Morphological changes occurring in naturally weathered micas in seven residual soils from Ontario were studied. Splitting, exfoliation of the (100) planes, surface coats, filling of spaces with soil material as well as cracking and crumbling of (001) surfaces were the main morphological changes found to be taking place during natural weathering of mica particles. The significance of the observed feature in relation to the general theory of mica weathering is discussed.

Organic acids are commonly found in soils and sediments, playing an important role in the alteration and weathering of minerals and influencing a series of geochemical processes such as soil fertility, metal cycling and pollutant migration. In order to better comprehend the reaction mechanisms of different layered silicate minerals with organic acids, three minerals with various structure types, namely montmorillonite, kaolinite and muscovite, were investigated in this work. In particular, the effects of interfacial reactions with oxalic acid on the crystal structure, chemical composition, morphology and specific surface area of minerals were compared. The composition and structure of montmorillonite, kaolinite and muscovite during the interfacial reaction with oxalic acid were characterised using powder X-ray diffraction (XRD), scanning electron microscopy (SEM) and inductively coupled plasma-optical emission spectroscopy (ICP-OES) methods. It was shown that Si4+ and Al3+ were dissolved gradually during the interfacial reactions and that the changes in the properties of minerals depended on structural characteristics. After 300 days of the interfacial reactions with oxalic acid, the dissolution percentages of Si4+ and Al3+ in montmorillonite, kaolinite and muscovite were 12.7%, 8.4%, 3.8% and 62.1%, 30.7%, 6.1%, respectively. Moreover, the lamellar morphology of montmorillonite was destroyed upon the interfacial reaction with oxalic acid, and irregular particles with sizes of ~100–500 nm were formed on the surface. The diameter of kaolinite flake particles decreased from 400–1500 nm to 50–400 nm, and the surface of rod-shaped particles was ruptured. The small particles disappeared from the muscovite surface, and the initially sharp edges became blunted. The specific surface area and the total pore volume of montmorillonite and kaolinite increased after the interfacial reaction with oxalic acid, whereas the opposite results were obtained for muscovite. The differential dissolution of the minerals during their interfacial reaction with oxalic acid was mainly related to the differences between cation occupancies, structural types, chemical bond strengths and specific surface areas.

Muscovite has been used increasingly as a substrate in flexible electronics and fillers in high-performance nanocomposites. Muscovite-based interfacial interactions play a crucial rule in material fabrication. Hansen solubility parameters (HSPs) have proven useful in characterizing molecular interactions within/between condensed phases. The present study aimed to determine the HSPs of raw muscovite (RM) and to investigate solvent dispersion mechanisms of RM. To achieve this, the solubilities of RM in 17 solvents were evaluated by dispersion tests, and the HSPs of RM were calculated as the center of the optimal solubility rotated-ellipsoid in HSP space, which included all good solvents, had the smallest number of outliers, and had the smallest volume. The resulting dispersion, polar, and hydrogen bonding components of RM were 18.301, 2.366, and 3.727 MPa1/2, respectively. By considering the HSPs and Kamlet-Taft's solvatochromic parameters of solvents, we concluded that the low polarity of RM is due to hindered K+/H+ exchange on the RM surface, resulting from limited water/moisture contact. For solvent dispersion of RM, essential conditions include strong dispersion forces and weak polar forces, finely tuned to match the surface property of RM at a certain hydration level. The HSPs of RM determined from dispersion tests were restricted to predicting/characterizing RM-based interfacial phenomena in an environment with strictly controlled water/moisture content. The HSP calculation method proposed herein was applicable to any clay mineral.

The crystal structures of a reddish-purple, Mn-bearing muscovite-2M1 (alurgite variety) and a reddish-brown, Mn-bearing phlogopite-1M (manganophyllite variety) were refined to final residuals of 2.7% and 3.1%, respectively. The refinements were carried out in space groups C2/c and C1 for alurgite and C2/m and C2 for manganophyllite. The C1 and C2 subgroup refinements gave atomic coordinates consistent with the parent space group refinements. No cation ordering was found in either specimen, and the structures are very similar to those of muscovite and phlogopite. Residual areas of positive electron densities were found between the tetrahedral cations and neighboring oxygens in difference maps of both minerals. Those of alurgite were examined in detail to show the correlation between the residual densities and covalent bonding in the tetrahedra. The valence of the Fe present was determined by Mössbauer spectra as Fe3+ in both samples and of the Mn by optical spectra as Mn3+ in the alurgite but as Mn2+ in the manganophyllite.

