Carbonates and Sulfates Carbonates CO 3 2 groups
Carbonates and Sulfates
Carbonates • (CO 3) -2 groups bond in a trigonal arrangement • Fairly strong bonds, but weaker than CO 2 bonds • No sharing of oxygens • Breaks down into H 2 O and CO 2 in presence of H+1 cations • Dominant non-silicate by volume, esp. calcite (Ca. CO 3)
Remember Linus Pauling’s Rules 2. Electrostatic Valency Principle Anisodesmic – a structure in which anions are more strongly bound to one molecular cation than any other cations Occurs when bonding to molecular groups Ex: (CO 3)2 - C 4+ [CN = 3] Each O 2 - contributes 4/3 e. v. to each C 4+ Leaves 2/3 e. v. to bond with other elements, ex: Ca 2+
Remember Linus Pauling’s Rules 2. Electrostatic Valency Principle Anisodesmic – a structure in which anions are more strongly bound to one molecular cation than any other cations Occurs when bonding to molecular groups Only 2/3 e. v. to bond with other elements, ex: Ca 2+ Means the O 2 - is bonded more strongly to C than to Ca
Calcite Group • Hexagonal: 32/m – Space group R 3 c • Includes five major isostructural minerals: – – – Calcite: Ca. CO 3 Magnesite: Mg. CO 3 Siderite: Fe. CO 3 Rhodochrosite: Mn. CO 3 Smithsonite: Zn. CO 3 • X 2+ cations and CO 32 - groups in modified halite structure, made rhombic, not cubic
Calcite Group • Hexagonal: 32/m – Space group R 3 c • Includes five major isostructural minerals: – – – Calcite: Ca. CO 3 Magnesite: Mg. CO 3 Siderite: Fe. CO 3 Rhodochrosite: Mn. CO 3 Smithsonite: Zn. CO 3 • X 2+ cations and CO 32 - groups in modified halite structure, made rhombic, not cubic X 2+ cations in octahedral coordination with 6 oxygens
Calcite – Ca. CO 3 • Incredibly abundant as dominant limestone mineral • Highly variable habit – Prismatic, rhombohedral, scalenohedral, and more • Commonly twinned, producing lamellae along {0112}, or along {0001} • Definitive Mohs mineral 3 • Shows classic optical property of double-refraction • “Baked” to make lime for cement – Ca. CO 3 Ca. O + CO 2
Magnesite – Mg. CO 3 • Rhombohedral crystals, but more commonly cryptocrystalline or in compact masses • Typically white to tan in color • Similar appearance to chert, but softer • Derived from alteration of Mg-rich metamorphic and igneous rocks by fluids containing carbonic acid • Most famous sample locations in Greece, Austria, and Brazil • Used to make bricks for lining furnaces – Also source for “magnesia” used in industrial chemicals
Siderite – Fe. CO 3 • Typically brown color, but with a white streak • Rhombohedral, but often with curved faces or in globular concretions • Often in a mixture with clay minerals, or as concentrically-layered concretions • Formed by replacement of Ca by Fe from fluids flowing in limestone • Extensively mined in the UK as “clay ironstone, ” but not elsewhere • Often found in iron-sulfide deposits and with iron ores and silver-bearing minerals
Rhodochrosite – Mn. CO 3 • Pink color is diagnostic • Rarely in distinct crystal form, usually massive, granular, or compact • In solid solution with Siderite, and partially with Magnesite and Smithsonite • Similar to rhodonite (Mn. Si. O 3), but softer (4 vs 6) • Found in hydrothermal veins with metallic (esp. Mn) ores
Smithsonite – Zn. CO 3 • Waxy green to brown, bulbous appearance common – Blue-green caused by copper – Yellow variety: “Turkey fat ore” • Rarely in distinct crystal form, usually reniform or botryoidal – Sometimes stalactitic • In partial solid solution with Rhodochrosite, Siderite • Sometimes used as a zinc ore • Often ornamental uses
