Naturally occurring van der Waals materials

The exfoliation of two naturally occurring van der Waals minerals, graphite and molybdenite, arouse an unprecedented level of interest by the scientific community and shaped a whole new field of research: 2D materials research. Several years later, the family of van der Waals materials that can be exfoliated to isolate 2D materials keeps growing, but most of them are synthetic. Interestingly, in nature, plenty of naturally occurring van der Waals minerals can be found with a wide range of chemical compositions and crystal structures whose properties are mostly unexplored so far. This Perspective aims to provide an overview of different families of van der Waals minerals to stimulate their exploration in the 2D limit.


INTRODUCTION
The mechanical exfoliation of graphite to isolate single layers of graphene 1 has been closely followed by intense experimental efforts to exfoliate other layered bulk crystals to achieve a broad catalog of two-dimensional (2D) materials with complementary properties to those of graphene [2][3][4][5] . The exploration of other 2D materials is motivated not only by the need of finding materials whose properties are optimal for a certain specific application but also by the fact that along with this exploratory process one might often find interesting (and sometimes even unexpected) physical phenomena.
The goal of this manuscript is to present an overview, rather than a comprehensive study, of different families of naturally occurring van der Waals materials (many of whom are almost unexplored so far), to provide a starting point for future works on these natural 2D materials and to stimulate further studies on other layered minerals. Moreover, from the study of naturally occurring layered minerals, one might get inspired to develop new synthesis approaches to grow synthetic materials whose structure mimics that of a natural layered mineral family.
In the following, we will introduce illustrative examples of natural van der Waals materials belonging to different mineral families: natural elements, sulfides, sulfosalts, oxides, silicates, phosphates, and carbonates.

NATURAL ELEMENTS
Highly crystalline graphite mineral rocks are found in nature and are currently used by many research groups to fabricate single-layer graphene by mechanical or liquid-phase exfoliation routes. For example, natural graphite source has been employed in some of the seminal graphene papers to unravel unexplored physical phenomena like a new type of quantum Hall effect or the fundamental relationship between the transparency of graphene and the fine structure constant [34][35][36] . Figure 1a shows a schematic of the crystal structure of graphite where the different layers, which are held together by weak van der Waals interaction, are visible. Each layer is formed by covalently bonded sp 2 hybridized carbon atoms arranged in a honeycomb lattice. Figure 1b shows a photograph of a natural graphite piece and 1c displays an optical microscopy image of a few-layer graphene flake extracted from the mineral shown in Fig. 1b by mechanical exfoliation and transferred onto a 285 nm SiO 2 /Si substrate by all-dry transfer [37][38][39] .
Other examples of natural elemental van der Waals materials, although much less abundant than graphite, are native bismuth, antimony, selenium, and tellurium (see Fig. 1d, k). Although bismuth and antimony forms 2D layers (slightly puckered), selenium and tellurium constitute an interesting example of van der Waals material with a quasi one-dimensional structure. In fact, these materials are formed by the van der Wals interaction between neighboring helical wires of Se-ot Te-atoms forming a crystal. Regarding the electrical properties, bismuth is a metal (becomes a direct band gap semiconductor with 0.2-0.3 eV gap in the single-and bilayer limit) 40 , antimony a semi-metal (becomes an indirect band gap semiconductor with 2.28 eV in the singlelayer limit) 41 , and selenium and tellurium are semiconductors with band gaps of 0.31 eV (indirect) and 2 eV (direct) [42][43][44] . Although there are recent works on atomically thin bismuth, antimony, selenium, and tellurium (and the interest of the community in these systems is rapidly growing), to our knowledge, all these works employ synthetic materials as the starting point for the exfoliation or synthesis of the nanolayer systems that they study 40,[42][43][44][45][46][47][48][49][50] . For example, a hydrothermal synthesis method have been used to grow tellurium nanosheets with thickness down to 0.4 nm (one unit cell). These nanosheets were applied in fieldeffect transistors that show a high p-type mobility of up to 600 cm 2 V −1 s −1 and a high anisotropic in-plane electrical transport (anisotropy ratio~1.5) and photodetectors with a responsitivy of 8 A W −1 for 2.4 µm and a cutoff wavelength of 3.4 µm 44,51 .
Selenium nanosheets as thin as 5 nm were also synthesized by vapor transport and they have been applied in phototransistors showing a p-type character with mobilities up to 0.26 cm 2 V −1 s −1 and photoresponsivities reaching 260 A W −142 .

