Transcript Slide 1
Kaolinite Al2Si2O5(OH)4 By Dominic Papineau Named after the locality Kao-Ling, China The chemical and structural properties of kaolinite Chemical formula of kaolinite: Al2Si2O5(OH)4 Kaolinite is part of the kaolin subgroup of phyllosilicates, which also include the polymorphs dickite, nacrite, and halloysite. The structure of kaolinite consists of a Si2O5 sheet bonded on one side to a dioctahedral gibbsite layer. The gibbsite layer consists of aluminum octahedrons coordinated by hydroxyl groups. c a b The physical properties of kaolinite Color: usually white, brownish white, grayish white, yellowish white. Luster: dull Cleavage: perfect in one direction (not seen in massive specimens) Hardness: 1.5 to 2 Density: 2.1 to 2.6 g/cm3 Crystal habit: Clay-like texture with no visible crystalline affinities. Crystal habit can be seen with high magnification microscopy such as atomic force microscopy. The optical properties of kaolinite Montmorillonite is biaxial negative 2VZ = 24 - 50° = 1.553 – 1.563 = 1.559 – 1.569 = 1.560 – 1.570 = 0.007 (usually first order interference colors) Weakly pleochroic The crystallographic properties of kaolinite Crystal system: triclinic Point Group: 1 Unit cell parameters: a = 5.14 b = 8.93 c = 7.37 = 91.8° = 104.5° = 90.0° Z=2 V = 327.35 Calculated density = 2.62 Space group: P1 The crystal structure of kaolinite c a b Silicon tetrahedra Aluminum octahedron Oxygen atom In brief: T – O sheets The formation of kaolinite The chemical weathering of orthoclase or microcline from igneous or metamorphic rocks occurs, kaolinite clay is formed. Kaolinite can also form from the chemical weathering of aluminosilicate minerals from igneous or metamorphic rocks. Reactions involving volcanic glass, hydrothermal processes, and transformations of other clay minerals can also lead to the formation of kaolinite. The formation of kaolinite The transformation of potassium feldspar to kaolinite 2 KAlSi3O8 + 9 H2O + 2 H+ Al2Si2O5(OH)4 + 2 K+ + 4 H4SiO4 The equation is electrically balanced The K+ and H4SiO4 are removed by flowing water An abundance of water is required for the reaction to proceed Acidic conditions are required The reaction rate generally doubles by a 10oC temperature increase Hence the formation of kaolinite is favored by tropical climatic conditions The stability of kaolinite versus some other silicates Microcline-kaolinite Go = +7.103kcal Kaolinite-gibbsite Al2Si2O5(OH)4 + 5H2O 2Al(OH)3 + 2H4SiO4 Go = +12.755kcal Muscovite-kaolinite 2KAl3Si3O10(OH)2 + 12H2O + 2H+ 3Al2Si2O5(OH)4 + 2 K+ Go = -11.059kcal The stability of different minerals in equilibrium with K+, H+, and H4SiO4 in aqueous solution at 25oC and 1atm pressure The stability of kaolinite versus montmorillonite Kaolinite-K-montmorillonite 7Al2Si2O5(OH)4 + 8SiO2 + 2K+ 6[(Al2)(Si3.67Al0.33)O10 (OH)2]K0.33 + 7H2O + 2H+ Go = +35.511kcal K- to Na-montmorillonite 3[(Al2)(Si3.67Al0.33)O10 (OH)2]K0.33 + Na+ 3[(Al2)(Si3.67Al0.33)O10 (OH)2]Na0.33 + K+ Go = +0.413kcal The stability of kaolinite, K-montmorillonite, and Na-montmorillonite in the presence of amorphous silica Occurrences of kaolinite Locations on Earth: Occurs in clay beds everywhere on the planet. It is one of the major constituents of soils. Presence on Mars: Most likely, but has not been reported yet. Use Kaolinite is widely used in ceramics