geosciences Article Mineralogical and Chemical Investigations of the Amguid Crater (Algeria): Is there Evidence on an Impact Origin? Gian Paolo Sighinolfi 1, Maurizio Barbieri 2,* , Daniele Brunelli 1 and Romano Serra 3 1 Dipartimento di Scienze Chimiche e Geologiche, Università di Modena e Reggio Emilia, 41125 Modena, Italy; giampaolo.sighinolfi@unimore.it (G.P.S.); [email protected] (D.B.) 2 Dipartimento di Scienze della Terra, Università la Sapienza, 00185 Roma, Italy 3 Dipartimento di Fisica e Astronomia, Università di Bologna, 40126 Bologna, Italy; [email protected] * Correspondence: [email protected] Received: 14 December 2019; Accepted: 16 March 2020; Published: 18 March 2020 Abstract: Mineralogical and chemical investigations were carried out on intra-craterial bedrocks (Lower Devonian sandstone) and regolithic residual soil deposits present around the Amguid structure, to discuss the hypothesis of its formation through a relatively recent (about 0.1 Ma) impact event. Observations with an optical microscope on intra-craterial rocks do not unequivocally confirm the presence of impact correlated microscopic planar deformation features (PDFs) in quartz crystals. Field observations, and optical and instrumental analysis (Raman spectroscopy) on rocks and soils (including different granulometric fractions) do not provide any incontrovertible pieces of evidence of high energy impact effects or products of impact (e.g., high pressure—temperature phases, partially or totally melted materials, etc.) either in target rocks or in soils. A series of selected main and trace elements (Al, Fe, Mg, Ni, Co and Cu) were analysed on rocks and soils to evaluate the presence in these materials of extraterrestrial sources. Comparative chemical data on rocks and soils suggest that these last are significantly enriched in Fe-poor Mg-rich materials, and in Co, Ni and Cu, in the order. A large number of EDAX-SEM analyses on separated soil magnetic particles indicate an abnormally high presence of Al-free Mg-rich sub-spherical or drop-like silicate particles, showing very similar bulk chemistries compatible with forsterite olivine. Some particles were found associated with a Ni-rich iron metal phase, and this association suggests a specific extraterrestrial origin for them. Electron microscope analysis made on a large number of soil magnetic particles indicates that 98% of them are terrestrial phases (almandine garnet, tourmaline and Fe-oxides, in abundance order), whereas, only a few grains are of questionable origin. One of the Mg-rich silicate particles was found to be a forsterite (Mg = 0.86) Mn-rich (MnO: 0.23%) Cr-free olivine, almost surely of extraterrestrial sources. Electron microprobe analysis of three soil particles allowed identification of uncommon Cr-rich (Cr2O3 about 8%) spinels, poorly compatible with an origin from terrestrial sources, and in particular from local source rocks. We propose a specific extraterrestrial origin for sub-spherical olivine particles characterised by quite similar magnesian character. Excluding any derivation of these particles from interplanetary dust, two other possible extraterrestrial sources should be considered for them, i.e., either normal micrometeorite fluxes or strongly un-equilibrated, or the Vigarano type Carbonaceous (CV) chondrite meteorite material. In this case, further studies will confirm an impact origin for Amguid, as such magnesian olivine components found in soils might represent the only remnants of a vaporised projectile of ordinary non-equilibrated meteoritic composition. Keywords: impact crater; chemical data; rocks; soil; instrumental analysis. Geosciences 2020, 10, 107; doi:10.3390/geosciences10030107 www.mdpi.com/journal/geosciences Geosciences 2020, 10, 107 2 of 15 Geosciences 2019, 9, x FOR PEER REVIEW 2 of 15 1. Introduction Europeans discovered the circular structure known as the Amguid crater (26°05’17” N, 04°23’49” Europeans discovered the circular structure known as the Amguid crater (26 05 17 N, 04 23 49 E, Figure 1) in 1948 and it was confirmed from an aircraft in 1954. Jean‐Philippe◦ le0 Franc00 made◦ 0 the00 E, Figure1) in 1948 and it was confirmed from an aircraft in 1954. Jean-Philippe le Franc made the first first scientific description of it in 1969 [1]. One of the best‐preserved impact craters on Earth—the scientific description of it in 1969 [1]. One of the best-preserved impact craters on Earth—the Amguid Amguid crater in Algeria—is also one of the hardest to access crater in Algeria—is also one of the hardest to access (https://www.wondermondo.com/amguid-crater/): (https://www.wondermondo.com/amguid-crater/): FigureFigure 1.1. AmguidAmguid crater,crater, Algeria.Algeria. OwingOwing toto thethe toughtough accessibility,accessibility, relativelyrelatively