Surface textures of detrital pyroxenes in coastal dune sands (western Gulf of Mexico, Mexico): Implications for their preservation and geoenvironmental processes

This work investigates how the surface textures and morphology of pyroxene grains evolve during their source‐to‐sink history. This study applies to detrital clinopyroxenes concentrated in coastal dune sands of the Gulf of Mexico which were sourced in the Trans Mexican Volcanic Belt then transported and deposited in environments subject to limited chemical weathering. The composition and morphology of the pyroxenes was characterised using single‐grain geochemical analysis and surface texture imagery with a novel approach based on the compactness property to assess the shape of minerals. This reveals heterogenous diopside‐augite populations, displaying mineral morphologies dominantly controlled by impact breakage along cleavages, little physical abrasion along their edges and with limited evidence for chemical weathering. Mechanical surface textures dominate over mechanical/chemical and chemical surface textures. These mechanical surface textures are preserved primarily as flat cleavage surfaces and rounded edges inherited from fluvial‐intertidal and aeolian transport, respectively. Mechanically/chemically induced surface textures are preserved as elongated depressions. Chemical surface textures are sparse and mostly represented by mammillated textures that suggest local dissolution under subaqueous conditions. The scarcity of chemical surface textures is attributed to frequent fragmentation of the clinopyroxenes along cleavages and limited chemical weathering during transport of the observed populations. Clinopyroxene grains in the coastal dune sands primarily retain surface characteristics from fluvial transport. Although the breakage of minerals along cleavages can obscure their original morphology under a weathering‐limited erosion regime, this study shows how surface textures and morphology of pyroxene grains is used to determine episodes of transport and deposition close to volcanic environments. The use of the compactness property as a shape descriptor measurement of particles provides an alternative approach to observe how clinopyroxene remains unaltered despite the high energy conditions of the coastal area.

Hence, surface textures on clinopyroxenes offer the opportunity to identify some of the physical and chemical mechanisms that enable pyroxene grains to survive in a sandy coastal area emplaced within a volcaniclastic environment.
Along with the microtextural record, it is well known that the morphology of detrital minerals (e.g.sphericity/roundness/angulosity) are inherited grain properties determined by mechanical and chemical processes during the sedimentary cycle (Costa et al., 2013;Folk, 1980;Grandstaff, 1978;Immonen, 2013;Krinsley & Wellendorf, 1980;Resentini et al., 2018;Zhao & Wang, 2016).In addition, the morphology of pyroxenes can be primarily controlled by their weak cleavage surface boundaries and vulnerability to chemical weathering under natural and artificial conditions (Andò et al., 2012;Garzanti et al., 2015aGarzanti et al., , 2015b;;Le Pera & Morrone, 2020;Velbel, 2007).However, there are only a few studies providing detailed constraints on these processes and their record in the morphology of pyroxenes.This paper addresses this issue by investigating the composition and morphology of detrital pyroxenes collected in fine-grained, well-sorted, litho-quartzose modern sand dunes deposited under moderate chemical weathering within the Gulf of Mexico coastal plain in Mexico, which record volcaniclastic supply from the nearby Trans Mexican Volcanic Belt (Mejía-Ledezma et al., 2020;Mendieta-Lora et al., 2018).Quantitative analyses to determine the composition of the pyroxene were carried out by energy dispersive spectrometers (EDS) and wavelength-dispersive spectrometers (WDS) with a peak counting time of 10 s for Na and K. To determine the morphology of the pyroxenes the compactness shape descriptor property of objects was used to assess the shape of a sand-sized mineral closely related to a circle in the Euclidean domain or a polygon in the digital domain (Bogaert et al., 2000;Bribiesca, 2000;Li et al., 2013;Montero & Bribiesca, 2009;Osserman, 1978;Roduit, 2019).Hence, compactness offers an alternative conditions.The scarcity of chemical surface textures is attributed to frequent fragmentation of the clinopyroxenes along cleavages and limited chemical weathering during transport of the observed populations.Clinopyroxene grains in the coastal dune sands primarily retain surface characteristics from fluvial transport.
Although the breakage of minerals along cleavages can obscure their original morphology under a weathering-limited erosion regime, this study shows how surface textures and morphology of pyroxene grains is used to determine episodes of transport and deposition close to volcanic environments.The use of the compactness property as a shape descriptor measurement of particles provides an alternative approach to observe how clinopyroxene remains unaltered despite the high energy conditions of the coastal area.

