A fine-grained soft rock with a layered structure that spits easily. Angular from mechanical breakage along bedding planes. The flat, angular surfaces make broken specimens especially suitable for scratch tests to determine hardness. Sedimentary: Formed from compacted layers of clay, silt, and mud particles deposited in quiet water environments and cemented over time (therefore a 'clastic sedimentary' rock). Source location: Tasman Sizes: Choose from dropdown menu: Small specimen, Hand specimen, Shelf specimen, or Extra large shelf specimen. Note: Images are examples of specimens only. Sizes and shapes will vary. More detail Structure Shale is a fine-grained clastic sedimentary rock with a laminated, fissile structure. It is composed mainly of clay minerals (kaolinite, illite, smectite), quartz, and feldspar, with thin parallel beds or laminations less than a couple of centimetres thick. Type Clastic sedimentary (mudstone). Physical properties of shale Colour: Can have a wide range of colours, including black, grey, brown, red, green, or yellow, depending on mineral composition and organic content. Freshly exposed shale appears more vibrant, while weathered outcrops are dull and earthy-toned. Lustre: Has a dull, earthy lustre. Hardness: A soft rock, ranking around 1 to 4 on the Mohs scale of mineral hardness (can be easily scratched with a knife blade or copper coin). Density: Ranges from 2.2 to 2.8 grams per cubic centimetre. Fracture: Exhibits a splintery, fissile fracture, breaking easily along thin bedding planes into flat, plate-like shards or sheets. General properties of shale Composition: Consists mainly of clay minerals (kaolinite, illite, smectite), quartz, and feldspar, with variable amounts of organic carbon, carbonate minerals, iron oxides, and sulfide minerals. The proportions of these minerals can vary, resulting in different types of shale (e.g. silty shale, clay shale, sandy shale). Fine-Grained Texture: Has a fine-grained (clay and silt-sized particles less than 0.063 mm) texture due to deposition in quiet water environments such as lakes, lagoons, or deep marine settings. Fissility: The defining characteristic of shale is its fissility — the ability to split readily into thin, parallel layers or sheets due to aligned clay minerals and laminations. Durability: Relatively soft and brittle compared to igneous rocks like granite; susceptible to weathering and erosion, especially when wet. Heat Resistance: Exhibits moderate heat resistance but can deform or crack under high temperatures due to its clay mineral content. Porosity: Has relatively low porosity and permeability, though this varies with compaction and mineral composition; can act as a seal rock for hydrocarbon reservoirs. Uses of shale in New Zealand and worldwide Oil and Gas Extraction: Shale is a major source rock for hydrocarbons; oil and gas can be extracted from organic-rich shale formations. In New Zealand, oil shales occur in the Nevis and Waitati valleys (Otago), at Orepuki (Southland), and in the East Coast Basin (Waipawa and Whangai formations), where unconventional shale oil and gas exploration is ongoing. Construction: Used as a construction material for building foundations, road sub-bases, and fill material, though less durable than harder rocks. Brick and Cement Manufacture: Shale is quarried and processed to manufacture bricks, tiles, and cement, particularly in regions with abundant shale deposits. Ceramics: Used in the ceramics industry for making pottery, tiles, and other ceramic products due to its clay mineral content. Industrial Applications: Used as a raw material in various industrial processes, including as a filler in paints, plastics, and rubber. Geological Studies: Studied by geologists to understand sedimentary processes, depositional environments, and the history of Earth's crust; shale often contains fossils and organic matter that provide insights into past climates and ecosystems.
