Chapter 7 basic mineralogy

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Chapter Basic Mineralogy Mineral: a naturally occurring, inorganic substance with a characteristic internal structure and a chemical composition that is either fixed or varies within certain limits http://www.oum.ox.ac.uk/thezone/minerals/i ndex.htm Ta ble 7-1 Mineral cl asses Class Chemical characteristics Exa mples Borates Vario us ele ments in co mbination with boron Bora x [Na2 B O 10H O] Carb onates Metals in co mbination with carb onate 2 ( CO ) Calcite [CaCO ] Cerrusite [Pb CO ] Ha lides Alkali metals or alkaline earths in co mbination with halogens (F, Cl, Br, I) Halite [NaCl] Fluorite [CaF 2] Hydro xides Metals in co mbination with hyd ro xyls (OH -) Brucite [Mg(OH) 2] Native elements Pure co mpound of a meta llic or non metallic ele ment Gold [Au] Graphite [C] O xid es Metals in co mbination with o xygen Hematite [Fe 3O ] Phosphates, arsenates, vanadates, chro mates, tungstates & molybdates Vario us ele ments in co mbination with the ZO radical where Z = P, As, V, Cr, W , Mo Apatite [Ca5 (PO )3 (F,Cl,OH)] Carnotite [K 2(UO (VO 4) 3H O] Scheelite [CaWO ] Silicates Metals in co mbination with silic a tetrahedra 4 ( SiO4 ) for ming thre e d imensional networks, sheets, chains and isolated tetrahedra Quartz [SiO ] Forsterite [MgSiO 4] Orthoclase [KAlSi 3O ] Sulfates Alkaline earths or metals in c o mbinatio n with Barite [BaSO ] 2 sulfate ( SO ) Epso mite [MgSO 7 H 2O] Sulfides One or more meta ls in co mbination with Pyrite [FeS2 ] reduced sulfur or che mically similar elements Galena [PbS] (As, Se, Te ) Skutterudite [CoAs3 ] Ionization potential: a measure of the energy required to remove an electron from an atom and place it at an infinite distance from the nucleus Electronegativity: a measure of the ability of an atom to attract electrons (The smaller the electronegativity, the less likely the atom will attract electrons—it will most likely donate them instead.) A Measure of electronegativity of elements as seen in the periodic table Ta ble 7-2 Electroneg ati vities Ion Electronegativity Z Ion Electronegativity H+ 2.2 33 As5 + 2.18 + Li 0.9 34 Se Be2+ 1.5 35 Br - B 3+ C 4+ N 5+ 3.0 O2 - 3.4 - Z 11 F 2.0 2.5 3.9 + Na 2+ 12 Mg 13 Al 3+ 14 4+ 15 16 Si P 5+ S 2- 0.9 3.1 0.8 1.0 21 Sc 3+ 1.3 22 Ti4 + 1.5 23 3+ 24 1.6 Cr 2+ 25 Mn 26 Fe 2+ 27 28 29 Co Ni 2+ 2+ Cu + 1.6 42 Nb Mo 6+ 2+ 46 Rh Pd 2+ Ag + 48 Cd 2+ 49 In + 47 Sn 2+ 51 Sb 5+ 52 Te 2- 50 53 55 - I Zn 1.6 Ga 3+ 1.8 32 4+ 2.0 Pr 3+ Nd 3+ 70 1.25 3+ - Tm Yb 71 Lu 3+ 1.0 72 Hf 4+ 1.3 73 5+ 1.5 6+ 1.7 7+ 1.9 74 Ta W 75 Re 2.2 76 Os + 2.2 77 6+ 2.2 2.28 2.20 1.93 78 79 Ir 4+ 2.2 Au + 2.4 2+ 1.9 Pt 1.69 80 Hg 1.78 81 Tl3 + 1.8 82 2+ 1.8 3+ 1.9 1.96 Pb 2.05 83 Bi 2.1 84 Po 4+ 2.0 85 5+ 2.2 + 0.7 1.10 Ce 69 1.24 3+ 2.10 0.89 58 60 2.16 2+ 3+ 59 1.6 Cs La 3+ 31 0.95 2.66 Ba 1.9 0.82 + 57 30 Ge 41 56 2+ 1.23 Er3 + 1.33 1.8 1.9 Ho 68 1.22 1.5 1.8 67 2.96 Y 45 Cl 2.55 Zr 4+ 2+ K+ 1.22 3+ 40 Ru + 1.9 Dy 3+ 39 5+ Electronegativity Ion 65 3+ Tc 19 3+ Sr 44 2.5 V 2+ 43 17 Ca Rb 1.6 - 20 38 + 1.3 2.1 2+ 37 2- Z 0.79 1.12 1.13 1.14 87 At Fr 88 Ra 2+ 0.9 89 Ac3+ 1.1 90 4+ 1.3 4+ 1.5 91 92 Th Pa U 6+ 62 Sm 3+ 3+ 1.17 93 Np 64 Gd + 1.20 94 Pu 4+ 1.7 1.3 1.3 Ta ble 7-3 Percent ionic character of a single chemical bon d Difference in electronegativity Ionic character, % Difference in electronegativity Io nic character, % 0.1 0.5 1.7 51 0.2 1.8 55 0.3 1.9 59 0.4 2.0 63 0.5 2.1 67 0.6 2.2 70 0.7 12 2.3 74 0.8 15 2.4 76 0.9 19 2.5 79 1.0 22 2.6 82 1.1 26 2.7 84 1.2 30 2.8 86 1.3 34 2.9 88 1.4 39 3.0 89 1.5 43 3.1 91 1.6 47 3.2 92 http://skywalker.cochise.edu/wellerr/mineral/fluorite/fluoriteL.htm Example 7-1 The