Lingunite-a high-pressure plagioclase polymorph at mineral interfaces in doleritic rock of the lockne impact structure (sweden)

ABSTRACT Lingunite nanocrystals and amorphous plagioclase (maskelynite) are identified at the contacts between augite and labradorite wedge-shaped interfaces in the doleritic rocks of the

Lockne impact structure in Sweden. The occurrence of lingunite suggests that the local pressure was above 19 GPa and the local temperature overwhelmed 1000 °C. These values are up to 10

times higher than previous values estimated numerically for bulk pressure and temperature. High shock-induced temperatures are manifested by maskelynite injections into microfractures in

augite located next to the wedges. We discuss a possible model of shock heterogeneity at mineral interfaces, which may lead to longer duration of the same shock pressure and a concentration

of high temperature thus triggering the kinetics of labradorite transformation into lingunite and maskelynite. SIMILAR CONTENT BEING VIEWED BY OTHERS THE FORMATION OF IMPACT COESITE Article

Open access 06 August 2021 POIRIERITE, A DENSE METASTABLE POLYMORPH OF MAGNESIUM IRON SILICATE IN SHOCKED METEORITES Article Open access 22 January 2021 THE RESPONSE OF ZIRCON TO THE EXTREME

PRESSURES AND TEMPERATURES OF A LIGHTNING STRIKE Article Open access 15 January 2021 INTRODUCTION Shock localization is an important issue in many geoscientific fields. _In situ_ shock

localization can be determined, on a large scale by seismic investigations1,2,3, and at the micrometer scale by using petrological imprints in minerals and rocks4,5,6. For instance, extreme

shock deformations conserved in meteorite impacts can be uncovered through identification of high-pressure polymorph transformations and microstructures such as, the shock-induced

graphite-diamond phase transition in gneiss from the Ries Crater, Germany6. Theoretical studies suggest that the propagation of shock waves across heterogeneous interfaces, such as the

interface of different minerals with impedance contrast, may lead to longer duration of the same pressure and higher magnitude of temperature, thus, promoting the kinetics of nucleation of

the high-pressure polymorph6,7,8. The high pressure-temperature (P-T) phase transitions in plagioclase are especially sensitive to monitor shock-induced phase transitions in stony

meteorites, which resemble in their composition to terrestrial mafic rocks. In particular, lingunite, a high P-T polymorphs of sodic plagioclase has been observed in stony

meteorites9,10,11,12,13 or synthesized in anvil cells by compressing sub-millimeter sized material14,15,16. Transformation of plagioclase into lingunite is accompanied by amorphization of

the plagioclase17. It is noteworthy that, even though there are 188 confirmed impact structures on Earth18, many with feldspar bearing target rocks, there is only one short report on

silicate-hollandite in a target rock of the Manicougan impact structure19. To our knowledge, transformation of plagioclase into high pressure polymorphs, due to reverberations of shock-wave

at heterogeneous mineral interfaces has not been reported so far. Here we report lingunite and amorphous labradorite (maskelynite) formed probably due to concentration of shock induced T in

shocked doleritic basement rocks at the Lockne impact structure, Sweden. This is one of the rare reports on high P-T phase transitions of plagioclase into lingunite at heterogeneous mineral

interfaces in impactites. GEOLOGICAL SETTING AND SAMPLE DESCRIPTION The Lockne impact structure (c. 14°40′E, 63°00′N) is a well-known impact structure with c. 7.5 km wide inner crater (Fig.

1). The Ordovician (455 Ma) impact occurred in a marine environment with Proterozoic metavolcanic, granitic and doleritic basement rocks overlain by Cambro-Ordovician shale and limestone as

target rocks20. Numerical modelling indicated formation of the Lockne crater in a 500–700 m deep sea due to a c. 600 m wide projectile, travelling at 15 km/s from the east at 45° from the

horizontal21,22,23. Numerical estimates reveal that in 4 to 10 km from the crater centre, the bulk shock pressure and temperature (P-T) range between 0.1 to 3 GPa and 0 to 127 °C21. Chromite

grains discovered in middle Ordovician limestone in south Sweden were interpreted to represent relics of a fossil L-chondritic meteorite related to a major parent body breakup in the

asteroid belt at Ordovician time24,25. Fragments of this global event enhanced the meteorite flux on Earth for several orders of magnitude in the late solar system history24,25. RESULTS

EVIDENCES OF SHOCK-INDUCED DEFORMATION The dolerites at Lockne impact structure consist of olivine, augite, labradorite and bytownite (Fig. 2). A peculiar microstructure composed of several

alternating augite and labradorite wedges (white frame in Fig. 2) is abundant (11 out of 14 dolerite samples; Fig. 1). Such complex, comb like structures have neither been observed at grain

boundaries of the other silicates (e.g. between olivine and plagioclase), nor in unshocked rocks from more distant doleritic rocks in the surrounding of the Lockne impact structure e.g.,26.

