KINETICS AND MECHANISM OF HYDROTHERMAL CRYSTALLIZATION OF 2M 1 MUSCOVITE: AN EXPERIMENTAL STUDY

Abstract---A hydrothermally altered rhyolitic hyaloclastite from Ponza island, Italy, has four alteration zones with unique clay assemblages: (1) a non-pervasive argillic zone characterized by smectite; (2) a propylitic zone with interstratified illite-smectite (I-S) containing 10-85% illite (I); (3) a silicic zone composed of I-S with -->90% I and pure illite; and (4) a sericitic zone with I-S ranging from 66% I to pure illite. Atomic force microscopy reveals abrupt changes in particle morphology with illitization, including initial changes from anhedral plates to laths and then to eubedral plates and hexagonal plates. I-S particles progressively thicken with illitization and mean particle area (basal plane) remains constant from pure smectite to I-S with 80% I. However, particle area increases from 90 to 100% illite. Computer modeling of I-S structural forms indicates octahedral cation ordering progressively changes from cisvacant smectite to interstratified cis-and trans-vacant l-S, and then to trans-vacant illite. In addition, polytypes progressively change from 1M d to 1M, and then to 2MI illite. Electron-microprobe and X-ray fluoresence analyses show that I-S chemistry progressively changes during illitization, evolving toward a phengitic composition with --0.89 fixed interlayer K + per O~0(OH)2. Octahedral Mg 2+ shows little change with illitization, varying from 0.3 to 0.5 cations per O~0(OH)2. The layer charge of smectite is --0.38 equivalents per OI0(OH) 2.On the basis of abrupt changes in morphology and progressive changes in polytype and chemistry, smectite illitization on Ponza involved a dissolution and recrystallization mechanism with multiple stages of nucleation and crystal growth. In this multi-step model, temperature of alteration provided the major control for the layer composition, polytype, and morphology of I-S crystallites. Other factors that may play a secondary role include: K + availability, water-rock ratio, and permeability. Alternatively, the mechanism of I-S and illite formation at Ponza and other hydrothermal environments may occur by direct precipitation of I-S crystallites from rhyolite glass and may not involve progressive reactions of smectite precursors.

Section: Structural Changes and Temperatures Of Alterationsupporting

confidence: 67%

Section: Mechanism Of Smectite Iuitization On Ponzamentioning

confidence: 82%

Reaction Mechanisms of Smectite Illitization Associated with Hydrothermal Alteration from Ponza Island, Italy

Ylagan

2000

“…Poorly oriented slides were prepared by using acetone and scanned air-dried at 0.25~ from 20 to 37 ~ 20. Quantification of mica polytypes was difficult because published methods using the intensity ratio 2.79 ~J2.58 (Maxwell and Hower, 1967;Mukhamet-Galeyev et al, 1985;Wilcoxon et al, 1990) produced 2M1 values in excess of 100%. More realistic results were obtained by calibrating against a pure 2MI (pegmatitic muscovite) mica as an external standard, assuming a linear calibration line (cf.…”

Section: Methodsmentioning

confidence: 99%

“…More realistic results were obtained by calibrating against a pure 2MI (pegmatitic muscovite) mica as an external standard, assuming a linear calibration line (cf. Mukhamet-Galeyev et aL, 1985).…”

Section: Methodsmentioning

confidence: 99%

Clay Mineralogy and Illite Crystallinity of the Atoka Formation, Arkoma Basin, and Frontal Ouachita Mountains

Spötl

1993

Abstract--Clay mineralogy (including illite crystallinity) was studied in Pennsylvanian synorogenic sediments (Atoka Formation) in the subsurface of the Arkoma Basin and the adjacent Ouachita thrust belt. Vitrinite reflectance values range from >-0.8% at the surface up to as high as 4.7% Ro at the base of the Atoka Formation. The mineralogy of the <2 #m fraction of the mudrocks is fairly monotonous and composed of illite (< 10% interstratified smectite), Fe-chlorite, kaolinite, quartz, and traces of feldspars. Kaolinite is common at shallow levels and "'disappears" in most wells at a thermal maturity of 1.9-2.1% Ro, suggesting its possible use as an independent paleothermal indicator in this basin. Illite crystallinity (IC) values are fairly high (0.3-0.5 ~ 20) and show little variation throughout the entire maturity range. In addition, no relation was observed between vitrinite reflectance and illite crystallinity, indicating that IC is not a useful paleothermal indicator in these rocks. Illite is almost exclusively of the 2Ml polytype, suggesting a predominantly detrital origin. Incipient metamorphic and low-grade metamorphic mudrocks in the Ouachita thrust belt to the east of the Arkoma Basin are regarded as the source rocks for the clays of the Atoka Formation. Rapid transportation and deposition by turbidity currents probably played a key role in protecting these unweathered micas from pervasive alteration in the terrestrial environment.