Aragonite Group • Orthorhombic: 2/m 2/m • Typically includes a large cation bonded with CO 3 groups • Includes four major isostructural minerals: – Aragonite: Ca. CO 3 – Witherite: Ba. CO 3 – Strontianite: Sr. CO 3 – Cerussite: Pb. CO 3 groups perpendicular to caxis in offset planes, pointing in opposite directions X 2+ cations in unusual ninefold coordination with surrounding oxygens, nearly hexagonal closest packing
Aragonite Group • Orthorhombic: 2/m 2/m • Typically includes a large cation bonded with CO 3 groups • Includes four major isostructural minerals: – Aragonite: Ca. CO 3 – Witherite: Ba. CO 3 – Strontianite: Sr. CO 3 – Cerussite: Pb. CO 3 groups perpendicular to c-axis in offset planes, pointing in opposite directions X 2+ cations in unusual nine-fold coordination with surrounding oxygens, nearly hexagonal closest packing
Aragonite – Ca. CO 3 • Orthorhombic • Commonly in acicular pyramids, tabular habit, or in pseudohexagonal twins • Polymorph of calcite – Less stable under atmospheric conditions • Major mineral in most seashells – Also at hot springs, and as fibrous crusts on serpentines
Phase Diagram of Ca. CO 3
Dolomite Group • Hexagonal: 32/m – Space group R 3 c • Includes two major isostructural minerals: – Dolomite: Ca. Mg(CO 3)2 – Ankerite: Ca. Fe(CO 3)2 • X 2+ cations and CO 32 groups in modified halite structure, made rhombic, not cubic Differing X 2+ cations in alternating layers in octahedral coordination with 6 oxygens
Dolomite Group • Hexagonal: 32/m – Space group R 3 c • Includes two major isostructural minerals: – Dolomite: Ca. Mg(CO 3)2 – Ankerite: Ca. Fe(CO 3)2 • X 2+ cations and CO 32 groups in modified halite structure, made rhombic, not cubic Differing X 2+ cations in alternating layers in octahedral coordination with 6 oxygens
Dolomite – Ca. Mg(CO 3)2 • Often in curved crystals, typically coarsely granular • Colors range from pink to grey to white • Effervescent in heated HCl • Found worldwide as a replacement product in limestones – Also a hydrothermal vein mineral with Pb and Zn ores in limestone
Solid Solution of Dolomite with Mg and Ca carbonates
Malachite – Cu 2 CO 3(OH)2 Azurite – Cu 3(CO 3)2(OH)2 Most-commonly found in association with each other
Malachite – Cu 2 CO 3(OH)2 • Monoclinic: 2/m • Hydroxyls bonded in structure • Cu 2+ is in octahedra coordinated to both O 2 - and (OH)1– Cu. O 2(OH)4 & Cu. O 4(OH)2 • Octahedra link edges to form chains parallel to c-axis
Azurite – Cu 3(CO 3)2(OH)2 • Monoclinic: 2/m • Hydroxyls bonded in structure • Cu 2+ is in square, coplanar groups with 2 O 2 - and 2(OH)1 - – Each (OH) is shared by 3 Cu 2+ – Each O (from trigonal CO 3 group) bonds to one Cu 2+ • Square planar groups link to form chains parallel to b-axis
Sulfates • Dominant anion (SO 4) -2 in a tetrahedral arrangement • Covalent bonds with oxygens – Not shared between groups • Can have covalent bonds with water or hydroxyls (OH) -1 • Fairly common • Distinct hydrous and anhydrous species, often low hardness • Key groups: Barite, Anhydrite, Gypsum
Remember Linus Pauling’s Rules 2. Electrostatic Valency Principle Anisodesmic – a structure in which anions are more strongly bound to one molecular cation than any other cations Each O 2 - contributes 1 ½ e. v. to S 6+ Occurs when bonding to molecular groups Only ½ e. v. from each O 2 - to bond with other elements Ex: (SO 4)2 - S 6+ [CN = 4]
Barite Group • Orthorhombic: 2/m 2/m – Space group Pbnm – lots of glide planes • Includes three isostructural largecation minerals: – Barite: Ba. SO 4 – Celestite: Sr. SO 4 – Anglesite: Pb. SO 4 • Large X 2+ cations in highly-distorted twelve-fold coordination with oxygens from seven SO 4 groups