Figure 3 presents detailed SEM observations of the area of the white rectangle in Fig. 2. Boundaries of both labradorite and augite wedges are slightly curved near the labradorite grain

(Fig. 3a). Microfractures in augite wedges are at high angles to the wedge shaped structures and are filled with labradorite from adjacent wedges (s. the curved arrow Fig. 3b). This feature

is similar to injected labradoritic melt into shock induced fractures in pyroxene described from shocked Shergottites27. Microfractures, which are interpreted to be shock-induced, occur in

augite as well as in olivine grains (Fig. 2) cf.61. Augite and labradorite grains and their corresponding wedges have comparable chemical composition (Supplementary Table S1). The chemical

composition at the labradorite wedge interfaces represent a mixture of augite and plagioclase (Supplementary Table S1). This is due to beam overlap, and because electron exited volume is

larger than the electron beam size in SEM-EDX analysis. Therefore we applied additional Raman and high-resolution transmission electron microscopy (HRTEM) studies to investigate the

structural and crystallographic changes at the wedge margins. EVIDENCES OF HIGH-PRESSURE TRANSFORMATION IN PLAGIOCLASE Raman spectra from augite, labradorite and their wedge interface were

acquired for evidence of phase and/or structural transformation (Fig. 4a). The spectra from bulk labradorite and augite minerals are comparable to that of labradorite and augite reported

elsewhere cf.28,29,30 and are in good agreement with our petrographical results (Fig. 2). The spectrum from the augite-labradorite wedge interface shows active bands corresponding to both

labradorite and augite (Fig. 4a)28,29,30. This is because the wedges are dipping with respect to the tilted thin-sections and the laser beam excited both the labradorite as well as augite

below the surface. Beside these bands, the spectrum from the interface shows additional bands at 670, 765, 820 and 926 cm−1. The intense 820 cm−1 and weak 670 cm−1 bands are neither

characteristic of augite nor labradorite cf.28,29,30. However, these bands compare well with the _K_-lingunite spectrum reported31 by Liu _et al._31 (Fig. 4a; Supplementary Figure S2). The

820 cm−1 band is most intense in the lingunite spectrum and varies between 760 and 900 cm−1. The band, characteristic of SiO6 octahedral stretching vibrations, corresponds to splitting of

A1g mode31. The broad shape of the 820 cm−1 band may be attributed to either a deformed crystal lattice and/or semi-amorphous material with many, variably oriented nanocrystals32. Such

changes may also contribute to the weak and broad 765 cm−1 band, as it is the strongest band of the undeformed lingunite31 and compares closely, within few wave numbers, to that of

KAlSi3O8-hollandite (_K_-lingunite) observed in Sixiangkou L6 chondrite9. The broad 926 cm−1 is similar to that of either diaplectic glass33 or maskelynite34. However, the spectrum of

diaplectic glass and maskelynite are very similar33,34. Thus, Raman spectrum alone cannot discriminate between diaplectic glass and maskelynite and additional HRTEM analysis is employed.

Further important evidence for impact-induced deformation in the labradorite wedge is manifested by the evolution of 282 cm−1, 480 cm−1 and 508 cm−1 bands (Fig. 4b). These bands are assigned

to the external lattice modes, a mixed Si-O-Si (or Si-O-Al) bending/stretching, Ag vibrational mode respectively28, is studied as a function of distance from the bulk labradorite grain

(Fig. 4b). The fact that the three active bands are absent in the typical Raman spectrum of shock-densified anorthite glass (cf. Reynard _et al._34) indicates that, the wedges may be

composed of crystalline labradorite and amorphous labradorite may be present only along the wedge interface. Firstly, the bands show a gradual and systematic shift either to a higher or a

lower value on moving towards the wedge tip (Fig. 4b), which may be attributed to anisotropic straining, common in triclinic minerals such as labradorite35. Secondly, the bands broaden

towards the wedge tip (Supplementary Table S2); for example the full width at half maxima (FWHM) of the bands near the wedge tip (pt. 3 in inset of Fig. 4b) is 12 to 15% higher than that

near the bulk labradorite grain (pt. 1 in inset of Fig. 4b). Similar to the shifts in active bands, the shoulder at 250 cm−1 also demonstrates shifts to lower value (Fig. 4b). The gradual

broadening and shift may imply that the labradorite in the wedge is deformed and the deformation increases towards the wedge tip. In this context, it is important to note that Liu _et al._31

reported deformed crystal lattice showing similar Raman spectrum (Supplementary Figure S2) from static experiments. Obviously, after loading and release in the static experiments, the

crystal lattice gets sufficient time to relax back elastically to its original state. In contrast, the high-rate meteorite shock impact provokes rapid plastic deformation6,7. The crystal,

thus, retains some residual strain, which is represented in the present Raman results, for example the position and broad shape of the 820−1 cm band (Figs 2 and 4a). To validate these

results HRTEM investigations were done. NANOSCALE DEFORMATION AT AUGITE-LABRADORITE WEDGES CONTACT ZONE Further structural investigations were done at the nm-scale using HRTEM. Such

investigations may answer the question “What type of shock induced lattice defects, if present, accompany the deformation in lingunite and labradorite?” For this purpose focused ion beam

