Silikatlar Chemical Properties of Minerals CHEMICAL PROPERTIES OF MINERALS CHEMICAL PROPERTIES OF MINERALS mineral properties Atom its chemistry and its crystalline structure SO chem. comp. smallest subdivision of an element (from H to Lr) imp idealized str of an atomIonic Radii and Ionic Potential The effective atomic radii, R, of an atom is defined as ½ the distance between two covalently bonded nuclei of the element R increases down the periodic table and decreases across a row Atomic radius --- used to calculate the charge-to-radius ratio (Z/R) or ionic potential (IP) IP decreases ---- down the periodic table and increases across a rowAtomic Number number of p Atomic Mass (p + n) Atomic Weight (p + n) of an element a whole number & element abundance compared to O=16 Isotope same atomic number but different atomic mass and different n 92 U 238 92 U 235 Ion charged atoms Cl - anion Na + cation Valence number of electron lost or gained (e.g. O 2- Fe 3+ ) Molecule combined atoms with different bonds Molecular Weight total atomic weights of its components Radical strong bonding between two or more atoms (e.g. (NH 4 ) + (OH) - (SiO 4 ) 4- (CO 3 ) 2- ) Acid compound of H + with an anion like Cl - (HCl) a negative radical (CO 3 ) 2- (H 2 CO 3 ) Base compound of cation like Na + with a negative radical (OH) - NaOHSalt by a reaction of an acid and a base HCl + NaOH ›NaCl+H 2 O Water of Crystallization some minerals contain free H 2 O molecule or (OH) radical within its X-structure e.g. Gypsum CaSO 4 . 2H 2 O, Colemanite CaB 3 O 4 (OH) 3 H 2 O shown as + H 2 O in chemical analyses water absorbed as moisture on minerals shown as – H 2 OREPRESENTATION OF MINERAL FORMULA REPRESENTATION OF MINERAL FORMULA mineral formula expressed as elemental cation-anion pairs OR simple compounds such as oxides Calcite CaCO 3 CaO.CO 2 Dolomite CaMg(CO 3 ) 2 CaO.MgO.2CO 2 Kyanite Al 2 SiO 5 Al 2 O 3 .SiO 2 Orthoclase KAlSi 3 O 8 K 2 O.Al 2 O 3 .6SiO 2 Oligoclase (MgFe) 2 SiO 4 MgO.FeO.SiO 2 Apatite Ca 5 (PO 4 ) 3 (F,Cl,OH) 9CaO. Ca(F,Cl,OH) 2 .3P 2 O 5 Magnetite Fe 3 O 4 FeO.Fe 2 O 3 or Fe 2+ O. Fe 3+ 2 O 3 CALCULATION OF CHEMICAL FORMULA OF MINERALS CALCULATION OF CHEMICAL FORMULA OF MINERALS quantitative chemical analysis basic information for atomic formula v wt% (metals or oxides) a listing of what elements present add up close 100±1% oxygen bearing minerals wt% of oxidesSteps in calculation Steps in calculation A. From Elemental Analysis 1. Compare max. wt% for possible anion (!) and name major mineral group as Sulphide, Sulphate, Silicate, Carbonate, Phosphate etc. 2. Consider and group the cations according to their valency as Na + , K + , Mg 2+ , Fe 2+ , Zn 2+ Al 3+ , Fe 3+ Si 4+ , Ti 4+ etc. 3. Divide wt% values by at. wt. given and multiply by 1000 4. Omit trace and minor elements or oxides 5. Add atomic proportions of possible substitutions, i.e. Zn ?Fe 6. Determine atomic proportions as approximate whole numbers 7. Write chemical formula cations first anion last (Zn,Fe)S 8. If required recalculate atomic percentage of cations in the formula 9. Write chemical formula cations percentages first anion last (Zn 87 Fe 13 )S replace each other in chemical formula Elemental Analysis calculation calculation from from Oxide Analysis Mineral Mixtures recalculate oxide analyses to cations per given number of oxygensWt% Atomic Weights Atomic Proportions X 1000 Atomic Ratios Fe 18.25 55.85 327 Mn 2.66 54.94 48 Zn 44.67 65.38 683 1060 ?1 Cd 0.28 112.40 2 In 0.01 114.82 0.1 (omitted) S 33.57 32.07 1047 1047 ?1 ? 