SULFIDES
Molybdenite mineral (with formula MoS 2 ) is very abundant in nature in its 2H polytype, in which the different layers are stacked in ABA manner, and high-quality large crystals are easily available.  1 Elemental van der Waals minerals. a, d, f, h, j Three-dimensional representation of the crystal structure of graphite 146 , native antimony 147 , native bismuth 147 , native selenium 148 , and native tellurium 149 . b, e, g, i, k Pictures of mineral rocks of graphite, native antimony 150 , native bismuth 151 , native selenium 152 , and native tellurium 153 , respectively. c Optical microscopy image of a few-layer graphene flake mechanically exfoliated from the bulk graphite mineral shown in b.
Bulk molybdenite is an n-type indirect band gap semiconductor with a band gap of ≈1.3 eV, which becomes a direct semiconductor with 1.85-1.90 eV gap when thinned down to a single layer 52,53 . Figure 2a-c show, respectively, the crystal structure of 2H-molybdenite, a picture of a piece of 2H-molybdenite mineral, and an optical microscopy image of a flake mechanically exfoliated from the mineral and onto a 285 nm SiO 2 /Si substrate.
Although way less abundant than the 2H polytype, molybdenite can also be found in nature in its 3R polytype. In fact, 3Rmolybdenite is typically found as a micro-mineral (small pieces inside a matrix of another rock). This mineral differs from the 2Hmolybdenite in the way the layers are stack on top of each other, having a ABCA stacking (see a representation of its crystal structure in Fig. 2d). This polytype is interesting because of its particular stacking of the layers that yields to a broken inversion symmetry even in bulk form (while in the case of the 2H polytype only an odd number of monolayers does not present inversion symmetry) 67,68 . The lack of an inversion center symmetry is particular important for valleytronics because it introduces Kvalley dependent optical selection rules. More specifically, the direct interband transitions in the vicinity of the K+ (K−) point of the hexagonal Brillouin zone are coupled to right (left) circular photon polarization states, allowing to control the properties of the material through circular polarized light 67,68 . Nonetheless the experimental works reported so far on 3R-MoS 2 are based on synthetic crystals and not naturally occurring minerals 69,70 . Figure  2e, f show a picture of 3R-molybdenite micro-crystals on the surface of a quartz rock and a optical microscopy image of the resulting mechanically exfoliated 3R-molybdenite flakes, after transferring them to a 285 nm SiO 2 /Si surface.
Tungstenite mineral (with formula WS 2 ) is also found in nature in its 2H polytype as a (rare) micro-mineral. This is probably the reason why although tungstenite is naturally available, most works used synthetic WS 2 instead and only few examples of exfoliation of natural tungstenite have been reported in the literature. Tungstenite, similar to molybdenite, is an n-type indirect band gap semiconductor in bulk (~1.3 eV), which becomes a direct band gap semiconductor when thinned down to a single-layer (with a gap value of 2 eV) 71 . The spin-orbit splitting of the valence band in single-layer tungstenite reaches 430 meV, which is approximately three times larger than in single-layer molybdenite (150 meV), and thus this material is very interesting for the incipient field of spinorbitronics 72-75 . Withers et al. 9 isolated WS 2 flakes with thickness ranging from single-layer up to four layers by exfoliation of natural tungstenite and studied its intrinsic electronic properties by fabricating field-effect devices on hexagonal boron nitride, showing a marked n-type behavior and mobilities up to 80 cm 2 V −1 s −1 at room temperature. Figure 2g-i shows the crystal structure, a picture of a natural tungstenite micro-mineral on the surface of a quartz rock and an optical microscopy image of mechanically exfoliated tungstenite flakes.