(FIB) sections prepared from bulk labradorite, as shown in Fig. 3a, are compared with those prepared from the labradorite wedge and the labradorite-augite wedge contact zone. The HRTEM image

of the bulk labradorite displays groups of individual atoms (Fig. 5a). The corresponding Fast Fourier Transformation (FFT) pattern (Fig. 5b) contains sharp spots whose reciprocal allocation

is in good accordance with the interplanar distances reported for labradorite36. Zone axis of the FFT pattern, i.e., the axis along the incident electron beam, compares closely with the

[−212] labradorite axis. The Inverse Fast Fourier Transformation (IFFT) pattern of {101} lattice planes of labradorite, which are also the active deformation (sliding) planes37, demonstrates

(101) lattice fringes without any amorphous polymorph in the bulk labradorite (Fig. 5c). Absence of amorphous polymorph implies that the FIB preparation did not produce any artefacts such

as material amorphization. In contrast to the well-resolved atomic domains in bulk labradorite (Fig. 5), the labradorite wedge demonstrates superposed and overlapping domains (Fig. 6). The

[010] zone axis is normal to the TEM section, i.e., along the incident electron beam (Fig. 6). The corresponding FFT pattern is composed of elongated diffraction spots and a diffuse

background (Fig. 6b). Firstly, the streaking shape of diffraction spots is quite common due the double diffraction38,39. For example, in the case of very perfect single crystals or

overlapping regular arrays of precipitates the diffraction symmetry conditions may be broken. As a result, the multiple electron scattering appears in from of forbidden satellites or double

diffracted spots located outside the main Laue spots. The presence of closely spaced twins may as well provoke streaking of the diffraction spots38,39. In this situation, the additional

spots should be accompanied by diffuse streaks, which also commonly appear outside the main Laue spots. In the case of the labradorite wedges, the additional spots are very weakly developed

(Fig. 6b). Furthermore, the material from the labradorite wedges is not a perfect single crystal but characterized by deformation features in form of shifted bands (Fig. 4) and overlapped

nano-domains (Fig. 6a). Therefore, double diffraction may not be the main reason responsible for the spot streaking. However the presence of twinned regions within the labradorite wedges

cannot be completely ruled out. Finally, the steaking of diffraction spots or the so called Laue asterism is commonly associated with the occurrence of continuous misorientations in the

strained crystals subdivided into sets of subgrains38. The asterism were abundantly observed in a broad range of materials including deformed metals38,39,40 and impactites41,42,43,44. For

instance, using micro X-ray diffraction (μXRD), Flemming44 observed similar diffraction asterism in strained quartz grains from La Malbaie quartzite, Quebec and in shocked clinopyroxenes

from Shergottite meteorite, NWA 3171. It appears reasonable to conclude that in case of the labradorite wedge (Fig. 6b), the distortion of the diffraction spots may be attributed to the

deformation-related asterism. This implies certainly that the labradorite in the wedges is much more deformed by shock metamorphism than the bulk labradorite (cf. Fig. 5b). This statement is

consistent with their electron optical appearance displaying mosaic domains (Fig. 6a). Furthermore, contrary to the undeformed bulk labradorite (Fig. 5c), the IFFT pattern of labradorite in

the wedges shows numerous edge dislocations within (101) planes (Fig. 6c) and confirms its deformed state. At the labradorite - augite wedge interface, the well-ordered augite crystal is

bordering amorphous matrix (Fig. 7a). Randomly oriented, 60 to 150 nm2 in size, nanocrystals are embedded in the amorphous matrix (Fig. 7a). These nano-crystals are rounded with serrated

grain boundaries, suggesting partial melting after crystallization. Augite demonstrates perfectly aligned planes, 2.98(3) Å from each other. This distance is in accordance with values

published for (−221) planes of augite36. Perfectly crystallized augite planes imply that the FIB preparation did not produce artefacts. We, therefore, used augite as an inner reference

material to investigate various phases of the nanocrystals. Figure 7b is a FFT pattern from the area containing nanocrystals embedded in the amorphous matrix. Here the diffuse Debye-Scherrer

rings are associated with a diffuse halo that indicates a crystalline and an amorphous (glass-like) material. Using augite interplanar distance of 2.98(3) Å as a reference, we obtained

Bragg diffraction rings corresponding to the interplanar distances of 2.62(3), 2.26(3), 1.73(3) and 1.33(4) Å. These distances are comparable to the interplanar distances between (101),

(211), (411) and (611) planes of lingunite10,13,15,16. The occurrence of lingunite in the contact zone is also confirmed by a selected area electron diffraction pattern (Supplementary Figure

S5, Table S3). Unlike the perfectly aligned (−221) planes of augite, (101) planes of lingunite show numerous edge dislocations (cf. Fig. 7a,c). Similar dislocations in (101) planes are

common in the labradorite wedge (Fig. 6c). The HRTEM results, revealing more intensely deformed labradorite in the wedge as compared to the bulk (Figs 5, 6, 7) are in a good agreement with

our Raman results (Fig. 4a). Randomly oriented and deformed lingunite nanocrystals embedded in an amorphous matrix (Fig. 7a) explain the broadening and shift of Raman bands (Fig. 4b). It is

significant to note that the lingunite nanocrystals and amorphous labradorite are detected only in a zone along the augite-labradorite wedges contact and not in the middle of the wedge or in

the bulk labradorite grain (Figs 5 and 6). Origin of the lingunite nanocrystals is, therefore, related and restricted to processes active at the augite-labradorite interface. DISCUSSION The

dolerites from the basement of the Lockne Impact structure are part of the Central Scandinavian Dolerite Group45. Previous workers have studied this dolerite in extensive

detail46,47,48,49,50, and few reports have suggested that the dolerite experienced thrust related very low grade metamorphism26,51. Greiling _et al._26 studied the same, but unshocked,