K E Y W O R D S
clinopyroxene, coastal dune sands, compactness, Mexico, surface textures, weathering way to describe the shape of the grains utilising the properties of sphericity and convexity (see Bufham, 2000;Zhao & Wang, 2016).This approach can be regarded as an alternative to Fourier coefficient image analysis (Diepenbroek et al., 1992;Resentini et al., 2018;Thomas et al., 1995) and fractal analysis of grains (Hyslip & Vallejo, 1997;Maria & Carey, 2002, 2007;Zhou et al., 2018) to quantify the shape of sediment particles.

| STUDY AREA
The study area is located in the state of Veracruz along the western Gulf of Mexico, Mexico.It is part of the coastal area and the Maya Terrane (20°12′,19°24′N;96°45′, 96°19′W;Centeno-García, 2016;Tamayo, 2013; Figures 1  and 2).The climate is defined as Am (warm-humid with a summer rainfall maxima) and Aw (sub-humid with summer rainfall; Kottek et al., 2006).Intense tropical storms are common from June to November (Tamayo, 2013), with low air temperatures between 17 and 25°C and highs of 22-30°C.The average precipitation ranges from 370 mm in summer to 60 mm in winter.The annual average humidity is about 80% (Tejeda et al., 1988).

| Coastal features and fluvial network
The Gulf of Mexico coastal plain is a wave-dominated, transgressive, low relief, narrow coastal plain, averaging 1.25 km in width (Ortiz-Pérez & Espinosa, 1991).Hinterland rivers are derived from the inactive volcanic complex of the Trans Mexican Volcanic Belt.Rivers developed as dendritic and subparallel arrangements of permanent and intermittent discharges (Figure 2).

| Physical oceanography and wind regime
The longshore drift is characterised by a northward component in spring-summer with velocities of 0.035-0.08m/s (Fernández-Eguiarte et al., 1992).The average wave height is 0.90 m with 3-8 s periods.The tidal regime in the western Gulf of Mexico is negligible.Tides of diurnal and mixed-diurnal types have an average tidal range of 0.30 m.Winds come from the north, east, north-east and south-east with maximum velocities of 4-5 m/s (Mejía-Ledezma et al., 2020;Pérez-Villegas, 1990).

| Geological setting
The Trans Mexican Volcanic Belt Neogene continental arc is a volcanic province ca 1000 km long with a width of ca 80 to 230 km (Ferrari et al., 2011;Servicio Geológico Mexicano, 2017).It follows an E-W direction in its central and eastern parts.In its western sector, the belt develops a WNW-ESE trend with an angle of ca 16° with the Middle America Trench (Demant, 1979;Gómez-Tuena et al., 2007).The Trans Mexican Volcanic Belt divides the central part of Mexico into three sectors, from the Mexican Pacific to the Gulf of Mexico coasts (Alaniz-Alvarez et al., 2002;Demant, 1979;Pasquaré et al., 1991).
In the Gulf of Mexico coastal plain, the rocks of the Trans Mexican Volcanic Belt are compositionally dominated by mafic, intermediate and acid rocks exposed as basalts, andesites, andesite-dacites and basalt-rhyolite tuffs of Miocene and Pliocene ages (Ferrari et al., 2011;Servicio Geológico Mexicano, 2017).Miocene andesites are the oldest Trans Mexican Volcanic Belt rocks exposed along the coast.Basalts and basanitic breccias represent upper Miocene to lower Pliocene rocks of mugearitic composition (e.g.Naalkaline structure exposed at the Palma Sola coastal area; Figure 1).These rocks contain phenocrysts of plagioclase, augite-diopside, oligoclase, opaque minerals and olivine suites (Ferrari et al., 2005;Gómez-Tuena et al., 2007).The upper Pliocene to Quaternary volcanic rocks are of calcalkaline composition (Ferrari et al., 2005;Gómez-Tuena et al., 2007;Negendank et al., 1985; Figure 1).Recent units are exposed as Plio-Quaternary alluvial deposits and dune fields occur along the coastal area (Ferrari et al., 2005).

