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The Chemistry of Color Why do minerals have different colors? Definition of a Mineral Minerals are naturally-occurring, inorganic substances with a definite and predictable chemical composition and crystalline structure. Mineral Properties Predictable physical properties Controlled by chemical composition and/or structure Mineral Properties Color Streak: Color of the mineral when powdered Luster: The character of light that is reflected Cleavage/Fracture: The way the mineral breaks Habit: The form in which the mineral grows Hardness: Resistance to being scratched Specific Gravity Miscellaneous Properties: e.g., smell, magnetism Generalized Atomic Structure Nucleus houses the massive particles (protons and neutrons) # Protons = Atomic Number # Protons + Neutrons = Atomic Mass Responsible for mass and density Electrons lie in orbitals that surround the nucleus # Electrons = # Protons If not, then it is an ION Responsible for bonding Ions Ions are electrically charged particles formed by the gain or loss of electrons Cations are positively charged Anions are negatively charged Metals are elements that readily form cations Non-Metals are elements that readily form anions Electron Orbitals Alkali Metals Alkali Earth Metals Transition Metals Metaloids Other Metals Non-Metals Halogens Noble Gases Decreasing Atomic Radius Decreasing Atomic Radius Electronegativity: measure of an atom’s ability to attract electrons in a chemical bond Ionic Bonds An atom with lower electro negativity transfers one or more electrons to an atom with higher electronegativity Metal with non-metal Results in charged ions Oppositely charged ions are attracted and thereby form a weak bond Ionic Bonds Ions are not actually “attached” and so bonds are relatively weak What property? Ions can be attracted by other charged ions or molecules such as water What property? Ionic Bonds Ions are not actually “attached” and so bonds are relatively weak Soft minerals (easily scratched) Ions can be attracted by other charged ions or molecules such as water What property? Ionic Bonds Ions are not actually “attached” and so bonds are relatively weak Soft minerals Ions can be attracted by other charged ions or molecules such as water Soluble in water Ionic Bonds and Solubility Electronegativty of H: 2.1 Electronegativty of O: 3.5 Uneven sharing of electrons Due to shape of water molecule, one end (O) http://upload.wikimedia.org/wikipedia/commons/0/05/H2O_molecule_scheme_of_dipole tends to be negative whereas the other end (H) tends to be positive Ionic Bonds and Solubility OH+ H+ H+ Na+ Cl- OH+ Cl- Na+ Na+ Cl- O- Ionic Bonds and Solubility H+ H+ H+ Cl- OH+ H+ H+ O- O- H+ H+ Na+ H+ OCl- H+ H+ O- H+ Common Ionic Minerals Halite (NaCl): Halide Table Salt, De-Icer Calcite (CaCO3): Carbonate Chalk, Plaster, Toothpaste, Antacids Gypsum (CaSO4•H2O): Sulfate Drywall, Cement, Plaster Covalent Bonds Two atoms with approximately equal electronegativity share one or more electrons Results in a merging of the electron clouds, and thereby forms a strong bond What property? Covalent Bonds Two atoms with approximately equal electronegativity share one or more electrons Results in a merging of the electron clouds, and thereby forms a strong bond Hard minerals Common Covalent Minerals Quartz (SiO2): Silicate Abrasive, Glass Feldspar (NaAlSi3O8 ): Silicate Ceramics, Household Cleanser Garnet (Fe3Al2Si3O12 ): Silicate Sandpaper Relative Bond Strengths Na Ion: +1 Bonds: 6 Bond Strength: 1/6 • Si Ion: +4 • Bonds: 4 • Bond Strength: 4/4=1 Covalent Bonds in Silicates Are Stronger than Ionic Bonds Metallic Bonds Delocalized sharing of free electrons among a lattice of metal atoms with little or no difference in electronegativity Results in one continuous electron cloud in which electrons are free to move Soft, malleable minerals that conduct both heat and electricity Metallic Bonds Delocalized sharing of free electrons among a lattice of metal atoms with little or no difference in electronegativity Results in one continuous electron cloud in which electrons are free to move What properties? Van der Waal’s “Bonds” Attraction between atoms, ions or molecules that have an imbalance in the distribution of electrons (dipole) Fleeting and fluctuating weak attractions Soft minerals with a low melting point Allows for the basal cleavage in micas Common Van der Waals Minerals Talc (Mg3Si4O10(OH)2 ): Silicate Baby Powder, Paint, Paper Graphite (C ): Element Pencil Lead, Dry Lubricant Chemical Bonding Why do minerals have different hardness? Definition of Light Light is electromagnetic radiation. Light exhibits properties of both a particle (e.g., ability to travel through a vacuum) and a wave (e.g, interference and polarization) For