Mesoproterozoic dolerite dykes from Orkulla, Sweden, about 200 km north from the Locke impact structure. However, none of these studies report the pyroxene-plagioclase wedges reported in

present study. Therefore, it is very unlikely that the augite-plagioclase wedges were a result of magmatism or metamorphism. Instead, we suggest that they formed due to passing of the

shock-waves. Occurrence of labradorite in microfractures within augite wedges (Fig. 3b) strongly suggests that the shock-wave induced injection of molten labradorite into the fractures of

augite. Previous studies on shocked L-chondrite and SNC meteorite52 and Shergottites27 elucidate that such microfractures develop during the decompression. Since the labradorite wedges are

barren of microfractures (Fig. 3), it is reasonable to presume that the labradorite melt injection solidified during the decompression stage. This idea of shock-induced transformations is

strongly supported by the observation of high-pressure plagioclase phase lingunite and labradorite glass (Figs 6 and 7). Transformation of plagioclase (labradorite) at the interface of

minerals with contrasting mechanical properties may be explained by the processes leading to longer duration of the high pressure regime and shear induced melting5,6,7,8,52. Theoretical

studies suggest that the propagation of shock waves across heterogeneous interfaces, such as the contact zone of minerals with impedance contrast, may lead to longer duration of the same

pressure, and higher magnitude of temperature thus promoting the kinetics of nucleation of the high-pressure polymorph7,8. In the present case the augite-labradorite interface representing a

heterogeneity, due to a large contrast in mechanical strength, ~15%, between augite and labradorite (Poisson’s ratio are 0.293 and 0.243, respectively), facilitate the formation of

lingunite and labradorite glass at a very small scale (Figs 6 and 7). It is a well-known fact that the duration of shock-wave is much longer in natural impact than in experiments7. Impedance

contrast leading to shock wave heterogeneity assisted in melting of labradorite. We predict that during the shock-wave imparted compression, labradorite melt was injected in the

pre-existing augite fractures forming alternating augite-labradorite wedges. Sharp and de Carli7 predict a similar situation where, due to shock heterogeneity former open cracks would become

larger melt veins. Moreover, similar injections of melt veins are also reported from Shergottites27. The labradorite melt, injected in the pre-existing augite fractures, solidified into

glass and lingunite nano-crystals during the decompression. The labradorite glass has a Raman spectrum similar to both maskelynite and diaplectic glass. However, the labradorite glass lacks

features such as cleavage, cracks and shock induced fractures, which are characteristic of diaplectic glass52. Similar to the offshoots of maskelynite into shock-induced fractures of

neighbouring olivine in Shergottites52, shock-induced fracture wedges in augite are filled with labradorite glass (Fig. 3b). Such offshoots are highly uncharacteristic of diaplectic glass.

We may thus conclude that the glass in the labradorite wedges and shock-induced fracture of augite is maskelynite. Maskelynite was first defined by Tschermak53,54. As suggested by Chen and

El Goresy52, and El Goresy _et al._27, maskelynite is not diaplectic plagioclase glass formed by solid-state transformation, but a dense quenched glass. We suggest that, shock-impedance at

mineral interfaces, with stark difference in densities, may induce reverbrations of the shock-waves at the interfaces, producing high temperatures at same high pressure magnitude leading to

melting of plagioclase. This was followed by injection of the dense liquid in shock-induced fractures in neighbouring augite followed by quenching under high pressure. The presence of

nanometer-sized lingunite islands in the labradorite glass strongly suggest longer duration in the decompression stage thus allowing lingunite nuclei to crystallize. Kubo _et al._17 discuss

that the breakdown/transformation of plagioclase into either a high P-T crystalline or amorphous polymorph depends upon the kinetics in such a way that, when the transformation into high P-T

crystalline polymorph is sluggish, plagioclase transforms into amorphous matter. A similar temperature difference is predicted between melt vein and host rock of the Tenham L6 chondrite55.