Orpiment and anorpiment are two minerals with the same chemical formula (As 2 S 3 ) but differing in their respective crystal lattices. Figure 2j, m display the crystal structure of these two materials, with orpiment belonging to the monoclinic crystal system, while anorpiment is the triclinic dimorph of orpiment 76 . It is worth mentioning that bulk anorpiment mineral was identified only 10 years ago and thus there is still scarce information about   157 , and getchellite 158,159 , respectively. b, e, h, k, n, q, t Pictures of mineral rocks of molybdenite 2H, molybdenite 3R, tungstenite 2H, orpiment, anorpiment, stibnite, and getchellite, respectively. d, f, i, l, o, r, u Optical microscopy images of few-layer flakes mechanically exfoliated from the bulk crystals shown in b, e, h k, n, q, t, respectively. this material, even in the bulk form 77 . In this bulk form, orpiment is an indirect wide band gap semiconductor with band gap in the range of~2.4-2.6 eV and a intense yellow color 76,78 . Recently, Manjón and colleagues 79 studied the vibrations, structure, and electronic properties of orpiment under high pressure, finding that pressure is able to tune its metavalent bonding thus turning orpiment from a semiconductor into an "incipient metal" with promising phase-change, thermoelectric and topological insulating properties. Mortazavi et al. 80 have calculated the mechanical properties and the mobility of orpiment nanosheets reporting a strong anisotropy in the mechanical properties: along one crystal direction orpiment is elastic and brittle, whereas along the perpendicular direction it shows superstretchability, similar to rubber. Interestingly, Steeneken and colleagues 81 isolated a single layer and few layers of orpiment by mechanical exfoliation and tested their strong vibrational and mechanical anisotropy, arising from its crystal structure, finding a Young's modulus of E a-axis = 79.1 ± 10.1 GPa and E c-axis = 47.2 ± 7.9 GPa along the two main crystalline directions 81 . This Young's modulus anisotropy is amongst the largest reported in the literature so far for 2D materials, only below that of black phosphorus 82 . Figure 2k Stibnite has a similar chemical composition to orpiment and anorpiment with Sb atoms replacing the As atoms to form the structure Sb 2 S 3 . Nonetheless, the crystal structure of stibnite strongly differs from that of orpiment and anorpiment. The S atoms are coordinated in a pyramidal fashion around the Sb atoms to form a chain structure with units given by Sb 4 S 6 ribbons (see Fig. 2p). Stibnite, similar to native selenium or tellurium, presents layers, which are not fully covalently bonded inside the plane, and thus their structure resembles more to that of molecular solids like rubrene. This peculiar crystal structure has motivated works studying the in-plane anisotropy of the optical and electronic properties of stibnite and related materials, although these works relied on artificially synthesized material sources rather than in stibnite mineral 83 . In bulk stibnite is a semiconductor with a gap of~1.6-1.7 eV (there is no consensus in the literature about the direct/indirect nature) with interest in photocatalysis and photovoltaics 76,78,[84][85][86] . Figure 2q, r show a picture of a stibnite mineral rock and an optical microscopy image of flakes exfoliated trom the buk mineral.
Getchellite is a relative of the orpiment and stibnite minerals. It has a formula of AsSbS 3 and it has a complex crystal structure that ressembles a mixture between those of orpiment and stibnite. Very recently, Wang et al. 87 reported the isolation of getchellite flakes (45 nm thick) by mechanical exfoliation of synthetic getchellite crystals and their characterization by transient absorption spectroscopy. They found that AsSbS 3 has a direct band gap of 1.74 eV and they determined the mobility of the charge carriers of 200 cm 2 Vs −1 .
Gehring et al. 88 reported the exfoliation of natural kawazulite mineral (with the approximate composition Bi 2 (Te,Se) 2 (Se,S)), a naturally occurring topological insulator. They isolated flakes by mechanical exfoliation with thickness down to few tens of nanometers and fabricated electronic devices with carrier mobility exceeding 1000 cm 2 V −1 s −1 . Moreover, angle-resolved photoelectron spectroscopy show a surface state with the typical Dirac-like conical dispersion, which verifies the topological insulator behavior of natural kawazulite. Athough tin sulfide minerals such as herzenbergite (with formula SnS, a puckered honeycomb structure, and a band gap of~1.0-1.1 eV) and berndtite (formula SnS 2 , a structure similar to that of 1 T MoS 2 polytype, and a band gap of~2.2 eV) are found in nature, we have not found works reported on exfoliation of these natural minerals but on the exfoliation of their synthetic counterparts [89][90][91][92] .