| Dune fields
Dune fields cover an extensive coastal area throughout the Gulf of Mexico coastal plain including transverse mobile and vegetated dunes with average heights of 2-8 m in the north-central coastal area, that is Istirincha (I) to Boca Andrea (BA).In contrast, the coastal dunes in the south, that is Villa Rica (VR) to Chachalacas (CH), are transversevegetated, parabolic and barchan coastal dunes with average heights of ca 10 m (Mendieta-Lora et al., 2018; Figures 1  and 3A,B,C).The dunes are exposed where chemical weathering action is lower than sand transport rates (Johnsson et al., 1993;Kasper-Zubillaga et al., 2022).
3.2 | Scanning electron microscopy analysis 3.2.1 | Sample preparation and geochemical analyses Samples were cleaned and rinsed with distilled water, avoiding chemical solutions that might remove adhering particles since they provide valuable information on the surface texture attributes of the grains (Vos et al., 2014).After cleaning, the sands were dried in an oven at 50°C.A small amount of the whole bulk sand sample (ca 0.5 g) was placed in a Petri-like dish under a binocular microscope objective 4X-Velab-VES1 for separation of pyroxene grains by handpicking based on their noticeable green and dark green colour and shape.Pyroxene grains ranging from 100 μm to 2 mm, suitable for assessing the prevailing surface textures (Vos et al., 2014), were separated from bulk samples of very fine to fine sand (from 2.5 to 3.0 ɸ).This grain size consideration was achieved after most of the grains observed were bigger than 100 μm (e.g.2.5-3.0 ɸ; very fine to fine sands).
A selection of separated crystals were mounted on organic resin slides, polished, and coated with graphite.
The centre of each pyroxene crystal was then analysed twice by electron microprobe (n = 66) (Figure 4A through E).The geochemical characterisation of pyroxene minerals in sand samples can be carried out by EDS or WDS.The EDS technique is relatively fast and enables almost simultaneous determination of the chemical elements that compose a mineral sample, although with lower precision.However, the WDS analysis is more suitable for ferromagnesian phases because more detailed information is obtained in terms of chemical composition that can be related to the source rock of the pyroxene grains.The WDS analysis is not as fast as EDS but has better precision since only one element can be monitored at a time in each spectrometer, although the parallel acquisition of n elements (n … number of spectrometers) is achieved.
An automated JEOL JXA8900-R electron microprobe with five WDS and one EDS was employed (Laboratorio Universitario de Petrología, Universidad Nacional Autónoma de México, UNAM).An acceleration voltage of 20 keV and 20 nA current with a 1.0 μm beam diameter was used.Quantitative analyses were carried out by WDS with a peak counting time of 10 s for Na and K, and 30 s for the other elements.Calibration was performed using international standards (United States Geological Survey; USGS) and internal standards (Division of Structure Probe, Inc.; SPI).Matrix effects were corrected for using the JEOL-ZAF protocol, with the maximum analytical error in oxides determined to be less than 3%.The quality of the data was assessed using kaersulite, olivine, diopside, cuprite and pentlandite standards.Only analyses with a total >96 wt% were considered valid (most of the analytical totals range between 98 and 101 wt%, as expected for pyroxenes).The structural formula of the pyroxenes was calculated according to the method described in Morimoto et al. (1988).
3.2.2| Grain shape assessment: Compactness measurements Quantitative measurement of compactness on pyroxene grains was obtained from petrographic images using the J Microvision v. 1.3.4software (n = 332 grains; Roduit, 2019).The compactness property of objects and particles has been widely used to quantify the extent to which a geographical area changes throughout space and time (Bogaert et al., 2000;Bribiesca, 2000;Chaudhuri et al., 2012;Li et al., 2013).To date, compactness measurements have been used to assess the shapes of objects without any attempt to employ them as shape descriptors in minerals.This study aims to discuss how pyroxene grains are mechanically and chemically altered during transport and deposition using the compactness shape property.Compactness for isoperimetric inequality ratios is defined as the ratio of the area of the object to the size of a circle with the same perimeter.
where C represents the compactness, A represents the area of the particle and P 2 represents the squared perimeter of any object (Li et al., 2013;Osserman, 1978).Compactness in grains is an alternative method implemented to quantify particle shape.Quantification involves determining the area of the enclosing sphere (4π A) that contains the particle's perimeter (P).Compactness measurements encompass (i) sphericity, defined as the ratio of the surface area of a sphere with the same volume as the given particle to the surface area of the particle (Wadell, 1932) and the (ii) convexity, defined as the ratio of the volume of a particle and the volume of the convex hull that is the smallest polygon that contains a set of points inside it where interior angles of the convex polygon are ≤180° (see Angelidakis et al., 2022;Graham & Yao, 1983;Montero & Bribiesca, 2009;Zhao & Wang, 2016) Hence, compactness compares the particle with its volume equivalent sphere whereas convexity is a measure of the smallest set of convex shapes of the particle approaching a polygon (Zhao & Wang, 2016).Roundness is excluded as a parameter to define compactness measurements since roundness measurements determine the corners and sharpness of the grains whereas compactness accounts for the overall shape close to a circle.Furthermore, shape compactness, sphericity and circularity are the same concepts in a continuous plane and 3D shape in Euclidean spaces (Montero & Bribiesca, 2009).Nonetheless, compactness values in the digital domain can be interpreted in terms of the shape of the clinopyroxene once the crystal suffers breakage and its morphology tends to a polygon rather than a circle (see Rosenfeld, 1974).To assess the compactness of pyroxene grains, the JMicrovision software allows binary images to be analysed accurately by pixel connectivity, extraction and digitalisation of the grains following the pixel codes arrangement based on Freeman (1961) and the empirical formula for perimeter (Vossepoel & Smeulders, 1982).In binary images, white regions represent the foreground pixels in 8-pixel connectivity and black regions the background pixels in 4-pixel connectivity.
A database of compactness resulted from: (a) the spatial calibration of the grains in microns; (b) the extraction operation to configure the grain images (image segmentation); (c) adjustment of the surface filters to ensure a 1000 pixel value to compute the descriptors of the grain's shape; (d) removal of the border and fill filters to avoid adjacent objects from the grain's border contour and surface holes and (e) the image threshold operation to convert the original greyscale image into a binary image (see Roduit, 2019).Compactness measurements range from 0.00 to 1.00, defined as low to high compactness (Li et al., 2013;Montero & Bribiesca, 2009;Roduit, 2019).Limitations with the compactness measurements of particles in the digital domain still remains a problem as they present fractal F I G U R E 4 (A) Geochemical composition of pyroxene grains by electron microprobe (n = 66) (Morimoto et al., 1988).Magmatic differentiation of source rocks based on the geochemical composition of calcic clinopyroxenes (see Leterrier et al., 1982) patterns which might lead to dissimilar compactness values (Bribiesca, 2000;Rosenfeld, 1974).To ensure as much as possible the consistency of this compactness data, a visual inspection calibration of the grains was performed providing a full catalogue of dissimilar shapes of clinopyroxene (see Supplementary data available at https://hdl.handle.net/20.500.12201/11352).
Additional shape parameters like sphericity, convexity and roundness were obtained to observe any significant changes in the grain shape patterns of the clinopyroxenes and evaluate the value of the compactness parameter relative to more traditional approaches.Sphericity values were obtained following the sphericity index Is = A I /A, where A I is the area of the maximum inscribed circle obtained from the width of the particle (minimum radius) and A the particle area of the covering circle computed by the length of the particle (maximum radius; see Mo, 2020) (Table 1).Convexity values were obtained from the JMicrovision software descriptor shape output data based on the convexity hull (Zhao & Wang, 2016; Table 1).Roundness values were obtained by visual inspection of the SEM images based on the chart by Resentini et al. (2018, from figure 4; Table 1; Supplementary data available at https://hdl.handle.net/20.500.12201/11352).