99.44 sulphide general formula (Zn,Fe,Mn,Cd)S SPHALERITE If atomic percentages of cations --- required Fe 327 Mn 48 Zn 683 Cd 2 total=1060 factor=100/1060XAP Fe 30.8 Mn 4.5 Zn 64.4 Cd 0.2 the formula (Zn 64.4 Fe 30.8 Mn 4.5 Cd 0.2 )S Example 1B. From Oxide Analysis steps 1 & 2 are same in step 3. divide wt% values by mol. wt.’s given and multiply by 1000 to avoid fractions to obtain Molecular Proportions steps 4 to 9 same again but remember mol. prop. of oxides Oxides of Lithophile Cations •SiO 2 TiO 2 (4+) •Al 2 O 3 Cr 2 O 3 Fe 2 O 3 (3+) • MgO MnO FeO (2+) •CaO (2+) •Na 2 O K 2 O H 2 O (1+)Example 2a (Weight percents to formula) Wt% Molecular Weights Molecular Prop X 1000 Molecular Ratios SiO 2 59.85 60.09 983 ?1 MgO 40.15 40.31 996 ?1 ? 100.00 silicate formula MgO.SiO 2 MgSiO 3 PYROXENE Example 2b (Formula to weight percents) Kyanite Al 2 SiO 5 the oxides –SiO 2 –Al 2 O 3 Molecular Ratios Molecular Weights grams oxide wt% SiO 2 1 60.09 60.09 37.08 Al 2 O 3 1 101.96 101.96 62.92 Formula weight 162.05 Example 2b Wt% Molecular Weights Molecular Prop X 1000 Molecular Ratios SiO 2 34.96 60.09 582 582 ?1 TiO 2 0.02 79.90 0.3 (omitted) Al 2 O 3 0.12 101.96 1 (omitted) Fe 2 O 3 0.03 159.70 0.2 (omitted) FeO 36.77 71.85 512 MnO 0.52 70.94 7 1190 ?2 MgO 27.04 40.31 671 ? 99.46 silicate formula 2(Mg,Fe,Mn)O.SiO 2 (Mg,Fe,Mn) 2 SiO 4 OLIVINE If atomic percentages of cations --- required FeO 512 MnO 7 MgO 671 FeO 43.0 MnO 0.6 MgO 56.4 Total=1190 Factor=100/1190XMP (Mg 56.4 Fe 43.0 Mn 0.6 ) 2 SiO 4 or Fo 57 Fa 43C. For Analysis of Mineral Mixtures same steps 1 to 9 mixture of mins make assumptions about # of mins pure end members of solid solutions series if same elements present in different min. phases calculations very difficult / impossible Example 3 Wt% Atomic Weights Atomic Prop. X 1000 Atomic Ratios Pb 4.43 207.20 21 Zn 11.56 65.38 177 Cu 26.54 63.55 418 ?1 Fe 23.84 55.85 427 ?1 In 0.01 114.82 0.1 (omitted) S 33.57 32.07 1065 1065 1044 867 ?2 ? 100.54 Formulas of minerals PbS Galena, ZnS Sphalerite and CuFeS 2 Chalcopyrite If percentage of minerals are required PbS 21X2 42 2.0% ZnS 177X2 354 16.8% CuFeS 2 rest 1712 81.2% 2108 100.0mins not pure substances extensive variation in chem. comp. due to substitution during solid solution specific atomic sites occupied in variable proportions by different atoms controlled by size of atoms or ions (<15% in size difference) valency of substituting ions (electric charge must be balanced) coupled substitution Si 4+ ?Al 3+ +Na + at HT due to thermal vibrations atomic sites larger SO greater possibility for substitution 3 types of substitution 1. Substitutional Solid Solution 2. Interstitial Solid Solution 3. Omission Solid Solution SOLID SOLUTION or ISOMORPHISM SOLID SOLUTION or ISOMORPHISM1. Substitutional Solid Solution - simple cationic or anionic substitutions e.g. Sylvite (KCl) --- Br - anion substitution --- K(Cl,Br) Complete Binary Solid Solution Series e.g. Olivine (Mg,Fe) 2 SiO 4 -Mg 2+ ?Fe 2+ substitution --- complete - end members --- Forsterite (Fo) --- Mg 2 SiO 4 --- Fayalite (Fa) --- Fe 2 SiO 4 - complete miscibility --- between end members - early formed crystals --- Fo-rich -as T v --- reaction with silicate melt --- richer in Fa by taking Fe 2+ ions into X-str - if cooling ---rapid zoninge.g. feldspars --- plagioclase series --- alkali feldspar series Ca 2+ +Al 3+ ? Na + +Si 4+ - plag series --- complete SSS --- coupled substitution --- end members - albite --- NaAlSi 3 O 8 - anorthite --- CaAl 2 Si 2 O Na + ?K + - alkali feldspar series --- at HT --- complete solid solution --- at LT --- only limited solid solution miscibility gap --- end members - orthoclase --- KAlSi 3 O 8 - albite --- NaAlSi 3 O 82. Interstitial Solid Solution - holes or voids in X-str --- filled by large ions or mol.’s to form interstitial substitution e.g. Beryl --- Be 3 Al 2 Si 6 O 18 H 2 O, CO 2 mol.’s / large K + , Rb + , Cs + ions enters into holes btw silicate rings --- entrance of neural H 2 O, CO 2 mol.’s --- no requirement for charge balance --- entrance of large K + , Rb + , Cs + ions --- charge balance required --- coupled substitutions Si 4+ ?Be 2+ +2R + or Si 4+ ?Al 3+ +R +3. Omission Solid Solution - two or more cations --- replaced by highly charged cation SO --- a vacancy (omission) --- in one of the atomic sites e.g1. amazonite --- KAlSi 3 O 8 (polymorph of K-feldspar) Pb --- substitute K as K + +K + ?Pb 2+ + e.g2. pyrrhotite --- Fe (1-x) S (x=0-0.2) (mineral with defect structure) Fe 2+ +Fe 2+ +Fe 2+ ?Fe 3+ +Fe 3+ +common solid solution series - Isometric (Cubic) system: Spinel group, Garnet group - Tetragonal system: Rutile group, Scheelite group - Hexagonal system: Apatite group, Corundum group, Carbonate group - Orthorhombic system: Aragonite group, Barite group - Monoclinic system: Pyroxene group, Amphibole group - Triclinic system: Feldspar group some physical properties of mineral groups in SSS --- more or less linear variation from one end member to another - melting point, boiling point, crystallization temperature - specific gravity - hardness - optical properties RI, etc.T v EXSOLUTION or UNMIXING EXSOLUTION or UNMIXING - opposite of solid solution - due to subs. of atoms or ions with different size - at HT --- enlarged atomic sites - at lower T --- atomic sites --- smaller - larger ions --- chemical disorder - SO --- at HT --- initially homogeneous SS --- with vsing T --- two distinct minerals with different chem. comps. --- bulk comp. --- same - exsolution lamellae --- crystallographically oriented e.g. alkali feldspars albite lamellae --- exsolved in orthoclase --- PERTHITE orthoclase lamellae --- exsolved in albite --- ANTIPERTHITEBlebby cpx Blebby cpx exsolved exsolved from opx from opx host, host, Skaergaard Skaergaard Intrusion whispy perthite lamellae as albite is exsolved from orthoclase Intrusion Opx with 2 lamellae of Opx with 2 lamellae of exsolved exsolved cpx, cpx, Bushveld Bushveld Intrusion Opx with lamellae of Opx with lamellae of exsolved exsolved plagioclase, plagioclase, Nain Nain anorthosite Intrusion anorthositePSEUDOMORPHISM PSEUDOMORPHISM - X of a min. --- altered --- internal structure / chem. comp. --- changed BUT ---external form --- preserved (pseudomorph) - types - exchange of one component e.g. limonite (FeO.OH.nH 2 O) after pyrite (FeS 2 ) X-form --- that of pyrite --- BUT --- different comp. - loss of a constituent e.g. cuprite (Cu 2 O) ? Native Cu - gain of a constituent e.g. anhydrite (CaSO 4 ) ? gypsum (CaSO 4 .2H 2 O) - partial exchange of constituents e.g. limonite (FeO.OH.nH 2 O) ? Goethite ( ?FeO.OH) - complete exchange of constituents e.g. fluorite (CaF 2 ) ? Qua (SiO 2 ) silicified wood (organic compounds --- replaced by silica)