SULFOSALTS
The sulfosalts are a mineral family with a general formula A m B n S p with A being commonly copper, lead, silver or iron, B being semimetals like arsenic, antimony or bismuth, or metals like tin, and S being sulfur. Among the layered sulfosalts, teallite is probably the simplest one. It has a formula of PbSnS 2 and its crystal structure largely resembles that of the compound SnS (a puckered honeycomb lattice very similar to that of black phosphorus but with different atomic species in the unit cell). Figure 3a shows a three-dimensional (3D) representation of the crystal structure of teallite. This particular crystal structure might result interesting for applications looking for semiconducting materials with strongly anisotropic in-plane properties. In bulk, teallite is a semiconductor with~1.6 eV of direct band gap 93 .
Recently, teallite has been mechanically exfoliated and its Raman spectra has been measured as a function of the incident light polarization at different temperatures finding a strong linear dichroism arising from its puckered structure 94 . Shu et al. 95 reported the growth of ultrathin synthetic tellite, as thin as 2.4 nm, by chemical vapor deposition finding a large anisotropy ratio in the charge carrier mobility (1.8) and photoresponse (1.25), and photoresponsivity up to 20 A W −1 and response times of milliseconds. Also, shear force based liquid-phase exfoliation of teallite has been demonstrated achieving suspensions with 11-22 layers in thickness and the exfoliated material has been subsequently employed in electrocatalytic reactions 96 . Figure 3b, c shows a picture of a teallite mineral rock and an optical microscopy image of a teallite flake mechanically exfoliated from the bullk mineral.
Some members of the sulfosalt family are composed of alternating layers with dissimilar chemical composition (i.e., they constitute naturally occurring van der Waals superlattices 97 ). Some illustrative examples of this sub-family are the franckeite, the cylindrite and the cannizzarite.
Franckeite bulk mineral (with an approximate formula Pb 5 Sn 3 Sb 2 S 14 ) has been recently exfoliated down to a single unit cell [98][99][100][101][102] . The crystal structure is composed of segregated Sn-rich, pseudo-hexagonal layers (with SnS 2 -like structure) alternated to Pb-rich, pseudo-tetragonal layers (with PbS-like structure) with a van der Waals gap between them (see Fig. 3d). Molina-Mendoza et al. 98 , Velický et al. 99 , and Ray et al. 101 demonstrated field-effect devices, photodetectors, and solar-cells with mechanically exfoliated flakes. These first works demonstrated that this exfoliated material is a p-type semiconductor with a band gap value of 0.6 eV 98 similarly to what has been reported for bulk franckeite 78 . Liquid-phase exfoliation with different solvents have been used to produce colloidal suspensions of flakes with thickness down to four unit cells 96,98,99,103 . Field-effect devices with performances very similar to that of mechanically exfoliated based devices, that show a strong p-type doping and a field-effect modulation up to a factor of 10, have been demonstrated from the liquid-phase exfoliation franckeite suspensions through dielectrophoresisbased assembly 103 . More recent works characterized the optical properties of exfoliated frankeite nanosheets 102,104 and it has been shown how covalent thiol-ene-like "click" chemistry can be used to decorate franckeite 105 . Also, Frisenda et al. 106 have recently studied the spontaneous symmetry breakdown in frankeite resulting from a spatial modulation of the van der Waals interaction between layers due to the SnS 2 -like and PbS-like lattices incommensurability. In fact, although franckeite superlattice is composed of a sequence of isotropic 2D layers, it exhibits a spontaneous rippling that makes the material structurally anisotropic leading to an inhomogeneous in-plane strain profile and anisotropic electrical, vibrational, and optical properties 106 . Cylindrite (with an approximate formula Pb 3 Sn 4 FeSb 2 S 14 ) is a mineral closely related to franckeite. It received its name because it often occurs as cylindrical crystals made up of rolled sheets. The structure, similar to franckeite, presents alternating segregated Snrich and Pb-rich layers (see Fig. 3g). In bulk cylindrite is a semiconductor with a band gap of 0.65 eV, very similar to that of franckeite 78 . Interestingly, bulk cylindrite presents intrinsic magnetic interactions, compatible with a spin glass-like system, which are assumed to be originated by its iron content 107,108 . Recently, Niu et al. reported the isolation of thin flakes of cylindrite (down to nine to ten layers) by mechanical and liquid-phase exfoliation, finding that the flakes of this material are heavily doped p-type semiconductors with a narrow gap (<0.85 eV). They also found intrinsic magnetic interactions (the magnetization shows an anomaly at~5 K that can be attributed to a slowing down of the spin dynamics in magnetically disordered systems, the hallmark of spin glass-like behavior) that are preserved even in the exfoliated nanosheets. Figure 3h, i show a picture of a cylindrite mineral rock and an optical microscopy image of cylindrite flakes exfoliated from the bulk rock.
Cannizzarite (with an approximated formula Pb 4 Bi 6 S 13 ) is a rare micro-mineral formed by alternating stacks of tetragonal PbS-like and hexagonal layers Bi 2 S 3 like (see the 3D representation of the crystal structure in Fig. 3j) 107,108 . Very little has been reported so far on this mineral, even in its bulk form. Figure 3k is a picture of a quartz rock covered by small micro-crystallites of cannizzarite, these crystals can be lifted up from the surface using a viscoelastic Gel-Film (by Gel-Pak) stamp. Figure 3l shows a cannizzarite flake after picking it up from the bulk rock and transfer it to a SiO 2 /Si substrate by an all-dry deterministic transfer method.