| Scanning electron microscopy imaging of pyroxenes
A total of 337 pyroxene grains from 32 samples were observed using a SEM-Jeol-JSM 6360LV at the Instituto de Ciencias del Mar y Limnología, UNAM, Mexico.
Grains were mounted on a specimen holder to be observed on an X-Y-Z tilt rotation stage and coated with gold for the best resolution (i.e. when compared to other metals or carbon; Krinsley & Doorkamp, 1973).A working distance of 15 mm was used for 200× magnification.A voltage of 10 kV was obtained using a tungsten filament in high vacuum mode.

| Quantitative assessment of surface textures
The percentage of total grain's pixel coverage area for each surface texture on the pyroxene crystal was assessed by the JMicrovision v. 1.3.4software.This was done by using one single polygon interaction around the image's external boundary to reduce the number of edges and preserve the grain's contour as much as possible (see Chaudhuri et al., 2012;Ramer, 1972;Roduit, 2007).Each polygon represents the surface area on the grain's total area equivalent to 100% of the pyroxene crystal.
The measurements were first obtained from a scale calibration set up in millimetres on the image.The contouring of each individual grain was achieved manually by drawing an iterative selection of a subset of points as vertices of a polygon on the silhouette image for each pyroxene image using the 2D setup tool from the J Microvision v. 1.3.4software (Roduit, 2019).The same contouring procedure was applied for each surface texture covering the grain's surface.Finally, for each surface texture, a percent proportion expressed as a percent value of the area covered is obtained for each pyroxene grain (Figure 5A through J).