mineral fluorite has the chemical composition CaF2 Calculate the ionic character of the bond between Ca-F From Table 7-2, the difference in electronegativity = 3.98 (F-) -1.00(Ca2+) = 2.98 From table 7-3, the bond is ~89% ionic http://web.arc.losrios.edu/~borougt/MineralogyDiagrams.htm Coordination number: the number of anions that surround a cation in an ionic crystal Radius ratio: the radius of the cation divided by the radius of the anion So, we seem to think that silica (SiO44-) has a coordination number of Let’s test this From appendix III, the ionic radius of Si4+ = 0.48 & O2- = 1.32 Then Rc/Ra = 0.48/1.32 = 0.36 If we were to check the corresponding radius ratios from figure 7-2, we would see that it fits nicely in the tetrahedral arrangement with a coordination number of Of course, we already knew that one! Forsterite: Mg2SiO4 http://www.minerals.net/Image/5/97/Olivine.asp The Unit cell is the basic building block for a crystal In order to understand this concept, think of the unit cell as being like a brick in a wall (if the wall is built by stacking bricks directly upon one another) Surface area effects Table 7-8 Surface area per unit mass of illite with a density of 2600 kg m-3-3 Nu mber of cubes Surface area of cube ( m2) Su rface area (m2 g -1) 2.3 x 10 -6 x 10 -2 (cm) x 10 6 x 10 -4 2.3 x 10 -4 x 10 -6 (1 µ m) x 10 x 10 -12 2.31 x 10 -7 (0.1 µ m) x 10 x 10 -14 3.1 x 10 -8 (0.01 µ m) x 10 x 10 -16 231 Length of side ( m) clay particles have a large surface to mass ratio The number of negative surface sites per area (NSA) is related to the surface area (SA),and the mole sites per unit mass (NSM) NSA = NSM / (1.66 E-6) (SA) And the NSM is used to calc CEC CEC = NSM (1.0 E5) units NSA – sites nm-2 NSM – mole sites g-1 SA – m2g-1 CEC – meq 100g-1 Example 7-6: Given a smectite with a negative surface charge of 0.8 mole sites kg -1, what is the CEC? CEC= (0.8 mole sites kg-1 ) (.001 kg g-1) (1 E5) = 80meq 100g-1 Determining ion-exchange properties Batch method Start with solution of known cation conc., ionic strength, and pH Add clay Compare cation conc before and after addition of clay Repeat at different pHs, ionic strengths, cation concentrations Use data to construct adsorption isotherms Adsorption Isotherms Represent partitioning of a particular species between an aqueous phase and solid particles (sorbate) Kd is the tangent to the isotherm found at the origin At high concentrations, precipitation keeps the aqueous concentration constant Figure 7-8 Representation of a typical adsorption isotherm showing the distribution of a species between an aqueous phase and a solid (sorbent) At very low concentrations, the distribution behaves ideally and can be represented by a unique value, Kd At higher concentrations, the partitioning deviates from ideality If precipitation occurs, the concentration of the species in solution will remain constant; i.e., the solution is saturated with respect to the particular species Column Test Method In this case, the sorbent is packed into a column and a volume of solution is passed through the column The concentration of the ion of interest in the original solution is compared to that in the eluent and the Kd is calculated using the following formula Kd = ((Ci – Cf)/Cf))(V/M) http://www.cresp.org/cresp-projects/waste-processing-special-nuclearmaterials/leaching-assessment-for-alternative-waste-forms/ Example 7-7 Ten grams of montmorillonite are placed in a column and 100ml of solution are passed through the column The initial solution has a zinc concentration of 20 mg L-1 and the eluent has a zinc concentration of 14.1 mg L-1 Calculate the Kd for zinc between the solution and the montmorillonite