OXIDES
The oxides are minerals in which the oxide anion (O 2− ) is bonded to one or more metal ions. Complex oxides such as silicates, carbonates and phosphates are traditionally classified separately. Figure 4a, b shows an example of one naturally occurring oxide: valentinite (with formula Sb 2 O 3 and orthorhombic structure) occurs as a weathering product of stibnite and other antimonybased minerals. It is a semiconductor with a wide band gap of 3.3 eV (no consensus about the direct/indirect nature of the gap) and dielectric constant of~3 that has recently been used in energy storage applications [109][110][111][112] . Other example of oxide mineral is the birnessite (see Fig. 4c, d), with approximate formula (Na,Ca,K) 0.6 (Mn 4+ ,Mn 3+ ) 2 O 4 ·1.5H 2 O, which has been exfoliated by liquid-phase expoliation technique 13,113 . In bulk, birnessite is a wide band gap semiconductor with an indirect gap at~2.1 eV, a direct gap at~2.7 eV, a dielectric constant of~5 [114][115][116] , and interesting for supercapacitors and photoelectrochemistry applications [115][116][117][118][119][120] . Moreover, there are theoretical predictions that exfoliated monolayer birnessite should exhibit intrinsic ferromagnetism with a Curie temperature of 140 K 109 . We are not aware of other experimental works reporting the exfoliation of other naturally occurring oxide minerals.

