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Folk classification

The Folk classification is a descriptive system for sedimentary rocks, devised by American geologist Robert L. Folk in the mid-20th century. It provides a standardized nomenclature to convey key textural and compositional information about rocks like sandstones and limestones, emphasizing components such as grains, matrix, cement, and maturity, without needing a full petrographic description. Widely used in sedimentary petrology, the system employs hierarchical naming conventions and triangular diagrams, such as the QFL (quartz-feldspar-lithic fragments) plot for clastic rocks, to classify based on relative abundances.^[1] Folk initially developed the sandstone classification in the 1950s, with refinements published in 1968, while the carbonate scheme was introduced in 1959 and revised in 1962. These frameworks distinguish between allochemical (grain-supported) and orthochemical (chemically precipitated) elements in carbonates, and assess textural maturity in clastics.^[2] The approach complements alternatives like the Dunham classification and remains influential for interpreting depositional environments and provenance.^[3]

Overview

Definition and purpose

Folk classification is a petrographic system developed for the descriptive naming and categorization of sedimentary rocks, encompassing both siliciclastic and carbonate varieties, through detailed examination of their modal composition and texture.^[4] It relies primarily on thin-section analysis using a petrographic polarizing microscope to quantify components such as grain types, matrix, and cement via point-counting methods.^[4] This approach allows for precise identification of mineralogical and textural attributes that reflect the rocks' formative processes.^[5] The primary purpose of the Folk classification is to provide a standardized nomenclature in sedimentary petrology, facilitating interpretations of provenance (the source area of detrital grains), diagenesis (post-depositional alterations like cementation and recrystallization), and depositional environments (such as fluvial, marine, or lagoonal settings).^[4] By emphasizing compositional and textural parameters, it enables geologists to infer tectonic influences, transport histories, and environmental energies, which are crucial for applications in resource exploration, particularly hydrocarbons.^[4] For instance, in siliciclastic rocks, it aids in assessing textural maturity stages from immature (poorly sorted, angular grains) to supermature (well-sorted, rounded grains), while in carbonates, it highlights depositional dynamics through grain-to-matrix ratios.^[5] A key distinction in the system lies between siliciclastic rocks, which are dominated by framework grains including quartz, feldspar, and rock fragments, and carbonate rocks, which are defined by allochems (grains like ooids or fossils), micrite (microcrystalline mud matrix), and sparry cement.^[4] Siliciclastics derive from mechanical weathering and transport of terrigenous material, whereas carbonates often form through chemical precipitation or biogenic accumulation.^[4] This separation ensures tailored descriptors for each rock type's unique petrogenetic pathways.^[6] The classification originated in the 1950s from the work of Robert L. Folk, a professor at the University of Texas at Austin, who introduced foundational schemes for both clastic and carbonate systems through seminal publications.^[5]^[7]

Historical development

Robert L. Folk (1925–2018) was an American geologist and professor at the University of Texas at Austin, where he spent much of his career developing influential classification systems for sedimentary rocks.^[8] His work focused on petrographic analysis to standardize the description of clastic and carbonate rocks, emphasizing texture, composition, and depositional processes.^[9] Folk's initial classification for clastic rocks appeared in his 1951 paper, which proposed stages of textural maturity based on grain rounding, sorting, and matrix content to describe the progressive modification of sediments during transport and deposition.^[10] He revised and expanded these ideas in his 1968 book Petrology of Sedimentary Rocks, incorporating maturity concepts that integrated compositional stability with textural refinement, such as distinguishing submature from supermature sandstones.^[11] The carbonate classification system was introduced in Folk's 1959 paper, where he coined the term "allochems" for biogenic and inorganic grains to differentiate skeletal, oolitic, and peloidal components from the surrounding matrix.^[12] This was refined in 1962 with a textural scheme that distinguished micrite (fine-grained lime mud) from spar (coarser calcite cement), enabling a spectral subdivision of limestone types based on relative abundances.^[13] Folk's systems drew from earlier grain-typing efforts, such as Amadeus W. Grabau's 1904 broad categorization of sedimentary rocks into clastic and non-clastic groups and Paul D. Krynine's 1948 megascopic classification emphasizing field identification of mineral components.^[14]^[15] A key innovation was his advocacy for modal analysis through point counting on thin sections, which provided quantitative data on grain populations to support provenance and diagenetic interpretations.^[11] No major updates to the frameworks occurred after the 1970s, yet their simplicity and adaptability have ensured widespread adoption in sedimentary petrology.^[16]

