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Chapter 4.1 – Minerals and Their Properties • Minerals are the building blocks of rocks (which are usually mixtures). • Minerals are pure substances - elements or compounds, with specific chemical formulas that do not vary or only vary within a limited range. • About 3000 minerals are known. • Only about 10 are common in Earth's crust. • Most minerals form in the geosphere, but some minerals form in the hydrosphere or even the biosphere. What is a Mineral? The 3 criteria: 1) Naturally occurring substance, not manmade. 2) Inorganic – not made of anything that is/was living. 3) Solid and Crystalline in form. What does “crystalline” mean? The particles in minerals (atoms, ions, or molecules) are arranged at specific “lattice” positions. The shape of a mineral crystal can reflect the internal structure but often does not. What does “crystalline” mean? Remember…a mineral can also be crystalline at the atomic scale without exhibiting crystal-like shapes. It is still a “crystal”. Mineral Properties help you to identify minerals! Characteristic property - A physical or chemical property that is helpful in identifying a material (in this case, minerals). Ex. – Diamond’s extreme hardness and luster Ex. – Graphite’s greasy feel, softness, and metallic luster. Mineral Properties help you to identify minerals! Physical property – A quality that can an be measured without a chemical reaction occurring. Ex. Color, crystal shape, luster, hardness, density (specific gravity) Chemical property – reflect how something chemically reacts with something else; when tested, a chemical reaction will occur if it has that chemical property. Minerals Properties are caused by… The properties (characteristics) of minerals are determined by the structure (arrangement) and chemical composition of the particles inside the mineral. Diamond Graphite (C) (C) Minerals Composition • Mineral composition refers to the ratios of elements present in the crystal – the chemical formula or chemical symbol. • • • • • Pyrite: FeS2 Quartz: SiO2 Calcite: CaCO3 Copper: Cu Sulfur: S Minerals: Characteristic Properties • Color- One of the most obvious mineral properties, but NOT the most useful. Many minerals come in a large variety of colors. • Streak- The color left behind when mineral is scraped on unglazed porcelain (may be the same or a different color than the crystal). • Luster- The manner in which light reflects off surface of a mineral metallic vs. nonmetallic (glassy, pearly, dull, earthy, silky). Minerals: Characteristic Properties • Cleavage - the smooth breakage of a mineral along relatively weak planes of the crystalline structure. • Fracture – The rouch and irregular breakage of a mineral (choncoidal fracture). Basal or platy cleavage Prismatic cleavage Cubic cleavage Rhombohedral cleavage Octahedral cleavage Describe the Cleavage B A D C Minerals: Characteristic Properties Hardness- The scratch-resistance of a mineral based on a scale from 1 to 10. Moh’s Scale of Hardness Hardness Test Warning! Don’t confuse a scratch with a streak. You can’t rub a scratch off! Minerals: Characteristic Properties Crystal Form (or Crystal Habit) The geometric shape of a well-formed crystal (the growth shape if there was room to grow). More Characteristic Properties of Minerals • Reactivity to acid- The chemical response of a mineral to hydrochloric acid, causing the mineral to fizz (effervescence). • Double Refraction – The ability to split light rays to produce two images. • Magnetism- The attraction to a magnet. • Density/Specific gravity- How heavy a mineral seems (for its size): High, medium, or low as determined by hefting. • Fluorescence – the ability to glow in the dark after exposure to ultraviolet light. • Irridescence – the ability to produce a play of colors. HCl Test CaCO3 + 2HCl CaCl2 + CO2 + H2O Wash the specimen and dry it. Wash your hands too! Identifying Feldspars Alkali Series- Wavelets Identifying Feldspars Plagioclase Series- Striations Non-Silicate Minerals Some noteworthy non-silicate minerals are: • Native elements- Composed entirely of one element (copper, gold, diamond). • Carbonates- Contain CO32- (calcite, CaCO3). • Sulfates- Contain SO42- in their structures (gypsum, CaSO4· 2H2O) • Sulfides- Contain S2- (but no O) in their structures (galena, PbS; pyrite FeS2) • Oxides- Contain O2- bonded to metal atoms (hematite, Fe2O3; magnetite, Fe3O4) Chapter 4.2 - The Silicate Minerals • Five minerals (out of 3000) account for 90% of the minerals in Earth's crust. • These are: – Quartz – Feldspar – Pyroxene – Amphibole – Mica All contain Silicon and Oxygen! The Silicate Minerals • The silicate minerals all contain Si and O covalently bonded together to form a silicon-oxygen tetrahedron: Silicate Mineral Structures • The basic structure for silicate minerals is a silicon-oxygen tetrahedron. • Contains 1 silicon at the center, and 4 oxygen atoms at the corners. • Each tetrahedron is negatively charged (SiO44-). The extra negative charge (-4) means that positive metal ions like Mg, Ca, Fe, Al etc. are needed for charge balance (i.e., MgSiO3). The metal ions go in between the silicate tetrahedra. Different arrangements of the silicate tetrahedra 5 major, different silicate structures. Isolated Tetrahedra- No oxygen atoms are shared between tetrahedra. Examples: Olivine, Garnet, Topaz Single Chain silicates- 2 oxygen atoms on each tetrahedron are shared. Example: Pyroxene Pyroxenes Enstatite: MgSiO3 Ferrosilite: FeSiO3 Double-chain silicate- Alternating 2 and 3 oxygen atoms on each tetrahedron are shared. Example: Amphibole (hornblende) Amphiboles Hornblende: Ca2(Mg,Fe,Al)5(Al,Si)8O22(OH)2 Sheet silicates- 3 oxygen atoms on each tetrahedron are shared. Examples: Biotite, Muscovite, Talc, Clays Micas Biotite: K(Fe,Mg)3AlSi3O10(F, OH)2 Micas Muscovite: KAl2(AlSi3O10)(F,OH)2 Talc (Mg,Fe)3Si4O10(OH)2 Framework silicates- All 4 oxygen atoms on each tetrahedron are shared. Examples: Quartz, Feldspar Quartz (SiO2) (Fixed composition) Feldspar Group Minerals K-Feldspar: KAlSi3O8 Albite: NaAlSi3O8 Anorthite: CaAl2Si2O8 (Somewhat variable composition) Gemstones What properties of minerals make certain specimens suitable for jewelry (gemstones)? Gemstones • • • • Attractive color/transparency. High hardness. Good cleavage. Rarity. Gemstones Have “Special” Names Mineral name: Quartz Gemstone name: Amethyst Gemstones Have “Special” Names Mineral name: Olivine Gemstone name: Peridot Gemstones Have “Special” Names Mineral name: Corundum Gemstone name: Ruby or Sapphire