--- title: "Properties of schist, sandstone, phyllite" subject: "Engineering Geology I" chapter: 4 key: "semester_3/engineering_geology_i:properties-of-schist-sandstone-phyllite" source: https://www.bctengineer.com/q/semester_3/engineering_geology_i/properties-of-schist-sandstone-phyllite site: BCT Engineering --- # Properties of schist, sandstone, phyllite Engineering Geology I · Chapter 4: Petrology - Asked in: 2073 Shrawan, 6 marks - Video: [What is schist? How is schist formed? What are the general characteristics of schist?](https://www.youtube.com/watch?v=AiPdCrCxz-0) — Professional Geology Club - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/engineering_geology_i/properties-of-schist-sandstone-phyllite - Whole chapter: https://www.bctengineer.com/md/semester_3/engineering_geology_i/chapter-4 ## Question Describe the engineering properties, texture and structure of schist, sandstone and phyllite. ## Short answer Schist has crystalloblastic texture with schistosity, anisotropic low strength, splitting easily, and is poor for foundations and slopes. Sandstone is clastic, bedded, quartz with variable cement, strength 20-170 MPa, used as building stone. Phyllite is lepidoblastic, foliated, weak, weathers rapidly to clay, and causes slope failures. ## Answer #### Schist Medium to coarse grained metamorphic rock of medium to high grade. - **Texture**: crystalloblastic, lepidoblastic (flaky minerals visible to naked eye). - **Structure**: well developed foliation called **schistosity**. - **Minerals**: mica (muscovite, biotite), chlorite, talc, quartz, garnet, hornblende. - **Engineering properties**: strength is low and highly anisotropic. It splits along schistosity, so it is weak for foundations, slopes (sliding along foliation) and tunnels (overbreak, squeezing). It is not good as a building stone or aggregate. #### Sandstone Clastic sedimentary rock made of sand-size grains (0.06 - 2 mm) cemented together. - **Texture**: arenaceous, clastic, grains of quartz held by cement. - **Structure**: bedded, with cross bedding and ripple marks; jointed. - **Minerals**: quartz mainly, some feldspar and mica; cement of silica, calcite, iron oxide or clay. - **Engineering properties**: compressive strength 20 - 170 MPa depending on cement (siliceous strongest, argillaceous weakest), specific gravity 2.2 - 2.7, porosity 5 - 25 %, water absorption 1 - 10 %. Used as building stone, paving and aggregate. Weak cement type weathers and softens in water. #### Phyllite Fine grained low-grade metamorphic rock between slate and schist. - **Texture**: very fine, lepidoblastic, with silky sheen. - **Structure**: foliated (phyllitic cleavage), often wavy or crinkled. - **Minerals**: sericite mica, chlorite, quartz. - **Engineering properties**: weak and easily split along foliation, weathers rapidly to clayey soil, shows creep and slope failure. Poor foundation and tunnelling rock; widespread in the Lesser Himalaya of Nepal, where it causes landslides. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What is the characteristic structure of schist? A. Well developed foliation called schistosity B. Cross bedding and ripple marks C. Massive, non-foliated granular structure D. Fine slaty cleavage into thin sheets Answer: A. Schist is a medium to coarse grained metamorphic rock with flaky minerals aligned as schistosity. (Key point: Schist structure) 2. Why is schist weak for slopes and foundations? A. It is very porous and dissolves in water B. It splits along schistosity, so strength is low and anisotropic C. It is fully isotropic and brittle D. It has very high quartz content Answer: B. Weakness along foliation planes gives sliding on slopes and overbreak in tunnels. (Key point: Schist engineering behaviour) 3. Which factor mainly controls the compressive strength of sandstone? A. The colour of the rock B. The age of the rock only C. The type of cement binding the grains D. The grain size of feldspar only Answer: C. Siliceous cement gives the strongest sandstone and argillaceous the weakest. (Key point: Sandstone strength control) 4. What is the main engineering problem with phyllite? A. It is too hard to excavate B. It is highly soluble in acid C. It is massive and unjointed so it leaks D. It splits along foliation, weathers rapidly to clayey soil and causes slope failure Answer: D. Phyllite is weak, foliated and weathers to clay, so it is poor for foundations and tunnels and causes landslides in the Lesser Himalaya. 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