Clastic rock classification

QFL framework

The QFL framework, a core component of Robert L. Folk's classification system for siliciclastic sedimentary rocks, utilizes a ternary diagram to categorize sandstones and related deposits based on the modal composition of their framework grains.^[17] In this diagram, the vertices represent Q (quartzose grains, encompassing monocrystalline quartz, polycrystalline quartz, and chert), F (feldspars, including both potassium feldspar and plagioclase), and L (lithic fragments, comprising polycrystalline grains of sedimentary, volcanic, and low-grade metamorphic rocks).^[18] These categories focus exclusively on detrital framework constituents larger than 0.0625 mm, excluding matrix, cement, and accessory minerals to emphasize provenance and diagenetic history.^[19] To construct the QFL plot, petrographers perform point-counting analysis on thin sections, typically examining 100 to 300 framework grains to ensure statistical reliability, and normalize the percentages of Q, F, and L to total 100%.^[17] The resulting point on the equilateral ternary triangle determines the primary rock name according to predefined boundaries: for instance, quartzarenites dominate near the Q vertex (Q ≥ 90%), arkoses near the F vertex (F ≥ 25% with low L), litharenites near the L vertex (L ≥ 25% with low F), subarkoses in the intermediate Q-F field (typically 75–90% Q and 5–25% F), and sublitharenites in the Q-L intermediate (75–90% Q and 5–25% L).^[19] Hybrid names like feldspathic litharenite or lithic arkose apply to fields where two components are prominent.^[20] Superimposed on the QFL diagram are clan designations that group compositions into broader provenance categories: the quartzose clan for samples where Q constitutes ≥75% of total Q + F + L, the arkosic clan where F ≥25%, and the lithic clan where L ≥25%.^[19] These clans provide interpretive value for tectonic settings, with quartzose sands often linked to cratonic interiors, arkosic to dissected arcs or basement uplifts, and lithic to volcanic or orogenic sources, though such inferences require integration with other data.^[20] The framework's simplicity and emphasis on modal analysis have made it a standard tool in sedimentary petrology since its formalization.^[17]

Compositional and textural descriptors

In Folk's classification system for clastic rocks, compositional and textural descriptors serve as modifiers to the primary QFL-based nomenclature, incorporating accessory minerals and textural elements to provide more nuanced descriptions without altering the fundamental ternary plot assignments. These descriptors highlight minor but significant constituents such as matrix, mica, heavy minerals, and glauconite, which influence the rock's provenance, depositional environment, and early diagenetic history.^[4] The matrix, defined as clay-sized particles less than 1/16 mm in diameter, is a key textural descriptor that determines the suffix applied to the rock name. If the matrix constitutes less than 15% of the rock volume, the suffix "-arenite" is used, indicating a relatively clean, framework-supported texture (e.g., sublitharenite). Conversely, when matrix exceeds 15%, the suffix shifts to "-wacke," reflecting a mud-supported or muddy texture (e.g., sublithwacke). The general suffix "-ite" may be applied in broader contexts to denote the sandstone clan without specifying matrix abundance.^[4] Miscellaneous constituents, including mica, heavy minerals, and glauconite, are denoted as prefixes when they exceed typical trace amounts (generally >1-2% of the terrigenous fraction). For instance, mica (often biotite or muscovite) imparts a "micaceous" prefix (e.g., micaceous sublitharenite), signaling a metamorphic or recycled sedimentary source. Heavy minerals such as zircon and tourmaline, typically ranging from 0.1-1.0% of the terrigenous fraction, are similarly prefixed when abundant, while glauconite—a green, microcrystalline authigenic mineral—earns a "glauconitic" prefix (e.g., glauconitic arkose), indicating a marine shelf environment.^[4] A critical distinction in applying these descriptors is between transported (detrital) elements, which are eroded and deposited grains like quartz, feldspar, or mica flakes, and authigenic elements, such as diagenetic cements or overgrowths formed post-depositionally. Only detrital components are included in the QFL framework and primary modifiers, ensuring the classification reflects provenance rather than later alterations; authigenic features are noted separately to avoid misinterpretation of source characteristics.^[4] Additional textural terms like "feldspathic" (feldspar 5-25%) and "lithic" (rock fragments 5-25%) are used as prefixes or qualifiers to emphasize subordinate components beyond the dominant QFL pole, but they do not override the base classification. For example, a rock with significant but not dominant feldspar might be termed feldspathic litharenite. These terms enhance precision without shifting the rock into a different clan.^[4] Descriptors are assembled hierarchically, ordered by dominance or abundance, to form the complete name—starting with accessory prefixes (e.g., calcareous for carbonate cement if prominent), followed by textural qualifiers (e.g., lithic), and ending with the QFL-based clan and matrix suffix (e.g., calcareous lithic arkose). This sequential rule prioritizes the most influential features while maintaining consistency with the overall system.^[4]