understanding behavior of light in minerals we can focus on the wave properties of light Aspects of Light Amplitude (height of wave) corresponds to the intensity (brightness) of light Wavelength corresponds to the energy of light In the visible spectrum wavelength corresponds to color Wavelength of Light Visible light is only a tiny, moderate energy fraction of the electromagnetic spectrum Longer wavelengths (e.g., radio waves) have less energy Shorter wavelengths have more energy Absorption Results in Color (A Familiar Example from Biology) Why is the Visible Spectrum Visible? What Color? What is the color of a mineral that absorbs green and yellow light? What spectral colors are absorbed by a yellow mineral? What Color? What is the color of a mineral that absorbs green and yellow light? Purple What spectral colors are absorbed by a yellow mineral? What Color? What is the color of a mineral that absorbs green and yellow light? Purple What spectral colors are absorbed by a yellow mineral? All but yellow Absorption and Emission of Light Color and Compositon Certain elements are strong pigmenting agents and produce strongly colored specimens when they are present, whether as a part of the crystal lattice or as an impurity. These elements are termed the chromophores. Color and Compositon Chromium (Cr): orange, green Cobalt (Co): pink, purple, blue Copper (Cu): blue, green Iron (Fe): red, green, yellow Manganese (Mn): pink Transition Metals What Color? Rhodonite (MnCO3)? Crocoite (PbCrO4)? Malachite (Cu2CO3[OH]2)? What Color? Rhodonite (MnCO3)? Crocoite (PbCrO4)? Malachite (Cu2CO3[OH]2)? What Color? Rhodonite (MnCO3)? Crocoite (PbCrO4)? Malachite (Cu2CO3[OH]2)? What Color? Rhodonite (MnCO3)? Crocoite (PbCrO4)? Malachite (Cu2CO3[OH]2)? Color vs Streak If the color of a mineral is due to a major constituent of the mineral (%), the color and streak will be similar If the color of a mineral is due to a trace constituent of the mineral (ppm), then the streak will be pale or white Generalized Atomic Structure Nucleus houses the massive particles (protons and neutrons) # Protons = Atomic Number # Protons + Neutrons = Atomic Mass Responsible for mass and density Electrons lie in orbitals that surround the nucleus # Electrons = # Protons If not, then it is an ION Responsible for bonding Ions Ions are electrically charged particles formed by the gain or loss of electrons Cations are positively charged Anions are negatively charged Metals are elements that readily form cations Non-Metals are elements that readily form anions Electron Orbitals Alkali Metals Alkali Earth Metals Transition Metals Metaloids Other Metals Non-Metals Halogens Noble Gases Decreasing Atomic Radius Decreasing Atomic Radius Electronegativity: measure of an atom’s ability to attract electrons in a chemical bond Ionic Bonds An atom with lower electro negativity transfers one or more electrons to an atom with higher electronegativity Metal with non-metal Results in charged ions Oppositely charged ions are attracted and thereby form a weak bond Ionic Bonds Ions are not actually “attached” and so bonds are relatively weak What property? Ions can be attracted by other charged ions or molecules such as water What property? Ionic Bonds Ions are not actually “attached” and so bonds are relatively weak Soft minerals (easily scratched) Ions can be attracted by other charged ions or molecules such as water What property? Ionic Bonds Ions are not actually “attached” and so bonds are relatively weak Soft minerals Ions can be attracted by other charged ions or molecules such as water Soluble in water Covalent Bonds Two atoms with approximately equal electronegativity share one or more electrons Results in a merging of the electron clouds, and thereby forms a strong bond What property? Covalent Bonds Two atoms with approximately equal electronegativity share one or more electrons Results in a merging of the electron clouds, and thereby forms a strong bond Hard minerals Metallic Bonds Delocalized sharing of free electrons among a lattice of metal atoms with little or no difference in electronegativity Results in one continuous electron cloud in which electrons are free to move Soft, malleable minerals that conduct both heat and electricity Metallic Bonds Delocalized sharing of free electrons among a lattice of metal atoms with little or no difference in electronegativity Results in one continuous electron cloud in which electrons are free to move What properties? Natural Bonds Usually Share Characteristics Van der Waal’s “Bonds” Attraction between atoms, ions or molecules that have an imbalance in the distribution of electrons (dipole) Fleeting and fluctuating weak attractions Soft minerals with a low melting point Allows for the basal cleavage in micas