Long-range ordering of tetrahedral cations in micas is favored by phengitic compositions, by the 3T stacking sequence of layers, and by tetrahedral Si:Al ratios near 1:1. Phengites of the 1M, 2M1, and 2M2 polytypes are said to show partial ordering of tetrahedral cations, although the amounts of tetrahedral substitutions are small and the accuracies of determination are not as large as desired. The 3T structures of muscovite, paragonite, lepidolite, and protolithionite show tetrahedral ordering, as do the 2M1 brittle micas margarite and an intermediate between margarite and bityite. Muscovite-3T and margarite-2M1 are also slightly phengitic relative to their ideal compositions. Examples of octahedral cation ordering in micas are more abundant and are to be expected when cations of different size and charge are present. Octahedron M(1) with its OH,F groups in the trans orientation tends to be larger than the mean of the two cis octahedra as a result of the ordering of cations and vacancies. In some samples ordering has reduced the true symmetry to a subgroup of that of the ideal space group. If ordering in subgroup symmetry results in ordered patterns of different geometries but similar energies in very small domains, the average over all unit cells may simulate long-range disorder.

Transmission electron microscopy has been used to characterize coexisting pyrophyllite and muscovite in low-grade metamorphosed pelites from Witwatersrand and northeastern Pennsylvania. The Witwatersrand sample consisted largely of porphyroblasts of interlayered muscovite and pyrophyllite in a fine-grained matrix of the same phases. In both textures, muscovite and pyrophyllite occurred as interlayered packets (with apparently coherent interfaces) from about 300 Å to a few micrometers in thickness, with no mixed layering. Their compositions were determined with a scanning transmission electron microscope to be

$$(\Box{_{{\rm{0}}{\rm{.11}}}}{\rm{ }}{{\rm{K}}_{{\rm{1}}{\rm{.72}}}}{\rm{Na}_{{\rm{0}}{\rm{.17}}})(A}{{\rm{l}}_{{\rm{3}}{\rm{.91}}}}{\rm{ F}}{{\rm{e}}_{{\rm{0}}{\rm{.03}}}}{\rm{M}}{{\rm{g}}_{{\rm{0}}{\rm{.05}}}}{\rm{T}}{{\rm{i}}_{{\rm{0}}{\rm{.01}}}}{\rm{)(S}}{{\rm{i}}_{{\rm{6}}{\rm{.11}}}}{\rm{ Al}}{{\rm{l}}_{{\rm{1}}{\rm{.89}}}}{\rm{)}}{{\rm{O}}_{{\rm{20}}}}{{\rm{(OH)}}_{\rm{4}}}$$

$$(\Box_{{\rm{1}}{\rm{.90}}}{\rm{N}}{{\rm{a}}_{{\rm{0.06}}}}{{\rm{K}}_{{\rm{0}}{\rm{.04}}}}{\rm{)(A}}{{\rm{l}}_{{\rm{3}}{\rm{.94}}}}{\rm{F}}{{\rm{e}}_{{\rm{0}}{\rm{.01}}}}{\rm{M}}{{\rm{g}}_{{\rm{0}}{\rm{.05}}}}{\rm{)(S}}{{\rm{i}}_{{\rm{7}}{\rm{.94}}}}{\rm{A}}{{\rm{l}}_{{\rm{0}}{\rm{.06}}}}{\rm{)}}{{\rm{O}}_{{\rm{20}}}}{{\rm{(OH)}}_{\rm{4}}},$$

The pyrophyllite and muscovite in the Pennsylvania shale likewise occurred only as coexisting coherent to sub-parallel packets as thin as 200 Å, with compositions of

$$(\Box_{{\rm{1}}{\rm{.89}}}{\rm{N}}{{\rm{a}}_{{\rm{0.04}}}}{\rm{C}}{{\rm{a}}_{{\rm{0}}{\rm{.02}}}}{{\rm{K}}_{{\rm{0}}{\rm{.05}}}}{\rm{)(A}}{{\rm{l}}_{{\rm{3}}{\rm{.93}}}}{\rm{F}}{{\rm{e}}_{{\rm{0}}{\rm{.04}}}}{\rm{M}}{{\rm{g}}_{{\rm{0}}{\rm{.02}}}}{\rm{T}}{{\rm{i}}_{{\rm{0}}{\rm{.01}}}}{\rm{)(S}}{{\rm{i}}_{{\rm{7}}{\rm{.92}}}}{\rm{A}}{{\rm{l}}_{{\rm{0}}{\rm{.08}}}}{\rm{)}}{{\rm{O}}_{{\rm{20}}}}{{\rm{(OH)}}_{\rm{4}}}$$