Here, extremely high temperatures in the veins triggered the formation of lingunite nanocrystals embedded in an amorphous matrix55. It has been shown that KAlSi3O8 and NaAlSi3O8 transform at

1026 °C into lingunite at 12 GPa and 21 to 24 GPa, respectively9,14,15. Previous numerical estimates predict that, in the studied area, 4 to 10 kms from the crater centre, the shock P-T

range between 0.1 to 3 GPa and 0 to 127 °C21. Sharp and de Carli7 suggest that, the initial peak pressure, and therefore the shock temperature, in the shock front can vary by as much as an

order of magnitude from grain to grain or even within a single mineral grain, depending on details of the local environment. Generally theoretical dynamic estimates and numerical modelling

never predicted or discussed the possibility of selective shock-induced melting, fractional crystallization of dense monomineralic phases and quenching of high-pressure liquid

products27,35,56,57,58. The estimates of Lindstrom _et al._21 are, thus, very rough and do not account for microscale fluctuations in peak shock P-T. In summary, high-pressure plagioclase

polymorphs along interfaces between augite and plagioclase wedges have been observed in dolerite of the Lockne impact structure. No evidence was found in this study for a chemical exchange

between the phases at the mineral interfaces. More studies, however, are needed to investigate other heterogeneous mineral associations, which have experienced shock waves. Thus, it must be

underlined that a combination of Raman spectroscopy and HRTEM is a powerful tool to study shock-induced phase transitions providing valuable information on pressure–temperature–time history

of impact shock events. METHODS PETROLOGY From each oriented dolerite block sample oriented polished thin sections were prepared and petrological observations were carried out using an

optical polarizing Leitz Othoplan microscope. High-resolution morphological investigation was performed through a scanning electron microscope (SEM), LEO 1530 of Gemini, equipped with an

EDX-detector for chemical analysis. RAMAN SPECTROSCOPY In order to reveal possible shock-induced phase transformations, micro-Raman spectrometry was employed. A Bruker SENTERRA Raman

spectrometer (Bruker Optics, Ettlingen, Germany) based on an Olympus BX-51 microscope (OLYMPUS Co. Tokyo Japan) provided insights into structures on the micrometre scale. A frequency doubled

NdYAG Laser, λ = 532 nm, operated at 5 mW power, served as the excitation source. The beam was focused through an objective, Olympus 100X, NA 0.8, in a 1 μm spot on the sample surface. An

integration time of 80 s with 2 co-additions (2*40 s) was used to scan an area about 190*90 μm. The obtained spectra were processed and baseline corrected using Bruker OPUS® software Ver.

7.2. HIGH-RESOLUTION ELECTRON MICROSCOPY Nanostructural investigations on rock thin sections were carried out by a Philips CM 200 FEG/ST TEM operated at 200 kV. The Fast Fourier

Transformation (FFT) of HRTEM images were carried out using ImageJ software59. TEM sections were prepared using focused iron beam (FIB) milling station FEI Dual Beam Strata 400S with a Ga+

cathode. Here the area of interest was coated with a 3 μm thick conductive platinum layer. Thereafter, sample was cut from the thin-sections by a 30 kV, 6.5 nA Ga ion beam. Later, the sample

was thinned by a 30 kV, 26 pA Ga ion beam and two to three, 5 to 50 nm thick, windows were prepared to check FIB induced damage and amorphization. In the final step, the sample was

polished/cleaned with a 5 kV (71 pA ion current). The step is essential to remove its crust, which is commonly damaged by the high energy ion beam. We propose that the polishing was

competent as the augite, present above lingunite, and therefore closer to the ion beam source is crystalline (Fig. 7). It is well-known that plagioclases are sensitive to amorphization due

to irradiation by electron beam37. We avoided the amorphization by using sample holder cooled by liquid nitrogen, and smaller exposure time of about 10–20 ms. A condenser aperture of 100 μm

at a spot size of ≥300 nm was used for initial selective area electron diffraction (SAED) image and general TEM investigations. Usually much higher energy is transmitted to the sample during

high resolution TEM (HRTEM) investigations. Therefore, for HRTEM the beam was first focused at a location away from the area of interest and only then the investigations were carried out.

ADDITIONAL INFORMATION HOW TO CITE THIS ARTICLE: Agarwal, A. _et al._ Lingunite-a high-pressure plagioclase polymorph at mineral interfaces in doleritic rock of the Lockne impact structure

(Sweden). _Sci. Rep._ 6, 25991; doi: 10.1038/srep25991 (2016). REFERENCES * Dong, L. & Li, X. A Microseismic/Acoustic Emission Source Location Method Using Arrival Times of PS Waves for

Unknown Velocity System. _Int. J. Distrib. Sens. Networks_ 2013, 1–8 (2013). Article  Google Scholar  * Li, X. & Dong, L. An efficient closed-form solution for acoustic emission source

location in three-dimensional structures. _AIP Adv._ 4, 027110 (2014). Article  ADS  Google Scholar  * Dong, L., Li, X., Zhou, Z. et al. Three-dimensional analytical solution of acoustic

emission source location for cuboid monitoring network without pre-measured wave velocity. _Transactions of Nonferrous Metals Society of China_ 25(1), 293–302 (2015). Article  Google Scholar

  * Kenkmann, T., Hornemann, U. & Stöffler, D. Experimental generation of shock-induced pseudotachylites along lithological interfaces. _Meteorit. Planet. Sci._ 35, 1275–1290 (2000).

Article  CAS  ADS  Google Scholar  * Gibson, H. M. & Spray, J. G. Shock-induced melting and vaporization of shatter cone surfaces: Evidence from the Sudbury impact structure. _Meteorit.