| Geochemical composition of pyroxenes
Among the 66 analysed pyroxenes, 64 are clinopyroxenes ranging in composition between diopside and augite, and two are enstatites (Figure 4A).The Mg# (Mg/ Mg + Fe × 100) of the clinopyroxenes ranges between 70 and 92 (Figure 4B), consistent with the provenance of the minerals from basaltic to andesitic volcanic sources.Titanium increases gradually from approximately 0.003-0.05cpfu (cation per formular unit), or from 0.1 to 1.7 wt%, with decreasing Mg# (Figure 4B).This trend is consistent with crystallisation in mafic to intermediate melts without the abrupt appearance of Fe-Ti oxides during magmatic differentiation.Variability of Ti at a given Mg# suggests distinct sources for the clinopyroxenes.Accordingly, discrimination of magmatic series using the approach by Leterrier et al. (1982) indicates that the clinopyroxenes predominantly sourced in a volcanic arc, with a subset of grains originating from an alkaline source (Figure 4C,D,E).This is consistent with provenance of the studied volcaniclastic sand from the Trans Mexican Volcanic Belt.No obvious compositional difference was observed between the pyroxenes from distinct sampling localities.

| Compactness, sphericity, convexity and roundness measurements
The average compactness for all coastal sites for the clinopyroxene grains is 0.71 ± 0.10, the average sphericity value for most of the clinopyroxene grains is 0.46 ± 0. 20, and the average convexity value for most of the grains is 0.93 ± 0.06.Significant correlations for La Mancha, Villa Rica, Chachalacas, El Moro Farallon and Lechuguillas between compactness and sphericity for clinopyroxene grains are observed (Figure 6A,see  is determined for Boca Andrea, Istirinchá and Palma Sola (Figure S1).Nonetheless, a significant correlation is achieved for most of the samples (Figure 6A).Strong correlations between compactness and convexity are observed for each separate site and for all the sites together (Figure 6B; Figure S2).Finally, a lack of significant correlations exists between compactness and roundness for the clinopyroxene grains at each site and most of the studied areas (Figure 6C; Figure S3).To compare the values of compactness, sphericity and convexity, three images with dissimilar shapes are presented in Figure 7A through C. The contribution resulting from grouping mechanical, mechanical-chemical and chemical surface textures for most of the locations, that is Istirincha + Lechuguillas + El Morro + Palma Sola + Boca Andrea + Villa Rica + Farrallón + La Mancha + Chachalacas, is limited to coverage by the high percentage of flat cleavage surfaces (48 ± 30%), bulbous edges (24 ± 26%) and elongated depressions (10 ± 15%) compared to the rest of the surface textures area coverage.

| Sand petrology
Petrology of the dune sands from the Gulf of Mexico coastal plain is markedly controlled by their closeness to the Trans Mexican Volcanic Belt and the alluvial deposits in the south-eastern area of the coast.The presence of angular to subrounded monocrystalline quartz largely concentrated in the dune sands suggests that fluvial input, and probably aeolian input, increase quartz with angular and subrounded morphologies, respectively (Mendieta-Lora et al., 2018).However, subrounded quartz commonly occurs in coastal areas like Namibia after fluvial transport (Garzanti et al., 2012).Plagioclase, volcanic fragments with lathwork > microlithic > glassy textures and subordinate minerals like ilmenite, andesine, albite, augite and fayalite are derived from intermediate volcanic sources of the Trans Mexican Volcanic Belt (Gómez-Tuena et al., 2007;Mejía-Ledezma et al., 2020;Mendieta-Lora et al., 2018;Negendank et al., 1985).In the study area, plagioclase is commonly embedded within volcanic fragments with lathwork textures but does not occur as single crystals, probably because of the proximity of the source rock.This is similar to volcanic island environments, where plagioclase is more commonly observed in the groundmass of volcanic lithics (Le Pera et al., 2021).The scarcity of olivine grains in the dune sands is probably due to the lack of released monomineralic crystals after rapid fluvial and marine transport.This is suggested by observations from volcanic environments associated with small drainages, where durable retention of olivine occurs in the groundmass of volcanic lithics with glassy textures (Le Pera et al., 2021).This is distinct from the widely held idea that low concentrations of olivine in sands may be a direct consequence of depletion and even elimination of detrital olivine after long subaqueous transport (Garzanti et al., 2015a;Shukri, 1950).Alternatively, the scarcity of olivine grains could be due to their low abundance in the Tans Mexican Volcanic Belt, where differentiated (olivine-poor) igneous rocks are abundant (Gómez-Tuena et al., 2007)  and winds (Mejía-Ledezma et al., 2020;Mendieta-Lora et al., 2018;Figure 8).
In the Gulf of Mexico coastal sands, the clinopyroxene grains are derived from the sub-alkali to alkali basalts and calcalkaline to thoeleiitic basalts originating in the Trans Mexican Volcanic Belt (Ferrari et al., 2005; Gómez-Tuena 5.2 | Surface textures on pyroxene grains