Maturity and cementation indicators

In the Folk classification system for clastic sedimentary rocks, textural maturity refers to the progressive refinement of grain characteristics through transport, weathering, and diagenesis, encompassing reductions in clay content, improvements in sorting, and increases in roundness. Four stages are defined: immature sediments exhibit more than 5% clay matrix, poor sorting (standard deviation >1.0 φ units), and angular grains; submature stages show less than 5% clay but retain poor sorting and subangular shapes; mature stages feature minimal clay, good sorting (<0.5 φ units), and subrounded grains; and supermature stages display no clay, excellent sorting, and well-rounded grains with a mean roundness index of at least 0.35.^[21]^[4] These stages are exemplified by rock types such as supermature rounded and well-sorted quartzarenites, submature lithic arkoses, and immature "dirty" arkoses laden with matrix.^[21] Compositional maturity complements textural maturity by assessing the mineralogical stability of framework grains, reflecting the degree of chemical weathering and recycling in the source area. A high ratio of quartz to the sum of feldspar and lithic fragments (Q/F+L) indicates advanced maturity, as unstable minerals like feldspar and rock fragments are depleted through prolonged exposure, often signifying recycled sediments from older sedimentary sources. Conversely, a low Q/F+L ratio points to immature compositions dominated by first-cycle detritus from volcanic or rapidly eroding terrains.^[4] Quantitative evaluation of mineralogical maturity frequently employs the ZTR index, which measures the percentage of stable heavy minerals—zircon, tourmaline, and rutile—relative to the total transparent heavy mineral suite, with values exceeding 90% typical of supermature orthoquartzites.^[22] Cementation in sandstones provides insights into diagenetic history and is denoted as a suffix in Folk's nomenclature to specify the dominant binding material. Common cement types include siliceous cements, such as quartz overgrowths that enhance grain cohesion without altering primary textures; calcareous cements, primarily calcite that may fill pores or replace grains; ferruginous cements, involving iron oxides like hematite for reddish hues; and zeolitic cements, formed in alkaline environments. For instance, a rock might be named "silty quartzarenite cemented by calcite" to indicate both accessory silty matrix and the prevailing cement.^[4] Grain size descriptors are integrated as prefixes in the classification, drawing from the Wentworth scale adapted for sandstones, where terms like "very fine-grained" apply to grains between 0.0625 and 0.125 mm, emphasizing the dominant fraction to refine the overall name without overriding the QFL-based root.^[4]