Planet. Sci._ 33, 329–336 (1998). Article  CAS  ADS  Google Scholar  * El Goresy, A. et al. _In situ_ discovery of shock-induced graphite-diamond phase transition in gneisses from the Ries

Crater, Germany. _Am. Mineral._ 86, 611–621 (2001). Article  CAS  ADS  Google Scholar  * Sharp, T. G. & de Carli, P. S. Shock effects in meteorites. _Meteorites Early Sol. Syst. II_

653–677 (1988) (13-0-2014). at http://www.lpi.usra.edu/books/MESSII/9040.pdf. * An, Q. et al. Elucidation of the dynamics for hot-spot initiation at nonuniform interfaces of highly shocked

materials. _Phys. Rev. B_ 84, 220101 (2011). Article  ADS  Google Scholar  * Gillet, P., Chen, M., Dubrovinsky, L. & El Goresy, A. Natural NaAlSi3O8-Hollandite in the Shocked Sixiangkou

Meteorite. _Science._ 287, 1633–1636 (2000). Article  CAS  ADS  Google Scholar  * Langenhorst, F. & Poirier, J. P. Anatomy of black veins in Zagami: Clues to the formation of

high-pressure phases. _Earth Planet. Sci. Lett._ 184, 37–55 (2000). Article  CAS  ADS  Google Scholar  * Ozawa, S. et al. Jadeite in Chelyabinsk meteorite and the nature of an impact event

on its parent body. _Sci. Rep._ 4, 5033 (2014). Article  CAS  Google Scholar  * Baziotis, I. P. et al. The Tissint Martian meteorite as evidence for the largest impact excavation. _Nat.

Commun._ 4, 1404 (2013). Article  Google Scholar  * Tomioka, N., Mori, H. & Fujino, K. Shock induced transition of NaAlSi3O8 feldspar into a hollandite structure in a L6 chondrite.

_Geophys. Res. Lett._ 27, 3997–4000 (2000). Article  CAS  ADS  Google Scholar  * Ringwood, A. E., Reid, A. F. & Wadsley, A. D. High-pressure KAlSi3O8, an aluminosilicate with sixfold

coordination. _Acta Crystallogr._ 23, 1093–1095 (1967). Article  CAS  Google Scholar  * Liu, L. G. High-pressure phase transformations of albite, jadeite and nepheline. _Earth Planet. Sci.

Lett._ 37, 438–444 (1978). Article  CAS  ADS  Google Scholar  * Tutti, F. Formation of end-member NaAlSi3O8 hollandite-type structure (lingunite) in diamond anvil cell. _Phys. Earth Planet.

Inter._ 161, 143–149 (2007). Article  CAS  ADS  Google Scholar  * Kubo, T. et al. Plagioclase breakdown as an indicator for shock conditions of meteorites. _Nat. Geosci._ 3, 41–45 (2010).

Article  CAS  ADS  Google Scholar  * Spray, J. & Hines, J. Earth impact database. _Planet. Sp. Sci. Centre, Univ. New Brunswick, Canada_ (2014) (02-09-2014). at

http://www.passc.net/EarthImpactDatabase/. * Langenhorst, F. & Dressler, B. In _Large Meteor. Impacts III_ ( Kenkmann, T., Hörz, F. & Deutsch, A. ) 4046–4047 (Geological Society of

America, 2005). * Sturkell, E. F. F. The marine Lockne impact structure, Jämtland, Sweden: a review. _Geol. Rundschau_ 87, 253–267 (1998). Article  ADS  Google Scholar  * Lindström, M.,

Shuvalov, V. & Ivanov, B. Lockne crater as a result of marine-target oblique impact. _Planet. Space Sci._ 53, 803–815 (2005). Article  ADS  Google Scholar  * Ormö, J. Numerical modeling

for target water depth estimation of marine-target impact craters. _J. Geophys. Res._ 107, 1–9 (2002). Article  Google Scholar  * Shuvalov, V., Ormö, J. & Lindström, M. In _Impact

Tectonics_ ( Koeberl, C. & Henkel, D. H. ) 405–422 (Springer-Verlag 2005) 10.1007/3-540-27548-7_16. * Schmitz, B. et al. Determining the impactor of the Ordovician Lockne crater: Oxygen

and neon isotopes in chromite versus sedimentary PGE signatures. _Earth Planet. Sci. Lett._ 306, 149–155 (2011). Article  CAS  ADS  Google Scholar  * Schmitz, B. et al. Asteroid breakup

linked to the Great Ordovician Biodiversification Event. _Nat. Geosci._ 1, 49–53 (2008). Article  CAS  ADS  Google Scholar  * Greiling, R. O., Grimmer, J. C., De Wall, H. & Bjork, R. L.

Mesoproterozoic dyke swarms in foreland and nappes of the central Scandinavian Caledonides: structure, magnetic fabric, and geochemistry. _Geol. Mag._ 144, 525 (2007). Article  CAS  ADS 

Google Scholar  * El Goresy, A. et al. Shock-induced deformation of Shergottites: Shock-pressures and perturbations of magmatic ages on Mars. _Geochim. Cosmochim. Acta_ 101, 233–262 (2013).