| Interpretation
Collision points and remaining particles probably developed following processes similar to those that led to the formation of elongated depressions and abrasion fatigue surface textures observed in quartz (Immonen, 2013;Krinsley & Donahue, 1968;Kuenen, 1960;Vos et al., 2014) and ilmenite (Makvandi et al., 2015;Mejía-Ledezma et al., 2020) from this and other study areas (Costa et al., 2013;Krinsley et al., 1976).Both surface textures are probably produced by high energy collisions during aeolian transport analogous to the abrasion fatigue surface textures resulting from grinding particles (Costa et al., 2013;Mahaney, 2002;Vos et al., 2014).
Flat cleavage surfaces can form most of the surface texture on detrital clinopyroxene grains due to the high abundance of weak cleavages in these minerals (Le Pera & Morrone, 2020).As also commonly observed in quartz and ilmenite (Cheng et al., 2017;Itamiya et al., 2019;Mejía-Ledezma et al., 2020;Vos et al., 2014), cleavage surfaces observed in the studied clinopyroxenes indicate they are features produced during high-energy fluvial transport and grain-to-grain collisions in the intertidal zone.However, unlike quartz, flat cleavage surfaces in clinopyroxenes are almost exclusively observed in larger gains (>100 μm).
Upturned plates are surface textures typical of aeolian environments (Costa et al., 2013;Krinsley & Cavallero, 1970;Mahaney, 2002), but they are poorly developed in the studied clinopyroxenes.This suggests that rapid aeolian transport occurs from the beach deposits onto the dunes in the study area, without any time to evolve upturned plates as part of a long-term aeolian process.Alternatively, upturned plates (and other small textures) might have developed on the clinopyroxenes before being removed/obscured during the development of flat cleavage surfaces during aeolian transport.This process could hide some textural features commonly developed during vigorous subaqueous and aeolian multicycles of sand transport.A relatively significant average percentage increase in bulbous edges on the clinopyroxene surfaces at the Chachalacas site is a response to the dominantly aeolian transport with the mineral's physical attributes and larger grain sizes >150 μm (Folk, 1980;Garzanti et al., 2015a;Resentini et al., 2018;Vos et al., 2014).Bulbous edges have also been reported in quartz, ilmenite and zircon from aeolian environments and sandstones (Costa et al., 2013;Mejía-Ledezma et al., 2020;Moral-Cardona et al., 2005).
In summary, surface textures on the studied clinopyroxene grains preserve signals acquired predominantly during subaqueous transport in the fluvial to intertidal beach zone environments, with subsequent minor overprints during subaqueous to aeolian transport to coastal sand dunes.Cleavage breakage is common in larger clinopyroxenes and can obscure the details of these signals.
A mix of physical and chemical processes acting on the grains are represented as elongated depressions derived from the grain-to-grain collisions during sand saltation or creeping (Mahaney, 2002).Commonly, they are preserved together with bulbous edges in grains >150 μm in size.
Arcuate/circular/polygonal cracks result from physical or chemical weathering, including the crystallisation of salts (Krinsley & Doornkamp, 1973;Krinsley et al., 1976;Vos et al., 2014).They can be attributed to minor chemical weathering and/or early diagenesis with a short residence time of the grains under intense chemical weathering conditions.In addition, arcuate/circular/polygonal cracks have not been reported in quartz from coastal sand dunes (Vos et al., 2014), which may suggest efficient removal of these textures during the breakage and surface abrasion of grains affected by fast-paced multicycle transportation/ reworking (Mejía-Ledezma et al., 2020).
There is little evidence of incipient microbox and cornflake textures in the clinopyroxenes (Velbel & Barker, 2008), as chemical weathering is negligible despite their vulnerability to chemical alteration (Buchs et al., 2015;Morton & Hallsworth, 2007).Mammillated surface textures likely reflect subaqueous alteration, where surface material was dissolved to form pinnacle features similar to those observed previously on olivine grains (Grandstaff, 1978;Velbel & Ranck, 2008).
Early stages of weathering are evidenced by the preservation of denticulated surface textures, defined as retreating or remnants of initial void spaces (Velbel & Barker, 2008).In addition, little development of imbricated wedge marks on the surface of the studied clinopyroxene grains suggests that the minerals were affected by the dissolution of their original crystal boundaries rather than grain enlargement produced by crystal overgrowths.Similar observations were previously made for detrital garnet (Salvino & Velbel, 1989;Velbel et al., 2007), suggesting this may be a recurring feature of ferro-magnesian minerals not encountered as commonly in quartz.
Although the abundance of lenticular etch pits increases slightly at the Istrincha sampling site, these textures and solution pits are generally negligible on clinopyroxene grains in the study area.Lenticular etch pits typically result from the formation of pore voids, with the development of vermiform corrosion features within resistant material (Velbel, 2007;Velbel & Barker, 2008).In contrast, solution pits occur during chemical weathering occasionally set on imbricated wedge marks (Velbel et al., 2007).
Overall, limited chemical surface textures on the studied clinopyroxene indicates moderate chemical weathering of grains.Instead, the morphology of retrieved grains was mainly controlled by cleavage breakage and other mechanical features developed during the highly energetic fluvial, marine and aeolian transport of their sedimentary cycle.