$$({\rm{N}}{{\rm{a}}_{{\rm{0}}{\rm{.04}}}}{\rm{C}}{{\rm{a}}_{{\rm{0}}{\rm{.02}}}}{{\rm{K}}_{{\rm{2}}{\rm{.03}}}}{\rm{)(A}}{{\rm{l}}_{{\rm{3}}{\rm{.54}}}}{\rm{F}}{{\rm{e}}_{{\rm{0}}{\rm{.24}}}}{\rm{M}}{{\rm{g}}_{{\rm{0}}{\rm{.16}}}}{\rm{T}}{{\rm{i}}_{{\rm{0}}{\rm{.06}}}}{\rm{)(S}}{{\rm{i}}_{{\rm{6}}{\rm{.09}}}}{\rm{A}}{{\rm{l}}_{{\rm{1}}{\rm{.91}}}}{\rm{)}}{{\rm{O}}_{{\rm{20}}}}{{\rm{(OH)}}_{\rm{4}}}.$$

The compositions of these coexisting pyrophyllite and muscovite define a solvus with steep limbs and extremely limited solid solution. Illite is a white mica, intermediate in composition between pyrophyllite and muscovite, formed at much lower temperatures than muscovite. These relations show that illite is metastable relative to pyrophyllite + muscovite in all of its diagenetic and low-grade metamorphic occurrences. This further implies that illite precursor phases, such as smectite, are also metastable. The prograde reactions involving smectite, illite, and muscovite are therefore inferred to represent Ostwald-step-rule-like advances through a series of metastable phases toward the equilibrium states attained in the greenschist facies. “Illite crystallinity” can therefore be a measure of reaction progress, for which temperature is only one of several determining factors.

The orientation of the OH-vectors in hydroxyl groups of micas of different compositions, polytype modifications, and symmetry were calculated by the method of minimization of electrostatic energy. The orientations are strongly affected by the peculiarities of structures, and even slight deviations from the ideal dioctahedral periodicity can introduce a correction of as much as 10° to the value of the polar angle. Ordering of cations in octahedra lead to twisting of the OH-bond towards the octahedron with lower charge and larger dimension. The latter is less true for cation ordering in tetrahedra. Because of these factors and the dependence of the hydrogen position on the coordinates of heavy atoms, such calculations can give reliable results only if the structure has been refined with high precision. Different polytypes of micas displayed the same orientation of the OH-vectors with respect to the axes of a separate layer, if the stacking sequence did not introduce specific distortions of the 2:1 layer.

The sorption of U and Ra on finely ground biotite, muscovite, and phlogopite was adequately described by the Freundlich adsorption equation, (x/m) = KCn, at low U and Ra concentrations despite Ra precipitation at the higher temperature. Radium and U sorption-efficiency curves derived from the Freundlich constants generally showed decreased distribution coefficients in response to increasing temperature and increasing Ra or U concentrations. Temperatures investigated were 5°C, 25°C, and 65°C. Solution compositions used were 0.1 M NaCl and 0.01 M NaHCO, for U, and 0.01 M NaCl for Ra. Uranium initial solution concentrations ranged from 1.00 × 10−4 M to 4.00 × 10−7 M; the Ra initial solution concentration range was 6.80 × 10−7 M to 8.60 × 10−9 M.

In 0.01 M NaHCO3 solutions, anionic uranyl carbonate complexes were prevalent, and because they are weakly sorbed relative to free uranyl ion and uranyl hydroxy complexes, the result was a relatively low U sorption efficiency on biotite and phlogopite and excellent sorption efficiency on muscovite. Uranyl carbonate complexes decreased in solubility with increasing temperature, so that U sorption efficiency on biotite increased with increasing temperature. Sorption of uranyl ion and uranyl hydroxy cations on biotite decreased with incresaing temperature.

Crystal growth theory was applied to describe edge sites of phyllosilicates. Three face configurations were found to exist. One face has one tetrahedral site per tetrahedral sheet and two octahedral one-coordinated sites per crystallographic area ac sin β, where a and c are layer dimensions and β is the angle between them. The other two faces are similar except that they have one less octahedral site which is replaced by one SiIV-O-AlVI site in this same ac sin β area. A transfer of bonding energy from the remaining octahedral site to the SiIV-O-AlVI site is believed to neutralize all edge charge on faces containing these latter sites at normally encountered pHs (pH 3–9). A similar charge rearrangement along the edges results in an apparent decrease in the permanent charge of the mineral with an increase in edge area.