Barite Group • Orthorhombic: 2/m 2/m – Space group Pbnm – lots of glide planes • Includes three isostructural largecation minerals: – Barite: Ba. SO 4 – Celestite: Sr. SO 4 – Anglesite: Pb. SO 4 • Large X 2+ cations in highly-distorted twelve-fold coordination with oxygens from seven SO 4 groups
Barite Group • 12 -fold, but not a regular, isometric dodecahedron • Link to one O from layer above • Link to two O from each of four “coplanar” tetrahedra View perpendicular to a-axis, parallel to b and c • Link to one O from layer below
Barite Group • Four possible orientations of 12 -fold polyhedron View perpendicular to a-axis, parallel to b and c • Depends on layer and orientations of surrounding tetrahedra
Barite Group • Ba-centered polyhedron has a pentagonal base, but a hexagonal top • Base and cap connected by nine triangles and one single quatrilateral
Barite – Ba. SO 4 • In solid solution with Celestite and Anglesite, but rarely intermediate • Common gangue mineral in hydrothermal veins, and found in limestones • Commonly distinguished by its high specific gravity • Common use in drilling muds and as a source of barium • Barite roses formed out of multiply twinned and intergrown crystals, mined in
Celestite – Sr. SO 4 • In solid solution with Barite and Anglesite, but rarely intermediate • Usually colorless or pale blue • Usually found in limestone or sandstone outcrops in vugs or other cavities – Good localities in SE Michigan and NE Ohio at Clay Center • Also associated with gypsum, halite, sulfur, and fluorite • Used in making Sr-nitrate for fireworks and tracer bullets, and in refining beet sugar
Anglesite – Pb. SO 4 • In solid solution with Barite and Celestite, but rarely intermediate • Typically prismatic or tabular, with twins or striations – Also found in concentric layers around cores of galena (Pb. S) • Notably high specific gravity (6. 2 -6. 4) for a non-metallic • Common mineral in MVT lead deposits, formed by the oxidation of galena, or by its dissolution and
Anhydrite Group • Orthorhombic: 2/m 2/m – Space group Amma – lots of mirror planes • X 2+ cations in eight-fold coordination with oxygens from six surrounding SO 4 groups – Not cubic coordination – Distorted quasi-pentagonal dipyramid polyhedra
Anhydrite Group • Orthorhombic: 2/m 2/m – Space group Amma – lots of mirror planes • X 2+ cations in eightfold coordination with oxygens from six surrounding SO 4 groups Polyhedra around Ca 2+ are almost like pentagonal dipyramids: snub disphenoid Two possible rotations of polyhedron
Anhydrite – Ca. SO 4 • Distinct crystals rare – Typically massive • Pearly luster common • Usually bluish-grey • Distinguishible orthogonal cleavage • Changes to gypsum after low-T hydration • Associated with gypsum, but not as common a mineral
Gypsum Group • Monoclinic: 2/m • Layers parallel to [010] – SO 4 layers bonded to Ca 2+ – H 2 O bonded to Ca 2+ between layers • Ca 2+ in 8 -fold coordination – 6 oxygens, 2 H 2 O groups • Very weak bonds between H 2 O layers – Results in very strong cleavage along [010]
Gypsum – Ca. SO 4 • 2 H 2 O • Diamond-shaped, tabular crystals, twinned and beveled on edges – Alabaster, Satin Spar, and Celestite varieties rarer, more valuable • Distinct crystals fairly common, and range in size from microscopic to enormous • Dehydrates in stages – Loses 1 ½ H 2 O up to 65° C – Loses last ½ H 2 O ~95° C • Definitive Mohs mineral 2 • Common evaporite mineral
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