Implications of pyroxene breakage
The significant correlation between compactness versus sphericity and convexity values suggests that the shape of the clinopyroxene is controlled by its original form, an irregular polygon, rather than being abraded to adopt a circular form.This is because sphericity accounts for a ratio of the area of the particle related to an enclosed and inscribed circle and convexity is the ratio of the particle related to the convex hull (polygon).The low average value of sphericity and the high convexity value suggests an irregular shape for the clinopyroxene grains, closer to a polygon rather than a circle (see Zhao & Wang, 2016; Figure 7A-C).Compactness is not correlated to the equivalent diameter or particle size (R 2 = 0.11).Furthermore, since compactness correlation is higher to convexity (R 2 = 0.59) than to sphericity (R 2 = 0.11), it is clear that compactness measurements in the studied pyroxenes are generally determined by the irregular shape of the grain rather than its approximation to a circle.Conversely, compactness is not correlated to roundness.This is due to the fact that compactness is a measure of the global shape of the grain whereas roundness is a mesoscale shape descriptor that only quantifies the sharpness of the grains.
In detail, compactness measurements combine some of the key morphological characteristics associated with the larger-scale shape of the mineral grains.In this study, this is exemplified by changes in the relative control of sphericity and convexity on the compactness values between two groups of localities.At the Boca Andrea, Istirincha and Palma Sola sites, sphericity is poorly correlated to compactness (R 2 = 0.01-0.08),but these two parameters are correlated at the six other studied localities (Figure S1).In contrast, convexity at the Boca Andrea, Istirincha and Palma Sola sites is systematically better correlated to compactness than at other localities (R 2 = 0.70-0.95vs 0.20-0.63)(Figure S2).This supports stronger control of convexity relative to sphericity in the compactness values at Boca Andrea, Istirincha and Palma Sola.Compactness results show that most clinopyroxenes display low compactness (Table 1), suggesting limited abrasion of their edges and possible retention of their original forms despite their susceptibility to abrasion (Folk, 1980;Resentini et al., 2018).However, low compactness values can also be controlled by cleavages in the grains, which can facilitate breakage, rather than significant smooth abrasion, of clinopyroxenes under both subaqueous and subaerial conditions (Folk, 1980;Le Pera & Morrone, 2020).The preservation of surface textures like flat cleavage surfaces is plausible evidence of clinopyroxene breakage.Some grains might have achieved relatively higher compactness (e.g. at sites VR and CH) probably due to long aeolian transport and minor breakage, because clinopyroxene are abraded faster than most other heavy minerals (e.g.amphibole, garnet and zircon; Resentini et al., 2018).Conversely, unlike the results reported here, the behaviour of clinopyroxene along the coastal sands of South-West Africa draws attention since rounded clinopyroxene endures in beach and dune sands even after a long distance of littoral transport and fluvial control of the Orange River input seaward (Garzanti et al., 2014, figure 2;Garzanti et al., 2018).Alternatively, several studies outline how clinopyroxene endures in different areas with different climates and topographic settings (Garzanti et al., 2021(Garzanti et al., , 2022) ) with evidence of roundness and preservation of chemical surface textures (Garzanti, et al., 2015a).
However, in this study little evidence of abrasion of clinopyroxene grains remains after high energy subaqueous and subaerial transport.Breakage along clinopyroxene cleavage surfaces is probably the main reason for the low preservation of abraded grains.Nonetheless, these results show that angular to subrounded clinopyoxene grains indicate the dominance of river control over aeolian influence on the grains.Finally, compactness is not correlated to the equivalent diameter or particle size (R 2 = 0.11).