On the basis of such an analysis, lath-shaped illite can be described as a very fine grained dioctahedral mica in which the apparent deficient occupancy of the octahedral sheet, presence of excess water, and measurable cation-exchange capacity may in part be the result of a large ratio of edge area to total volume, with no other chemical or structural change in the mica layers. The increasing importance of edge charge relative to layer charge produces erroneous formulae for 2:1 phyllosilicates in very fine grained samples containing fewer than 2 of 3 octahedral sites occupied by cations, on the basis of a 22-charge half cell.

The characterization of cations associated with clay materials has generally been approached by the sequential use of specific chemical reagents. To avoid the disturbing effect of the chemical reagents on the state and location of compensating cations of clays and to get information in situ, far-infrared spectroscopy was used. The far-infrared vibrational spectra of the potassium cation in muscovite, phlogopite, and biotite were recorded before and after heating at the dehydroxylation temperature. The vibrational frequency of K in micas before dehydroxylation was found to be a function of the di- or trioctahedral character and of the Fe content. After dehydroxylation or deprotonation, shifts of the K absorption band to lower frequencies were observed for the heated muscovite, in which K exchangeability increased, and to higher frequencies for the heated biotite, in which K exchangeability decreased. These results suggest that the vibrational frequency of potassium is characteristic of the state of K in these minerals and of its ability to be exchanged.

Hydrothermally kaolinitized muscovite from the Otago schist of Brighton, New Zealand, has been studied by transmission and analytical electron microscopy (TEM and AEM) to determine the mechanism of alteration and to compare reactant-product relations for di- and trioctahedral micas. The muscovite is a primary metamorphic phase having a phengitic composition. It occurs as well-ordered, two- and three-layer polytypes, in grains as thick as 30 μm. Kaolinite occurs as packets of layers, each about 100–600 Å thick, which alternate with packets of muscovite or smectite-like layers. Most of the kaolinite is highly disordered in stacking sequence, although a one-layer polytype is also present, occurring as relatively thick sequences of layers. Phase boundaries between kaolinite and muscovite are invariably parallel to the 001 lattice fringes with no strain contrast; i.e., no transitions exist along layers. Parallelism of 00l and 11l reflection rows of both kaolinite and muscovite implies a topotaxial intergrowth. A smectitelike phase is also present, occurring as packets of wavy layers, which locally have periodic contrast that may reflect R1 ordering of illite/smectite. This material appears to be a direct, “along-layer” alteration product of muscovite. Electron diffraction data and lattice-fringe images imply that kaolinite alternates with micaceous phase(s) with some regularity; i.e., micaceous layers are separated by approximately equal numbers of kaolinite layers. Similar long-range periodicity occurs in contrast variations within packets of kaolinite layers.

The data collectively suggest that the alteration interface was self-perpetuating and that alteration proceeded rapidly along layers once it initiated in 2:1 layers at crystal edges or strained areas, with no observable component normal to the layers. They also suggest that smectite may have formed as an intermediate phase during the hydrothermal kaolinitization of muscovite. In the previous study of alteration of biotite in the same sample, “along-layer” transition boundaries were commonly observed, and a second, intermediate product phase was not detected, implying a relation between the alteration mechanisms and the chemical differences between reactants and products.

Diffusion of K during analytical electron microscopy (AEM) results in anomalously low count rates for this element. As the analysis area and specimen thickness decrease, count rates become disproportionally lower. Adularia and muscovite show different diffusion profiles during AEM; for muscovite a strong dependence of diffusion on crystallographic orientation has been observed. Conditions giving rise to reliable chemical data by AEM are the use of a wide scanning area (>800 × 800 Å) and/or large beam size to reduce the effect of diffusion of alkali elements, a specimen thickness greater than about 1000 Å, constant instrument operating conditions, and the use of a homogeneous, well-characterized standard sample. The optimum thickness range was obtained by determining the element intensity ratio vs. thickness curve for given operating conditions. The standard and unknown should have a similar crystal structure and, especially for strongly anisotropic minerals such as phyllosilicates, a similar crystallographic orientation with respect to the electron beam.