Kd = ((20 – 14.1)/14.1))(100 cm3/10g) = 4.18 cm3 g-1 Zeolites: a crystalline structure characterized by a framework of linked tetrahedra, each consisting of four O atoms surrounded by a cation This framework open cavities in the form of channels and cages These channels are usually occupied by H2O, but large enough to allow the passage of guest species Zeolites have relatively large CEC and are useful for a variety of environmental remediation processes Asbestos minerals: a group of silicate minerals that occur as long, thin fibers They have high tensile strength, flexibility, and heat and chemical resistance Asbestos minerals can be described by two different structures: chrysotile and amphibole Chrysotile structure: consists of a layer of silica tetrahedra bonded to a layer of octahedrally coordinated Mg ions Each Mg2+ is surrounded by four hydroxyl molecules and two oxygens The distance between the oxygens in the octahedral layer is slightly greater than the distance between the oxygens in the tetrahedral layer This results in the octahedral layer curling around the tetrahedral layer forming a scrolled tube Amphibole structure: consists of a strip of octahedrally coordinated cations sandwiched between two double silica chains The chains extend for an infinite distance The cations can be Na, Li, Ca, Mn, Fe, Mg, Al, and Ti Health Effects of Asbestos Exposure Asbestosis: a lung disease caused by asbestos particles deposited in the lungs through inhalation Over time, the lung encapsulates these fibers and hardens leading to a decrease in the efficiency of the O2/CO2 exchange Mesothelioma: a rare, diffuse malignant cancer of the lining of the lung and stomach It has a long latency period of 35 to 40 years Lung cancer: usually linked to smoking, however, some cases have been attributed to radon, second-hand smoke, or exposure to asbestos Crystalline and Amorphous Silica There are six polymorphs (same chemical composition, but different crystalline structure) of silica composition with a chemical formula of SiO2 Amorphous silica (opal, SiO2·nH2O) is found in siliceous oozes in the seafloor sediments and on land as preserved deposits of marine sediments or precipitated from geyer fluids that contain high amounts of dissolved silica Dissolution of Silica Minerals SiO2(s) + 2H2O → H4SiO4(aq) For quartz: logKsp = 1.8814 – 2.028 x 10-3T – 1560.46 / T For amorphous silica: logKsp = 0.338037 – 7.8896 x 10-4T – 840.075 / T Where T is the temperature in Kelvin When figuring the solubility in ppm, remember to multiply the Ksp (in units of moles/L) by the gram-molecular weight of silica (SiO2) regardless if you are calculating quartz or amorphous silica Example 7-8 Calculate the Ksp for quartz and amorphous silica at a pH of 5.5 and T = 25⁰C Which form of silica is more soluble? For Quartz: Log Ksp = 1.8814 – (2.028 x 10-3)(298.15) – 1560.46/298.15 = -3.96 Ksp = 10-3.96 For Amorphous silica: Log Ksp = 0.338037 – (7.8896 x 10-4)(298.15) – (840.075/298.15) = -2.71 Ksp = 10-2.71 Amorphous silica has the larger Ksp and is the more soluble form of solid silica Chapter Problem set due November 26: #s: 1, 9, 10, 14, 36, 49, 55, 57 http://agwired.com/2010/11/12/the-cost-of-thanksgivingdinner/ [...]... For 1:1 clays surface charge arises mostly from broken bonds at crystal edges Table 7- 7 Per manent negative surface charge of 2:1 clay minerals11 M ineral group 1 2 Charge ( mol sites kg -1) 2 Kaolinite 0.02 - 0.2 Illites 0.1 - 0.9 S mectites 0 .7 - 1 .7 Vermiculites 1.6 - 2.5 Data fro m Sposito (1989), Lang muir (19 97) Charge in moles of monovalent sites per kg of clay What is Cation Exchange Capacity... Basal spacing 7. 