“…The existence of a sequence of transformations between polytypes ofaluminous dioctahedral micas from 1Ma through 1M to 2M~, occurring as a function of increasing grade ofdiagenesis-metamorphism, has been accepted for many years (e.g., Yoder and Eugster, 1955;Velde, 1965;Kisch, 1983;Mukhamet-Galeyev et al, 1985). It has been suggested (Velde, 1965) that 1 M a n d 1Ma polytypes are metastable and that 2M~ is the stable polytype of dioctahedral white micas; however, in sedimentary rocks 1Md is the predominant form.…”

Section: Introductionmentioning

confidence: 99%

Transmission Electron Microscope Observations of Illite Polytypism

Grubb

1991

Abstract--Transmission electron microscopy (TEM), including selected area electron diffraction (SAED), has been used to identify polytypes in iUite, phengite and muscovite from samples representing a wide range of diagenesis and low-temperature metamorphism. Samples include Gulf Coast sediments, sediments from the Salton Sea region, California, the Martinsburg Formation at Lehigh Gap, Pennsylvania, the Kalkberg Formation at Catskill, New York, Otago Schists from southern New Zealand, pelites from the Gasp6 Peninsula in Quebec, Canada, shales and slates from Wales, sediments from the Barbados accretionary complex, and synthetic hydrothermal illite.Samples from rocks of lowest grades, including those representing a range of sedimentary diagenesis, invariably give SAED patterns with few, complex non-001 reflections which are diffuse and ill-defined and that represent largely disordered stacking sequences. Corresponding XRD patterns are consistent with 1M~ polytypism. The term 1Md is therefore retained for this material. Higher grade samples, including those in which slaty cleavage is developed, and detrital grains in low-grade sediments invariably give diffraction patterns of well-ordered 2-or 3-layer polytypes. Of all samples and localities studied, only one diffraction pattern, from a sample in the Gasp6 sequence, was found to be predominantly 1M. In none of the other sequences included in this study were any 1M or predominantly 1M electron diffraction patterns obtained for illite grains.Where illite is in its original state of formation, it is consistently IMd, whether it originates as a result of direct crystallization from solution or as a replacement of smectite. Where illite has apparently undergone subsequent change, presumably through dissolution and crystallization representing an Ostwaldstep-rule-like change, it occurs as a well-ordered 2-layer (inferred to be 2M,) or, less commonly, a 3T polytype. On the basis of this limited survey, the state of polytypism appears to directly identify iUite as either being in, or changed from, its initial state of formation. Key Words--Illite, Polytypism, Transmission electron microscopy. I N T R O D U C T I O NThe existence of a sequence of transformations between polytypes ofaluminous dioctahedral micas from 1Ma through 1M to 2M~, occurring as a function of increasing grade ofdiagenesis-metamorphism, has been accepted for many years (e.g., Yoder and Eugster, 1955;Velde, 1965;Kisch, 1983;Mukhamet-Galeyev et al., 1985). It has been suggested (Velde, 1965) that 1 M a n d 1Ma polytypes are metastable and that 2M~ is the stable polytype of dioctahedral white micas; however, in sedimentary rocks 1Md is the predominant form. Kisch (1983) suggested that the 1M to 2M~ transition is complete by the onset ofgreenschist-facies metamorphism.Subsequent to the description of the six simple mica polytypes (1M, 2M~, 2M2, 20, 3T, and 6//) by Smith and Yoder (1956), much research has been devoted to the structural and chemical factors controlling polytypism. The effect of compositional...