Carbonate rock classification

Allochemical and orthochemical components

In Folk's classification system for carbonate rocks, the primary textural elements are divided into allochemical and orthochemical components, which form the foundation for identifying and naming limestones based on their mineralogical and depositional characteristics.^[23] Allochems represent the grain or framework fraction, consisting of discrete particles larger than 1/16 mm (62.5 microns) that have been transported and deposited, often showing evidence of mechanical or biological sorting.^[23] These grains are analogous to detrital components in siliciclastic rocks but are predominantly biogenic in origin, emphasizing the biological productivity of carbonate environments over physical weathering of source terrains.^[24] Allochems are categorized into biogenic and non-biogenic types. Biogenic allochems include skeletal fragments such as whole or broken fossils (e.g., foraminifera, brachiopods, or coral pieces), which range from gravel to fine sand sizes and contribute to rocks like biomicrites when dominant in a micritic matrix.^[23] Non-biogenic allochems encompass ooids (spherical grains with concentric layers of calcite, typically sand-sized and formed in agitated shallow waters), peloids (rounded, silt- to sand-sized fecal pellets or reworked micrite clusters), and intraclasts (lithified fragments of penecontemporaneous carbonate mud eroded and redeposited nearby).^[23]^[24] These components highlight the in-situ biogenic accumulation and limited transport typical of carbonate systems, contrasting with the long-distance detrital transport in clastics.^[23] Orthochemical components, in contrast, form through in-place crystallization without significant transport and include micrite and spar. Micrite consists of lime mud smaller than 1/16 mm, primarily microcrystalline calcite that appears as a dull, opaque matrix under the microscope, often indicating deposition in low-energy settings where fine precipitates or biogenic breakdown products accumulate.^[23]^[24] Spar, or sparry calcite, refers to coarser, clear calcite crystals larger than 1/16 mm that fill voids or result from recrystallization of micrite, serving as a cement that binds allochems in grain-supported fabrics.^[23]^[24] Unlike the clay or quartz matrix in clastic rocks, orthochemicals in carbonates often precipitate directly from seawater or during diagenesis, underscoring the chemical precipitation aspect of carbonate formation.^[24] The relative abundance of these components determines the rock's texture and support framework. Rocks are classified as allochemical if allochems exceed 10% by volume, leading to grain-supported textures where allochems are the primary framework and sparry cement predominates over micrite.^[25]^[23] In contrast, mud-supported textures occur when micrite dominates (typically >50%), embedding allochems and indicating a finer-grained, lower-energy depositional environment, while spar dominance signals post-depositional cementation or recrystallization.^[24]^[25] Folk emphasized quantitative assessment through microfacies analysis, using point-counting techniques on thin sections to determine modal percentages of allochems, micrite, and spar for precise textural classification.^[23] This focus on biogenic allochems and orthochemical cements distinguishes carbonate classification from clastic systems, where detrital mineralogy and matrix clay content drive the framework, rather than biological and chemical precipitation processes.^[24]^[23]

Naming conventions

In Folk's classification system for carbonate rocks, names are constructed using a combination of prefixes indicating the dominant allochem types, a main term reflecting the interstitial material or fabric, and sometimes a suffix or modifier for grain size, ensuring a descriptive and hierarchical nomenclature based on modal composition.^[13] The main term is determined by the dominant allochems, which form the primary skeletal or granular components; for instance, if fossil fragments (bioclasts) predominate, the base name incorporates "bio-" as in biomicrudite, where coarse (>2 mm) fossil grains are set in a micritic matrix.^[13] Suffixes specify the nature of the interstitial material: "-sparite" denotes grain-supported fabrics cemented by coarse, clear sparry calcite that fills voids between allochems; "-micrite" indicates mud-supported textures where microcrystalline carbonate mud (micrite) constitutes the matrix; and a general "-ite" suffix applies when the matrix is ambiguous or mixed.^[13] Prefixes denote the type and relative abundance of allochems, such as "bio-" for bioclasts, "oo-" for ooids, "pel-" for peloids, and "intra-" for intraclasts, with multiple prefixes listed in decreasing order of abundance if two or more are significant.^[13] Qualifiers like "pelletoidal" may be added for specific textures, and allochems comprising less than 10% of the rock are typically omitted from the name.^[13] Grain-size modifiers such as "-rudite" (for coarse grains >2 mm) or "-arenite" (for sand-sized grains) are appended when relevant, as in biomicrudite for a mud-supported assemblage of coarse bioclasts.^[13] Key rules govern name assembly: allochems are listed in order of decreasing abundance to reflect compositional dominance, and if micrite exceeds 50% of the rock volume, the name defaults to a micrite variant regardless of minor sparry cementation.^[13] Sparry calcite is interpreted as a void-filling cement rather than a primary depositional component, prioritizing textural interpretation. Special cases include biolithite, applied to rocks consisting primarily of organisms in growth position that create a rigid framework, such as algal mats or corals; however, Folk's system emphasizes modal composition over fabric, so biolithite names may incorporate allochem prefixes if binding structures are not overwhelmingly dominant.^[13]