1  10  Va ria ble most ~ 15  Va ria ble 14.4  whe n fully hydrated Ethylene glycol Only taken up by halloys ite No effect T wo glyc ol laye rs , 17  One glycol layer, 14  Ca tion e xchange ca pacity (CE C) in meq/100 g c lay Nil 3 - 15 Low 10 - 40 High 80 - 150 High 100 - 150 Formula Al 2Si 2 O 5(OH) 2, little variation K 0 5-0 .7 5Al 2(Si,A l)2 O 10 (OH) 2 M +0 7( Y3+ , Y2+ )... uptake or release of ammonium (NH4+) ions when the clay is exposed to a 1 M ammonium acetate solution at pH 7. 0 Units for CEC = meq / 100g CEC(meq/100g) = NSM(mole sites/g)x105 If surface has a net positive charge then the AEC (anion exchange capacity) is measured Surface area effects Table 7- 8 Surface area per unit mass of illite with a density of 2600 kg m-3-3 Nu mber of cubes Surface area of cube... calculated using the following formula Kd = ((Ci – Cf)/Cf))(V/M) http://www.cresp.org/cresp-projects/waste-processing-special-nuclearmaterials/leaching-assessment-for-alternative-waste-forms/ Example 7- 7 Ten grams of montmorillonite are placed in a column and 100ml of solution are passed through the column The initial solution has a zinc concentration of 20 mg L-1 and the eluent has a zinc concentration... 2 microns in size May or may not be clay mineral General clay types Kaolinite, illites, smectites, vermiculite Kaolinite – 1 tetrahedral and 1 octachedral layer (1:1) -Limited subsitution of Al in the basic formula (Al 2Si2O5(OH)4) -net surface charge minimal, negligible CEC Illite – 2 tetrahedral and 1 octachedral layer (2:1) ….the octahedral sandwich -Al substitution for Si in tetrahedral layer -marginal... generally have a greater C.E.C montmorillonite The octahedral and tetrahedral layers are arranged in different ways with different amounts of elemental substitutions to produce different clay minerals Table 7- 5 Su mmary of the principal characteristics of the layered clay mineral grou ps* Ka olinites Illites S mectites Ve rmiculites Structure T etrahed ra l: Octa hedral 1:1 2:1 2:1 2:1 Octa hedral layer Di-octahe... of 2600 kg m-3-3 Nu mber of cubes Surface area of cube ( m2) Su rface area (m2 g -1) 1 6 2.3 x 10 -6 1 x 10 -2 (cm) 1 x 10 6 6 x 10 -4 2.3 x 10 -4 1 x 10 -6 (1 µ m) 1 x 10 1 8 6 x 10 -12 2.31 1 x 10 -7 (0.1 µ m) 1 x 10 2 1 6 x 10 -14 2 3.1 1 x 10 -8 (0.01 µ m) 1 x 10 2 4 6 x 10 -16 231 Length of side ( m) 1 clay particles have a large surface to mass ratio The number of negative surface sites per area... (SA),and the mole sites per unit mass (NSM) NSA = NSM / (1.66 E-6) (SA) And the NSM is used to calc CEC CEC = NSM (1.0 E5) units NSA – sites nm-2 NSM – mole sites g-1 SA – m2g-1 CEC – meq 100g-1 Example 7- 6: Given a smectite with a negative surface charge of 0.8 mole sites kg -1, what is the CEC? CEC= (0.8 mole sites kg-1 ) (.001 kg g-1) (1 E5) = 80meq 100g-1 Determining ion-exchange properties Batch... species between an aqueous phase and solid particles (sorbate) Kd is the tangent to the isotherm found at the origin At high concentrations, precipitation keeps the aqueous concentration constant Figure 7- 8 Representation of a typical adsorption isotherm showing the distribution of a species between an aqueous phase and a solid (sorbent) At very low concentrations, the distribution behaves ideally and... character from that of the major ion Clay Minerals and Surface Ion Exchange Clay mineral – fine-grained hydrous silicate composed of layers of tetrahedrally and octahedrally coordinated cations Figure 7- 5 Structure of the octahedral and tetrahedral layer Mg2+ in the octahedral layer = brucite Al3+ in the octahedral layer = gibbsite Al3+ can substitute for Si4+ in the tetrahedral layer Clays – any particle
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