ReferencesNov. 26, 2003Mineralogy 231Lecture 34: surficial & weathering materials: clay minerals; other sheet silicatesWednesday: crustal minerals: alkali feldsparsreadings: N 208-228; K 476-484, 549-554; DHZ, 396-430definitions of clay mineralsphysical: < 2 m grain sizechemical: based on absorptive, expansive propertiesclay may have only a minor amount of clay minerals in itexample: Michigan till, which is found in many places in LP of MichiganEJE analyzed a till which produced a fulgurite what is a fulgurite?analyses had 85-90% SiO2 why? key is in understanding of the A soil horizonidentification of clay minerals clay mineral fraction concentrated by suspending in water, settling for a fixed amount of timeusually identified with powder XRD measurements, glycolation, heat treating Table 13.3usually only one peak, (001), seen for smectites due to structural imperfectionsmore recently, clays characterized by TEM (Don Peacor’s research) K, Fig. 12.54fine grain size, chemical heterogeneities, submicroscopic interlayering of more than one clayclay minerals are submicroscopic in crystallite size, may appear as a fine aggregate in thin sectionproperties of clay mineralsmay expand or contract depending on humidity, and can be expanded readily with organic solventsdi-ethylene glycol (also used for protecting radiators in cars from freezing): a dangerous poisonexpansion of smectite from 14 Å basal spacing to 18 Å d-spacing for (001) peakexpandable clays often form bentonite layers of altered volcanic glass interbedded in sedimentsfamous story of digging a pond in bentonite only to have it expand into a bulgemany clays have large and variable amounts of H2O between the layersmany clays easily exchange cations such as Na, K, Ca into and out of layersmost clay minerals also have marked absorptive properties what is absorption?thixotropic properties of clays a major hazard in urban areas prone to earthquakeswhat are thixotropic properties? (thixis, to touch, Greek)occurrences of clay mineralsproduced by alteration of feldspars, micas, many other minerals, also altered volcanic glassclays also produced by alteration of other clay mineralssedimentary transport of clay minerals leads to sedimentary deposits enriched in claysclays found in small quantities in many rocks; in large quantities in shales, mudstonescompositions of clay mineralscan be separated into aluminous, ferrian, magnesian types with Ca, Na even Mg in interlayersH3O+ (hydronium) or NH4+ (ammonia) may substitute for Na, K, Ca in some clay mineralsdifficult to purify for meaningful chemical analysis: grain size too small for SEM and EMPstructures of clay mineralsTOT silicates often interlayered at or near unit-cell scales Fig. 13.14TOT layers with interlayer Ca, Na: smectite; TOT layers with interlayer K: illitecommon clay mineralskaolinite often included as a clay mineral due to fine grain size, but is not expandableillite: 10 Å basal spacing, “hydrated muscovite”, also does not expandchemically intermediate between pyrophyllite and muscovite with extra H2O, some Mg, FeAl2Si4O10(OH)2 – KAl2Si3AlO10(OH)2, substitution is [ ].Si = K.Alsmectites: strongly expandible clays: 13 – 18 Å basal spacing depending on thermal historyTOT + single layer of H2O: 14 Å, with two layers of H2O: 15 Åmontmorillonite: Al silicate with Ca and/or Na, OH, H2O, some Mg, Fe: dioctahedralnontronite: ferric dioctahedral smectitesaponite: Mg trioctahedral smectitevermiculite: clay equivalents of chlorite, not very expandableNesse includes chlorite as a clay mineral but not by most workersclay mineral reactions: weathering processesvolcanic glass + H2O -> clay mineralsK-feldspar + H+ + H2O -> smectite + K+; using simplified formula for smectite:2KAlSi3O8 + H2O + 2H+ -> Al2Si4O10(OH)2.H2O + 2SiO2 + 2K+a similar reaction may be written for illitesuch reactions are irreversible, most clay minerals probably metastable (Essene & Peacor, 1997)other sheet silicatesstilpnomelane Fig. on p. 257formula is in some question: K0.6(Mg,Fe2+,Fe3+,Mn)6Si8Al(O,OH)27.2-4H2O most commonly citedstilpnomelane has variable Mg/Fe2+/Mn, Fe2+/Fe3+, and may have variable O/OH/H2Ooccurs in low grade metapelites and metabasalts: in chlorite zoneresembles biotite optically but no bird's-eye extinction, cinnamon brown, can be olive greenoften more fibrous or contorted, found in lower grade rocks than biotitehas second cleavage perpendicular to basal cleavagestructure of stilpnomelane Fig. 13.16prehnite Fig. on p. 259formula: Ca2Al2Si3O10(OH) 2, often with some Fe3+ substituting for Alsecond order colors, perfect basal cleavage, also common in low grade metamorphic rockseasily confused with lawsonite which has two perfect cleavagesprehnite characteristic of prehnite-pumpellyite facies (200-300°C, below the greenschist facies)ReferencesEssene, E.J. & Peacor, D.R. (1997) Illite and smectite - metastable, stable, or unstable? Clays & ClayMinerals 45, 116-122.Moore, D.M. & Reynolds, R.C. Jr. (1987) X-ray diffraction and the Identification and Analysis of ClayMinerals, 2nd ed., Oxford Univ. Press, Oxford.Newman, A.C.D., ed. (1987) Chemistry of clays and clay minerals. Mineral. Soc. Monog. 6, 480 p.Velde, B., ed. (1995) Origin and Mineralogy of Clays. Springer,