Petrographic examples

In the Folk classification system for carbonate rocks, petrographic thin sections provide key insights into the relative abundances of allochems, micrite matrix, and sparry calcite cement, enabling precise nomenclature. For instance, a grain-supported limestone composed of approximately 40% bioclasts (such as shell fragments) and 30% ooids, with the remaining 30% consisting of sparry calcite cement and negligible micrite, is classified as an oobiosparite. This name reflects the dominance of ooids and bioclasts (listed from subordinate to major allochem) in a sparite fabric, where the coarse, equant calcite crystals fill intergranular pores.^[26]^[25] Another representative example is a mud-supported limestone with about 60% micrite matrix, 20% pellets, and 20% fossil fragments, termed a biopelmicrite. Here, the fine-grained, cryptocrystalline micrite envelops the allochems, indicating deposition in a low-energy environment with limited grain support. The equal contributions of pellets and bioclasts justify including both prefixes before "micrite."^[26]^[24] Thin-section observations under plane-polarized light (PPL) typically reveal ooids as spherical grains with concentric laminations of microcrystalline carbonate, often 0.5–1 mm in diameter, while bioclasts appear as irregular, polarized fragments of shells or echinoderm plates. Under crossed polars, sparry cement displays high-order birefringence with bright interference colors due to its coarse crystallinity (up to 0.5 mm or larger), contrasting sharply with the optically isotropic or low-birefringence micrite, which remains mostly dark and structureless.^[27]^[24] A conceptual gallery of allochems in thin section would highlight ooids' radial or tangential fabric in concentric layers, often with a quartz nucleus, and fossil fragments showing internal microstructures like trabeculae in corals or punctae in brachiopods. Pellet allochems appear as rounded, homogeneous microcrystalline bodies lacking internal structure.^[27]^[28] Common misclassifications arise when equating Folk terms directly with Dunham's textural scheme; for example, a sparry packstone (grain-supported with some micrite) might be misidentified as a biosparite if the micrite exceeds 10% and is overlooked, whereas Folk requires negligible micrite for the pure sparite designation.^[25]^[6]

Applications and critiques

Practical uses in sedimentary petrology

In sedimentary petrology, the Folk classification facilitates provenance analysis by quantifying the relative abundances of quartz (Q), feldspar (F), and lithic fragments (L) in clastic rocks, allowing geologists to infer source terrane characteristics. For instance, high feldspar content in arkosic sandstones, as indicated by QFL ratios exceeding 25% F, points to rapid erosion from granitic or volcanic uplifts with minimal weathering, such as in tectonically active continental blocks.^[29] This approach, rooted in point-counting modal analysis, has been applied to reconstruct sediment dispersal patterns in basins like the Cretaceous Bredasdorp Basin, where subarkosic arenites revealed sources from stable cratons and recycled orogenic debris.^[29] Similarly, low L percentages suggest derivation from mature, quartz-rich shields rather than dissected arcs.^[4] The classification also tracks diagenetic processes by identifying cement types, matrix alterations, and maturity indicators, which reveal burial history and fluid interactions. In sandstones, the presence of syntaxial quartz overgrowths or authigenic clays replacing feldspars signals progressive cementation under increasing burial pressures, often distinguishing eogenetic (shallow) from mesogenetic (deeper) phases.^[4] For carbonates, orthochemical components like sparry calcite cement versus micritic matrix help delineate neomorphic recrystallization or dolomitization events, with poikilotopic calcite indicating late-stage precipitation from migrating fluids. These descriptors enable reconstruction of diagenetic sequences, such as in the El Abra Limestones, where petrographic modes combined with cement fabrics traced early marine cementation to later meteoric dissolution. In interpreting depositional environments, Folk's scheme for carbonates uses allochems to distinguish water depth and energy regimes; ooid grainstones, rich in coated grains, typify high-energy, shallow-marine settings like tidal shoals due to agitation promoting concentric layering.^[4] Conversely, micrite-dominated mudstones suggest low-energy, deeper or quieter environments, such as lagoons or below wave base, where fine-grained carbonate mud settles without significant grain support.^[4] This allochem-based differentiation aids in mapping facies transitions, as seen in reef complexes where biosparites (fossil-rich sparites) mark platform margins.^[4] Field applications of the Folk classification rely on hand-sample approximations of grain types and textures for rapid assessment, while laboratory work employs thin-section petrography for precise modal analysis via point counting, typically sampling 100–300 points per slide to achieve statistical reliability.^[4] This contrast allows initial fieldwork to guide sampling, with lab verification refining classifications; for example, hand lenses can approximate QFL ratios in outcrops, but polarized light microscopy confirms lithic fragment identities.^[4] Modern software tools, such as PETROG, streamline point counting by overlaying digital grids on scanned thin sections, automating data entry and generating QFL ternary plots for efficient workflow.^[30] Contemporary extensions integrate Folk's petrographic framework with geochemical analyses, enhancing basin-scale studies; stable isotope ratios in sparry cements (e.g., δ¹⁸O and δ¹³C) complement allochem identifications to trace diagenetic fluid sources, distinguishing marine from meteoric influences in formations like the El Abra. This multidisciplinary approach, applied in rift basins, combines QFL-derived provenance with isotopic signatures to model sediment routing and burial evolution over geologic time.^[29]