fewfew visitorsvisitors havehave reachedreached thethe areaarea soso far.far. Hence,Hence, relativelyrelatively fewfew scientificscientific studiesstudies werewere publishedpublished afterafter itsits discovery.discovery. TheThe originorigin ofof thethe structurestructure isis correlatedcorrelated to to an an impact impact event event proposed proposed by Lambert by Lambert et al. [et1 ].al. The [1]. theory The oftheory an impact of an event impact is assumed event is byassumed some macroscopic by some macroscopic structural structural and morphological and morphological features of features the exposed of the crater exposed rocks crater and rocks evidence and inevidence them of in microscopic them of microscopic planar deformation planar deformation features (PDFs) features in mineral (PDFs) phases in mineral (quartz) phases present (quartz) in the possiblepresent in target the possible rocks, correlated target rocks, to shock correlated metamorphic to shock e ffmetamorphicects [1]. effects [1]. DuringDuring anan ItalianItalian expeditionexpedition mademade inin FebruaryFebruary 2011,2011, aa seriesseries ofof “in“in situ”situ” observationsobservations werewere dedicateddedicated toto findingfinding evidenceevidence ofof thethe possiblepossible impactimpact event.event. To thisthis aim,aim, wewe alsoalso collectedcollected aa setset ofof materials,materials, includingincluding substratesubstrate intra-craterintra‐crater rocksrocks andand soilsoil samplessamples fromfrom thethe surrounding surrounding area. area. ThisThis studystudy aimedaimed toto ascertainascertain thethe impactimpact originorigin ofof thethe structurestructure and,and, byby thethe meansmeans ofof mineralogicalmineralogical andand chemicalchemical datadata onon thethe collectedcollected materials,materials, shedshed lightlight onon thethe possiblepossible modemode ofof impactimpact andand naturenature ofof thethe impactorimpactor [[2].2]. 2.2. GeneralGeneral InformationInformation onon thethe AmguidAmguid StructureStructure TheThe morphologicalmorphological andand structuralstructural featuresfeatures ofof thethe AmguidAmguid structurestructure werewere describeddescribed inin detaildetail byby LambertLambert etet al.al. [[1].1]. TheThe 550550 mm [[3]3] crater crater is is formed formed in in Lower Lower Devonian Devonian sandstones. sandstones. ItIt hashas anan elevatedelevated rim of up to 65 m [3,4]. The bottom of the crater (observation on February 2011) is partially filled by very bright, and fine‐grained eolian silts and sands (Figure 2), and in part, is covered by detrital materials composed of fall‐back breccias, also containing a fine‐grained component. The near‐perfect Geosciences 2020, 10, 107 3 of 15 rim of up to 65 m [3,4]. The bottom of the crater (observation on February 2011) is partially filled Geosciencesby very 2019 bright,, 9, x FOR and PEER fine-grained REVIEW eolian silts and sands (Figure2), and in part, is covered by detrital3 of 15 materials composed of fall-back breccias, also containing a fine-grained component. The near-perfect preservationpreservation state state of of the the crater crater led led Lambert Lambert et et al. al. [1] [1 ]to to estimate estimate a a presumed presumed age age of of 0.1 0.1 Ma, although in in reality, itit isis probablyprobably less.less. FigureFigure 2. Image 2. Image of the of the Amguid Amguid structure structure (intra (intra-crater‐crater soils soils location). location). ThereThere are are a series a series of of distinct distinct sandstone sandstone bed bed outcrops outcrops on on the internal wallswalls ofof thethe crater, crater, with with a a dip dipthat that becomes becomes progressively progressively steeper steeper in the in upperthe upper parts parts of the of walls. the Accordingwalls. According to Belhai to and Belhai Sahoui and [ 4], Sahouithe upturned [4], the upturned sandstones sandstones and overturned and overturned strata observed strata at observed the North-North-West-West at the North‐North‐ (NNWW)West‐West and (NNWW)South-South-East and South (SSE)‐South parts‐East of (SSE) the elevated parts of rim the are elevated consistent rim with are macroscopic consistent with impact macroscopic deformation impactfeatures. deformation According features. to Lambert According et al. [1 to], furtherLambert evidence et al. [1], for further an impact evidence origin for of an Amguid impact isorigin provided of Amguidby up tois provided three sets by of PDFup to textures three sets in quartzof PDF crystals textures from in quartz intra-crater crystals rocks. from intra‐crater rocks. 3. Foundations3. Foundations of ofthe the Present Present Investigations Investigations and and Studied Studied Materials Materials TheThe complete complete absence absence