervious, clay soils are used for the impervious core of earth dams | | Gypsum | Soluble, soft | Dissolves, causes settlement and attacks concrete | | Pyroxene, amphibole, olivine | Dense, dark | Strong rocks (basalt, gabbro) good for aggregate, but alter easily | | Pyrite | Oxidises to sulphuric acid | Attacks concrete and steel | Thus, minerals decide the strength, hardness, solubility, weathering behaviour and suitability of rocks as foundation and construction material. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What are rock-forming minerals? A. Rare gemstones in veins B. Common minerals that make up most of the earth's crust rocks C. Minerals found only in soil D. Artificial minerals in concrete Answer: B. About a dozen groups such as quartz, feldspar and mica form over 90% of crustal rocks. (Key point: Definition) 2. Why is mica-rich schist or phyllite a problem in engineering? A. It is too hard to excavate B. It dissolves in water C. It has low shear strength and fails along foliation D. It reacts with cement to give gas Answer: C. Perfect cleavage and softness of mica give low shear strength, so failure occurs along foliation planes. (Key point: Mica) 3. What is the engineering problem with calcite at a reservoir? A. Its hardness wears turbines B. Dissolution forms cavities and channels causing leakage C. It swells on wetting D. It oxidises to acid Answer: B. Calcite dissolves in acidic water, forming solution cavities that leak from reservoirs. (Key point: Calcite) 4. What useful engineering role do clay minerals have despite causing swelling? A. They give high shear strength B. They make excellent aggregate C. They seal fractures and form the impervious core of dams D. They resist weathering Answer: C. Clay is plastic and impervious, so it is used as the impervious core of dams, though swelling clays cause landslides and low strength. 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