Kaolin floc structures of commercial utility are produced by adsorption of hexamethylenediamine. These flocs are largely face-to-face structures with small diameter platelets of colloidal kaolinite aggregated on larger platelets of kaolinite and muscovite. Because of its higher charge density, muscovite is more effective than kaolinite in scavenging colloidal particles.

Although earlier workers have been at odds as to where amines adsorb on kaolinite, the present work indicates that both edge and basal surfaces are involved. Evidence for this contention is two-fold: (1) Face-to-face and edge-to-face flocculation of hexamethylenediamine kaolins has been observed. (2) Polyesters have been observed adsorbed on basal and edge surfaces of ethylenediamine-treated kaolins.

The mechanism by which kaolinite particles are bonded together into floc structures is unclear. Bridging by hexamethylenediammonium ions is a probable mechanism up to satisfaction of the cation-exchange capacity; further amine addition continues to promote flocculation. From this observation it is inferred that, in addition to bridging, hydrogen bonding and, perhaps, van der Waals forces are instrumental in floc formation. Although rheologically inferior to the untreated equivalent, these chemically induced flocs of kaolinite provide loosely packed, randomly oriented structures in paper coatings. Improved brightness, opacity, and printability of paper coatings arise from this structure.

Analytical and high-resolution transmission electron microscopy of weathered plagioclase and K-feldspar provided microtextural and chemical data that suggest a sequential formation of weathering products. An alteration layer < 1 µm thick on feldspar surfaces had short-range order and was termed protocrystalline. Relative to the parent feldspars the protocrystalline layer was depleted in Ca, Na, K, and Si and significantly enriched in Fe. On plagioclase, the protocrystalline material was replaced by Ca-Fe-K-smectite, another protocrystalline material, and spherical halloysite. Abundant tubular halloysite on the corroded surface apparently formed by reprecipitation of components released by plagioclase dissolution. The K-feldspar was markedly more resistant to weathering than the plagioclase.

Recrystallization of the patchily developed protocrystalline rind produced Fe-bearing, aluminous smecrite, which was ultimately replaced by spherical halloysite and laths of kaolinite. Muscovite laths within plagioclase crystals were converted initially to illite by loss of K, then to randomly interstratified illite/smectite, and then to smectite that contained Mg, little K and Fe, and was more aluminous and contained less Ca than the smectite that originally replaced the plagioclase. Smectite was replaced epitactically by kaolinite. Kaolinite was the stable weathering product of the feldspars and muscovite in the profiles. It probably formed in equilibrium with a solution whose composition was no longer controlled by the microenvironment within the feldspar, but approached that of meteoric water.

Weathered perthite and mixed muscovite-kaolinite from a kaolinitic granite at Trial Hill in east Queensland and kaolinized sericitic alteration from a granite from the Ardlethan Tin Mine of New South Wales were examined by optical, scanning electron (SEM), and transmission electron microscopy (TEM) to determine the alteration process of muscovite to kaolinite and kaolinite to halloysite (7Å). Muscovite was found intimately interleaved with kaolinite in a variety of proportions on a sub-micrometer scale. The contact was generally parallel to the (001) layers of both minerals, and the thickness of the contact layer alternated between 10 and 7 Å over short distances. Where the kaolinite to muscovite contact was at an acute angle to the muscovite layers, a small angle existed between the layering of the two phases, consistent with a topotactic alteration of muscovite to kaolinite. One tetrahedral sheet in the muscovite appeared to have been removed over 50–100 Å, converting a 10-Å layer to a 7-Å layer. The mica near the contact with kaolinite was easily damaged in the electron beam and showed Al loss during analytical transmission electron microscopy; thus, H3O+ probably substituted for K+ in this transitional phase.

An SEM examination of completely weathered plagioclase showed kaolinite plates having attached, parallel, polygonal rods of halloysite (7Å), which had planar sides and a central void, partly fused with the surfaces of the kaolinite crystals. TEM study showed that the kaolinite altered to halloysite, and that, where the kaolinite was partly altered to halloysite, a series of sharp kinks were present in the kaolinite plate in which alteration had occurred. These kinks were interspersed with linear kaolinite relics, 0.1–0.2 μm long, which appear to have provided local rigidity to the clay packet. Apparently, the altered clay first curled into loosely wound spirals, which ranged in cross-section from triangles to irregular octagons, with pentagons and hexagons being most common. The tendency to pentagons and hexagons compares well with a statistical study of the angles, which were most commonly grouped around 120°. As alteration of the kaolinite relics progressed, the linear parts of the spiral lost their rigidity and became circular or oval shaped. The long axis of the halloysite spirals was parallel to the X axis of the kaolinite. Halloysite spirals formed most readily if they had space to curl; if space was not available, the halloysite formed sheaves. Rare, thin layers of muscovite were present projecting through kaolinite into halloysite. Where muscovite relics reached open spaces, the 10-Å structure expanded to 14 Å.