SILICATES
Silicates is a family of largely abundant minerals made out of silicate groups. These materials are wide band gap insulators and some of them are typically used as dielectrics for capacitors in the semiconductor industry.

Nesosilicates
Nesosilicates are silicates that have SiO 4 tetrahedra that are isolated and connected by interstitial cations. Kyanite is an illustrative example of nesosilicate mineral with formula Al 2 SiO 5 and intense blue color. Its structure is composed of "staircases" of Al octahedra linked by Si tetrahedra (see Fig. 5a) 121 . It is a wide gap insulator with a direct band gap of~5-6 eV and a dielectric constant of~9 [122][123][124] . High-resolution frictional AFM images of the surface of bulk kyanite has been recently reported. These friction measurements performed along the [001] and [010] directions on the kyanite (100) face provide similar friction coefficients μ ≈ 0.10 121 . Figure 5b shows a picture of a kyanite mineral rock and Fig. 5c is an optical microscopy image of a kyanite flake exfoliated from the bulk mineral.  Phyllosilicates: clays, micas, and chlorites These materials are silicates with a 2 : 5 Si : O ratio. This large family of silicates includes the clays, micas, and chlorites, and all the members of this family are hydrated with either water or hydroxyl groups attached.
Clays are hydrous aluminum or magnesium phyllosilicates. From clays, one can obtain organoclays, in which the original interlayer cations (such as Na + ) are exchanged with organocations (e.g., quaternary alkylammonium ions) leading to a different layer separation. Previous works showed that clay minerals can be exfoliated in liquid phase with the help of large polymers that intercalate in the crystal structure separating the layers. Talc (with formula Mg 3 Si 4 O 10 (OH) 2 ) is an illustrative example of the clays family. It is a wide band gap insulator (direct gap of 5.2 eV) with high dielectric constant (9.4) 125,126 . The isolation of talc nanosheets with thickness down to one single-layer has been recently reported by mechanical exfoliation 125 and liquid-phase exfoliation has been also used to prepare suspensions of talc nanosheets with an average thickness of nine layers 127 . Mechanically exfoliated talc flakes have been also used as substrates or encapsulating layers to fabricate graphene-based electronic devices where the electronic interaction between the talc and the graphene are exploited (the dielectric constant of talc is higher than other encapsulation layers such as hexagonal boron nitride) to modify the performance of the devices 128,129 . Figure 6a-c show the representation of the crystal structure of talc, a picture of a mineral rock, and an optical microscopy image of a exfoliated talc flake, respectively.
Apart from talc, liquid-phase exfoliation has been demonstrated to be a powerful route to isolate suspensions of other members of the clay family 127 . Figure 6d, e shows the crystal structure and a picture of bulk vermiculite (general formula Mg 0,7 (Mg,Fe,Al) 6 (Si, Al) 8 O 20 (OH) 4 ·8H 2 O, an insulator with a dielectric constant of~5) 130 and expanded vermiculite that can be readily exfoliated by liquidphase exfoliation technique 13 .
The mica family is formed by very closely related layered silicates such as muscovite (with formula KAl 2 (AlSi 3 )O 10 (OH) 2 ), biotite (with formula K(Mg,Fe) 3 131,132 . The exfoliation of layered materials with stronger interlayer forces has rarely been reported and mica is one of these few cases. In fact, muscovite mica has been successfully exfoliated down to a single-layer by mechanical exfoliation and their optical properties 10,12 , as well as their mechanical properties have been studied finding an optical method to determine its thickness and a Young's modulus of~200 GPa, higher than that of bulk muscovite (~175 GPa) 11 . Monolayer muscovite mica nanosheets can also be obtained by liquid-phase exfoliation and by weakening its layer attractions (enlarging the basal spacing by intercalation) prior to sonication 133 . Mechanically exfoliated muscovite mica flakes have been used as dielectric to fabricate graphene transistors with improved performance due to the high dielectric constant of mica and the atomically flat surface achieved after mechanical exfoliation 134 . Interestingly, the hydrophilic nature of muscovite mica in combination with the impermeability of graphene has opened up the possibility of studying the structure and the electronic properties of water confined a the mica/graphene interface [135][136][137] . Figure 7 summarizes the crystal structure sketch, optical images of the bulk minerals and microscopy images of the corresponding exfoliated flakes for muscovite, biotite, lepidolite, and fluogopyte micas.
Members of the chlorite family have a great range in composition resulting in a large variety of physical, optical, and X-ray properties. Clinochlore is an illustrative example of the chlorite family. This mineral, with formula (Mg,Fe 2+ ) 5 Al(Si 3 Al) O 10 (OH) 8 , resembles the layered minerals of the mica family but it presents a characteristic greenish color and a more plastic response to bending. To our knowledge, there are no reports of exfoliated chlorite materials in the literature so far. Figure 8 summarizes the crystal structure sketch, the picture of a clinochlore rock, and an optical microscopy image of a mechanically exfoliated clinochlore flake.  13 . e, f Reproduced from ref. 13 , with permission.

PHOSPHATES
Phosphates are minerals that contain the tetrahedrally coordinated phosphate (PO 4 3− ) anion. Vivianite is a hydrated iron phosphate with an approximate formula Fe 2+ Fe 2 Vivianite is known to be sensitive to visible light exposure which leads to a marked change in its color from colorless/pale green to dark green/brown. Bulk vivianite has been used as natural electron donor to effectively dechlorinate a variety of chlorinated organics, the principal and most frequently found contaminants in soil and groundwater that generates significant environmental problems 130 . We also are not aware of any works in the literature reporting the exfoliation of vivianite in atomically thin flakes. Moreover, we could not find experimental reports on the band gap of vivianite but a recent theoretical calculation suggest that vivianite would present an indirect band gap in the range of 3.3-4.6 eV and a paramagnetic ground state 138,139 . Figure 9 summarizes the crystal structure sketch, the picture of the vivianite rock and an optical microscopy image of a mechanically exfoliated vivianite flake.