Article  CAS  ADS  Google Scholar  * Mernagh, T. P. Use of the laser Raman microprobe for discrimination amongst feldspar minerals. _J. Raman Spectrosc._ 22, 453–457 (1991). Article  CAS 

ADS  Google Scholar  * Freeman, J. J., Wang, A., Kuebler, K. E., Jolliff, B. L. & Haskin, L. A. Characterization of natural feldspars by Raman spectroscopy for future planetary

exploration. _Can. Mineral._ 46, 1477–1500 (2008). Article  CAS  Google Scholar  * RRUFF database. RRUFF (2014) (05-03-2014). at http://rruff.info/. * Liu, L., Lin, C.-C., Yung, Y. J.,

Mernagh, T. P. & Irifune, T. Raman spectroscopic study of K-lingunite at various pressures and temperatures. _Phys. Chem. Miner._ 36, 143–149 (2009). Article  CAS  ADS  Google Scholar  *

Zhang, W. F., He, Y. L., Zhang, M. S., Yin, Z. & Chen, Q. Raman scattering study on anatase TiO 2 nanocrystals. _J. Phys. D. Appl. Phys._ 33, 912–916 (2000). Article  CAS  ADS  Google

Scholar  * Velde, B., Syono, Y., Kikuchi, M. & Boyer, H. Raman microprobe study of synthetic diaplectic plagioclase feldspars. _Phys. Chem. Miner._ 16, 436–441 (1989). Article  CAS  ADS

  Google Scholar  * Reynard, B., Okuno, M., Shimada, Y., Syono, Y. & Willaime, C. A Raman spectroscopic study of shock-wave densification of anorthite (CaAl 2 Si 2 O 8) glass. _Phys.

Chem. Miner._ 26, 432–436 (1999). Article  CAS  ADS  Google Scholar  * Fritz, J., Greshake, A. & Stoffler, D. Micro-Raman spectroscopy of plagioclase and maskelynite in Martian

meteorites: Evidence of progressive shock metamorphism. _Antarct. Meteor. Res._ 18, 96–116 (2005). CAS  ADS  Google Scholar  * ICSD. Inorganic Crystal Structure Database. _Card Cat. #009257,

#034194_ (2014) (25-03-2014). at https://www.fiz-karlsruhe.de/icsd.html. * Mussi, A., Cordier, P., Mainprice, D. & Frost, D. J. Transmission electron microscopy characterization of

dislocations and slip systems in K-lingunite: Implications for the seismic anisotropy of subducted crust. _Phys. Earth Planet. Inter._ 182, 50–58 (2010). Article  CAS  ADS  Google Scholar  *

Guinier, A. _X-ray diffraction in crystals, imperfect crystals, and amorphous bodies._ (Dover Books, 1994). * Hirsch, P. B., Howie, A., Nicholson, R. B., Pashley, D. W. & Whelan, M. J.

Electron Microscopy of Thin Crystals (1965). * Bespalova, I. V. & Teplyakova, L. A. X-ray diffraction analysis of reorientation regions formed in [111]-single crystal of aluminum under

compression. _IOP Conf. Ser. Mater. Sci. Eng._ 71, 012058 (2015). Article  Google Scholar  * Hörz, F. & Ahrens, T. J. Deformation of experimentally shocked biotite. _Am. J. Sci._ 267,

1213–1229 (1969). Article  ADS  Google Scholar  * Wdowiak, S. Y., Wdowiak, T. J., Gillis, D. C. & Dudley, M. A Search for Shocked Grains in the KT Boundary at Braggs, Alabama. In _LPI

Contrib._ 130–132 (1994). * Bohor, B. F., Modreski, P. J. & Foord, E. E. Shocked quartz in the Cretaceous-Tertiary boundary clays: Evidence for a global distribution. _Science._ 236,

705–709 (1987). Article  CAS  ADS  Google Scholar  * Flemming, R. L. Micro X-ray diffraction (μXRD): a versatile technique for characterization of Earth and planetary materials. _Can. J.

Earth Sci._ 44, 1333–1346 (2007). Article  CAS  ADS  Google Scholar  * Söderlund, U., Elming, S. Å., Ernst, R. E. & Schissel, D. The Central Scandinavian Dolerite Group-Protracted

hotspot activity or back-arc magmatism? Constraints from U-Pb baddeleyite geochronology and Hf isotopic data. _Precambrian Res._ 150, 136–152 (2006). Article  ADS  Google Scholar  * Brander,

L., Söderlund, U. & Bingen, B. Tracing the 1271–1246 Ma Central Scandinavian Dolerite Group mafic magmatism in Fennoscandia: U–Pb baddeleyite and Hf isotope data on the Moslätt and

Børgefjell dolerites. _Geol. Mag._ 148, 632–643 (2011). Article  CAS  ADS  Google Scholar  * Elming, S. S. Å. & Mattsson, H. Post Jotnian basic intrusions in the Fennoscandian Shield,

and the break up of Baltica from Laurentia: A palaeomagnetic and AMS study. _Precambrian Res._ 108, 215–236 (2001). Article  CAS  ADS  Google Scholar  * Gorbatschev, R., Solyom, Z. &