Comparisons with alternative systems

Folk's classification scheme for sandstones emphasizes modal petrographic composition through the QFL (quartz-feldspar-lithics) framework, providing a descriptive approach focused on mineralogical maturity and grain types, whereas Dott's 1964 system adopts a more genetic perspective by incorporating matrix content and depositional processes to categorize immature sandstones. For instance, a matrix-poor sandstone rich in rock fragments might be termed a litharenite under Folk's scheme due to its lithic dominance, but classified as a subgraywacke by Dott if it reflects low-maturity depositional conditions with minimal matrix.^[31] Folk's approach offers greater detail in assessing maturity stages, such as distinguishing arkosic from lithic varieties based on precise point counts, while Dott's prioritizes process-oriented terms like wacke or graywacke to infer transport and diagenesis.^[32] In carbonate rocks, Folk's system relies on textural and modal analysis of allochems (grains), micrite (mud matrix), and sparry cement, enabling detailed petrographic naming like biosparite for grain-supported biogenic fragments in sparry calcite.^[26] In contrast, Dunham's 1962 classification centers on depositional fabric and support mechanisms, dividing rocks into mud-supported (e.g., mudstone with <10% grains) or grain-supported (e.g., grainstone) categories to reflect energy levels during deposition.^[33] A mud-rich carbonate, for example, is designated micrite in Folk's modal terms if dominated by fine matrix, but mudstone or wackestone under Dunham if mud-supported with varying grain percentages (>10% for wackestone), highlighting Folk's focus on component proportions versus Dunham's emphasis on structural fabric.^[34] Folk's granularity suits thin-section studies for identifying specific allochem types, whereas Dunham's fabric-based method is more applicable to field or hand-sample assessments of depositional environments.^[24] Despite its strengths, Folk's scheme has notable limitations, including the subjectivity inherent in point-counting modal compositions, which can vary based on observer interpretation of grain boundaries and categories like chert (grouped with lithics in Folk but quartz in Dott).^[35] It largely overlooks depositional fabric in carbonates, potentially underemphasizing support structures critical for interpreting diagenetic history, and proves less adaptable for mixed siliciclastic-carbonate systems due to its segregated clastic and carbonate frameworks.^[33] Additionally, Folk's original design, rooted in marine sedimentary contexts, has been critiqued as somewhat outdated for non-marine settings where volcanic or pedogenic influences dominate grain assemblages.^[36] Folk's classification excels in providing a comprehensive, petrographically rigorous framework that remains widely taught in sedimentary geology courses for its descriptive precision without requiring interpretive genetic assumptions.^[27] As a purely observational system, it necessitates no major updates, allowing consistent application across diverse datasets.^[37] Modern applications often hybridize Folk's modal categories with scanning electron microscopy (SEM) to enhance identification of microfossils and fine-grained allochems, such as distinguishing biogenic versus detrital components in thin sections.^[38]

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[PDF] classification of carbonates
Orthochemical rocks are those in which the carbonate crystallized in place. Allochemical rocks have grains that may consist of fossiliferous material, ooids, ...Missing: components | Show results with:components
[36]
[PDF] The quest for an ideal classification of sandstones
Sandstone classification after Folk et al. (1970) showing the primary arenite triangle. According to the original authors the second and third order triangles ...
[37]
Petrographic classification of sand and sandstone - ScienceDirect.com
The classic QFL plot is subdivided into 15 fields - labelled by adjectives introduced long ago by K.A.W. Crook and endorsed by W.R. Dickinson and more recently ...Missing: original | Show results with:original
[38]
Sandstone Classification: Relation to Composition and Texture
Krynine (1948) and. Folk (1954) are also in error in combining metamorphic rock fragments and clay, for, as Pettijohn (1954) points out, the former is a labile ...Missing: limitations | Show results with:limitations
[39]
Earth system science applications of next-generation SEM-EDS ...
SEM-EDS mineral mapping can reveal the mineral identity, morphology and petrological context of each imaged grain, making it a valuable tool in the Earth ...