CARBONATES
Carbonates are the minerals containing the carbonate ion (CO 3 2− ). Malachite is a copper carbonate hydroxide with a formula Cu 2 CO 3 (OH) 2 . It has a dielectric constant on~7 but we could not find information about its band structure 124 . Figure 10 summarizes the crystal structure sketch, the picture of the malachite rock, and an optical microscopy image of a mechanically exfoliated malachite flake. Unfortunately, little can be found in the literature, apart from its crystal structure 140 , even for the properties of bulk malachite. It is noteworthy that although this overview of natural van der Waals minerals is most likely far from being complete, as we expect that there are many other layered mineral families not discussed in this perspective, we believe that it will constitute a good starting point to motivate the scientific community working on 2D materials to study these natural materials. Table 1 summarizes the main information highlighted over this perspective for the different minerals discussed here.

CONCLUSIONS AND DISCUSSION
In summary, we provided an overview over different mineral families containing members with layered structure that can be exfoliated by mechanical exfoliation. We discussed about the basic electronic and structural characteristics of these materials and we illustrate how thin flakes can be prepared by mechanical exfoliation of the bulk minerals. Most of the minerals discussed in this Perspective are easily available in specialized mineral shops or online auction sites at reasonable price (typically <50$ per mineral) making it easier the access to exotic van der Waals materials whose synthetic counterparts might be much more expensive (typically~500$ per crystal) or even not available for purchase (e.g., some sulfosalts). The study of naturally occurring layered materials, however, can present some challenges. One of the most important ones is the purity/quality of the natural van der Waals mineral as compared to the syntheric counterpart. One might naively think that natural minerals are more prone to have impurities than synthetic ones. However, in the literature we can find two clear counterexamples: plenty of high-quality results for graphene and MoS 2 are obtained with mechanically exfoliated graphite and molybdenite. In the case of MoS 2 , e.g., the highest mobility reported to date has been obtained with MoS 2 flakes extracted from natural molybdenite (SPI source) 6 . We believe, however, that minerals with a complex structure and chemical composition (e.g., the sulfosalts) will suffer more from uncontrolled impurities than their synthetic counterparts. Another important issue will be to systematically study the optical and electrical properties of mineral coming from different mines to   assess the impact of the parenting mineral source on the properties of the exfoliated 2D materials. We would like to note that this is a general important issue, not specific of naturally occurring van der Waals materials but of 2D materials research as a whole. Indeed, one would expect that the source of the parenting bulk layered material (even synthetic ones) might have a strong impact in the properties of the exfoliated flakes produced from it. Systematic studies trying to correlate the properties of exfoliated materials produced from different sources are still scarce [141][142][143] . Many of these layered minerals are almost unexplored so far and we believe that this overview can constitute a necessary first step to trigger further works on exfoliation of naturally occurring layered minerals to produce 2D materials.

Materials
The minerals used in this work come from the private collection of A.C.-G. Unfortunately, we cannot track back the original supplier for all the materials, as these pieces have been gathered along more than 10 years, in mineral shops around the world and online mineral auctions (e.g., eBay or e-Rocks).
Exfoliation of the minerals Mineral bulks were mechanically exfoliated with Nitto tape (Nitto SPV 224) and then transferred onto Gel-Film (Gel-Pak, WF 4× 6.0 mil), which is a commercially available polydimethylsiloxane substrate. The resulting flakes on the surface of the Gel-Film were transferred onto a SiO 2 /Si substrate (with 285 nm of SiO 2 capping layer) by means of an all-dry deterministic placement metod 37 .
Optical microscopy imaging of exfoliated flakes Optical microscopy images have been acquired with two upright metallurgical microscopes a Motic BA MET310-T and a Nikon Eclipse CI equipped with an AM Scope mu1803 and a Canon EOS 1200D camera, respectively. 3D representation of the crystal structures The 3D representations on the crystal structures included in the figures along the manuscript were produced with VESTA software 144 using the crystallographic data from the cited references in the figure captions. It is noteworthy that most of the crystallographic data of those references can be directly found in form of.CIF files in the American Mineralogist Crystal Structure Database, making straightforward its 3D representation 145 .

DATA AVAILABILITY
Data are available on request from the authors.