| CONCLUSIONS
Heavy minerals in backshore dunes of the Gulf of Mexico coastal plain are mostly clinopyroxenes (augite and diopside) derived from basalts and andesites of the Trans-Mexican volcanic belt.The cleavage properties of clinopyroxenes determines the low compactness values in clinopyroxene grains with breakage under vigorous subaquatic conditions generating minor abrasion along their edges.Flat cleavage surfaces and bulbous edges represent the dominant average surface coverage percentages of mechanical surface textures on retrieved detrital clinopyroxenes.The former texture suggests highly active fluvial transport and grain-to-grain collisions in the intertidal zone.In contrast, bulbous edges were produced by aeolian transport.In addition, slight preservation of elongated depressions associated with bulbous edges in the clinopyroxenes is produced during high energy aeolian transport.Chemical surface textures are almost non-existent on the studied clinopyroxenes except for rare mammillated surface textures derived from dissolution under subaqueous conditions.The limited preservation of chemical surface textures is probably due to fast and energetic transport during their sedimentary cycle, which led to their morphology being dominated by mechanical processes.Notably, these results clearly show that the cleavage properties of these minerals had a significant control on their shape.In this study, large conchoidal fractures on clinopyroxene >100 μm suggests fluvial transport dominates over intertidal control on the grains.However, cleavage may obscure some surface textures initially developed on the clinopyroxenes as the mineral breaks and its surface remains apparently unaltered.The use of the compactness shape descriptor is an alternative approach to understanding clinopyroxene behaviour during its source-to-sink history in coastal dune sands since it considers the overall shape of the grain, that is sphericity and convexity, rather than the measurements of the particles based solely on the sphericity or roundness as individual parameters.Compactness measurements of grain particles in the digital domain should be visually inspected referencing a complete catalogue of images to ensure compactness measurements are consistent with the shape of the grain particles leading to a more objective interpretation of the shape of detrital clinopyroxene grains in coastal dune sands.

ACKNO WLE DGE MENTS
Final draft of the manuscript was completed during a Sabbatical Year at the School of Earth and Environmental Sciences Cardiff University, Wales, United Kingdom.The authors thank PASPA/DGAPA/UNAM for their financial support during the Sabbatical Year's Project 'Preservación del piroxeno en ambientes sedimentarios recientes: Implicaciones geoambientales de su concentración en playas y dunas costeras de México y Nueva Zelandia'.We are truly grateful to Laura Elena Gómez Lizárraga and Carlos Linares López, for Scanning Electron Microscopy and Microprobe Analyses at the Instituto de Ciencias del Mar y Limnología and Laboratorio Universitario de Petrología, Instituto de Geofísica, respectively at the Universidad Nacional Autónoma de México, México.This manuscript has largely benefited from the constructive review by Eduardo Garzanti.

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Dominant north-westerly, south-easterly winds, long shore drift and fluvial network along the Gulf of Mexico coastal plain.Cazones and Actopan are perennial rivers whereas Boca de Ovejas, Palma Sola and El Viejón are intermittent rivers (modified from Kasper-Zubillaga et al., 2022).

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Significant correlations for all sites studied (A) compactness and sphericity, (B) compactness and convexity, and (C) lack of significance between compactness and roundness.Significant correlations ρ < 0.05.F I G U R E 7 Shape properties in clinopyroxene grains; (A) 'pristine' grain form with a relatively larger area (A) and perimeter (p) compared to grains B and C. Note the low compactness (C) and sphericity (S) values in 'elongated' shapes and a constant convexity value (Cvx) (Zhao & Wang, 2016).(B) Relative increase in compactness and sphericity values as the grain is less elongated and more compact.(C) Grain with the highest compactness and sphericity values as the grain approaches a semi-developed hexagonal form.et al., 2007; Negendank et al., 1985; Figure 4C,E).The survivability of clinopyroxene in the dune sands is likely a consequence of the proximity of the source rock and rapid fluvial input seaward although detrital pyroxene can sometimes endure after prolonged subaqueous and subaerial transport (Garzanti et al., 2012).
Table1for the number of samples).A lack of significant correlations Average and SD of compactness, sphericity, convexity and roundness in pyroxene grains (n = 332).
T A B L E 1