Plane-wave pseudopotential total energy calculations have been applied to investigate the structure and energetics of the Cs/K exchange into interlayer sites in muscovite mica. Novel muscovite structures were designed to isolate the effects of 2:1 layer charge, cation size/interlayer site shape, and tetrahedral Al/Si substitutions on the exchange. All atom and cell-parameter optimizations were performed with the intention to mimic the constant pressure, non-isovolumetric exchange conditions thought to be found at frayed-edge sites. Under conditions where the cell parameters are allowed to relax, the overall Cs/K exchange reaction is surprisingly close to isoenergetic. The forward reaction is more strongly favored with increasing layer charge. For the condition of zero layer charge and no interlayer site distortion, the difference in the optimal interlayer spacing for Cs relative to K is very small, indicating a baseline indifference of the muscovite structure to cation size. The presence of 2:1 layer charge or tetrahedral rotations arising from Al/Si substitutions clearly change this outcome. Analysis of the dependence of the interlayer spacing on layer charge shows that while the spacing collapses with increasing layer charge for K as the interlayer cation, the reverse is true for Cs. We attribute the contrasting behavior to inherent differences in the ability of these cations to screen 2:1 layer-layer repulsions. Such effects might be involved during exchange at frayed-edge sites where interlayer spacings are increased. This is known, from experiment, to be very selective for Cs. Overall, the exchange energetics are so low that the Cs/K exchange rate and degree of irreversibility are likely to be dominated by diffusion kinetics.

Cation exchange experiments were carried out on phlogopite, biotite, and muscovite using cryptand [222] as a complexing agent, dioxane as solvent, and Li as the exchanging cation. The results indicate greater than 90% exchange of the analyzed cations K+, Rb+, and Sr++ in phlogopite and biotite after two days. Similar results for the exchange in muscovite are observed but it is apparently slower. The amount of exchange observed for mica depends mainly on pH, time and the exchanging cation at a constant temperature. Residues of the mineral phase were also investigated by X-ray diffraction and a significant change of the interlayer spacing was detected. Some samples of the residue were analyzed for Li content. The compositions of the treated samples were in good agreement with the determined exchange of K+.

Three authigenic muscovite morphologies are associated with Norphlet Formation stylolitization observed in the Texaco Mobile Area Block 872 #1 well: l)large crystals of 1M muscovite, which grew in the stylolites with their c-axes parallel to the plane of maximum compressive stress; 2) fine-grained bundles of muscovite that occur as pore-fillings near stylolites; and 3) pods of fine-grained muscovite that exist within stylolite insoluble residue and that were precipitated as pore-filling muscovite before the host sandstone pressolved.

The population of large crystals of 1M muscovite grew at 51 ± 9 Ma, pore-filling muscovites precipitated at 77 ± 22 Ma, and muscovite pods have ages of 86 ± 16 Ma, as indicated by 40Ar/39Ar laser fusion. Apparent ages indicate that stylolitization was coincident with the beginning of organic maturation Zone 5 and could be the product of reservoir fluid pressure fluctuations induced by gas leakage. The lower Smackover Formation source/seal rock, acting as a pressure relief valve, could have been compromised by microfractures occurring during hydrocarbon generation and expulsion. Decreases in reservoir fluid pressure would have acted upon the sandstone framework by increasing the effective overburden pressure, thus making the rock more susceptible to pressure solution.

Stylolite frequency and quartz cement volume increase in the finer grained portion of the conventional core. Quartz cement volume correlates inversely to percent sandstone porosity. Apparent muscovite ages indicate that stylolitization occurred after hydrocarbon migration. Silica mobility was limited because pressure solution mineral products were precipitated from within grain films of irreducible water within the sandstone.

Stylolitization of quartz grains accounts for a minimum of 34% of the quartz cement in the upper cored section of the Norphlet Formation and minimum of 17% of the quartz cement in the lower cored Norphlet Formation. Quartz cement volumes are based on stylolite insoluble residue thickness and weight measurements of pyrobitumen within and nearby the insoluble residue seams. Stylolitization of K-feldspar and precipitation of muscovite can release additional silica which may have precipitated as quartz cement.