Johansson, I. The Central Scandinavian Dolerite Group in Jämtland, central Sweden. _Geol. Foereningan i Stock. Foerhandlingar_ 101, 177–190 (1979). Article  CAS  Google Scholar  * Hogmalm,

K. J. et al. The Ulvö Gabbro Complex of the 1.27-1.25 Ga Central Scandinavian Dolerite Group (CSDG): Intrusive age, magmatic setting and metamorphic history. _GFF_ 128, 1–6 (2006). Article 

CAS  Google Scholar  * Agarwal, A., Kontny, A. & Greiling, R. O. Relationships among magnetic fabrics, microfractures and shock pressures at an impact crater: A case study from Lockne

crater, Sweden. _J. Appl. Geophys._ 114, 232–243 (2015). Article  ADS  Google Scholar  * Warr, L. N., Greiling, R. O. & Zachrisson, E. Thrust-related very low grade metamorphism in the

marginal part of an orogenic wedge, Scandinavian Caledonides. _Tectonics_ 15, 1213–1229 (1996). Article  ADS  Google Scholar  * Chen, M. & El Goresy, A. The nature of maskelynite in

shocked meteorites: not diaplectic glass but a glass quenched from shock-induced dense melt at high pressures. _Earth Planet. Sci. Lett._ 179, 489–502 (2000). Article  CAS  ADS  Google

Scholar  * Tschermak, G. Die Meteoriten von Shergotty und Gopalpur. _Sitzber. Akad. Wiss. Wien Math.-Naturwiss. Kl. Abt. I_ 65, 122–146 (1872). Google Scholar  * Tschermak, G. Beitrag zur

Klassifikation der Meteoriten. _Sitzber. Akad. Wiss. Wien Math.-Naturwiss. Kl. Abt. I_ 88, 347–371 (1883). Google Scholar  * Xie, Z., Sharp, T. G. & DeCarli, P. S. High-pressure phases

in a shock-induced melt vein of the Tenham L6 chondrite: Constraints on shock pressure and duration. _Geochim. Cosmochim. Acta_ 70, 504–515 (2006). Article  CAS  ADS  Google Scholar  *

Melosh, H. J. _Impact cratering: A geologic process_ 1, (Research supported by NASA. New York, Oxford University Press (Oxford Monographs on Geology and Geophysics, 1989). * Artemieva, N.

& Ivanov, B. Launch of Martian meteorites in oblique impacts. _Icarus_ 171, 84–101 (2004). Article  ADS  Google Scholar  * Miyahara, M. et al. Evidence for fractional crystallization of

wadsleyite and ringwoodite from olivine melts in chondrules entrained in shock-melt veins. _Proc. Natl. Acad. Sci. USA_ 105, 8542–8547 (2008). Article  CAS  ADS  Google Scholar  * Rasband,

W. S. & Image J. _US Natl. Inst. Heal. Bethesda, USA_ (1997) (17-05-2014). at http://imagej.nih.gov/ij/. * Sturkell, E. F. F. & Ormö, J. Magnetometry of the marine, Ordovician Lockne

impact structure, Jämtland, Sweden. _J. Appl. Geophys._ 38, 195–207 (1998). Article  ADS  Google Scholar  * Agarwal, A., Kontny, A., Srivastava, D. C. & Greiling, R. O. Shock pressure

estimates in target basalts of a pristine crater: A case study in the Lonar crater, _India. Geol. Soc. Am. Bull._ 128, 19–28 (2016). Google Scholar  Download references ACKNOWLEDGEMENTS

Authors would like to thank D.C. Srivastava, K. Drueppel, Dr. C.W. Passchier and R. Vernon for their suggestions and E. Sturkell for guidance in field. We are grateful to E. Müller for the

careful FIB preparation and H. Strömer for the help during TEM. Deutscher Akademischer Austauschdienst, Germany (DAAD) is thanked for the financial support of the first author (file number:

A/11/76052). TEM and SEM investigations were funded from DFG project KO1514/8. Two anonymous reviewers and A. El Goresy are thanked for their constructive criticism. We are very indebted to

R.O. Greiling for the providing collection of thin sections from the Orkulla dykes. We are thankful to L.M. Alva-Valdivia for reading the manuscript and constructive criticism. AUTHOR

INFORMATION AUTHORS AND AFFILIATIONS * Division of Structural Geology and Tectonophysics, Institute of Applied Geosciences, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany Amar

Agarwal, Boris Reznik & Agnes Kontny * Department of Earth Science, Indian Institute of Technology, Roorkee, India Amar Agarwal * Laboratory of Paleomagnetism, Institute of Geophysics,

National Autonomous University of Mexico, 4510 Mexico DF, Mexico, Amar Agarwal * Institute of Functional Interfaces, Karlsruhe Institute of Technology, Eggenstein-Leopoldshafen, 76344,

Germany Stefan Heissler * Division of Technical Petrophysics, Institute of Applied Geosciences, Karlsruhe Institute of Technology, Karlsruhe, 76131, Germany Frank Schilling Authors * Amar

Agarwal View author publications You can also search for this author inPubMed Google Scholar * Boris Reznik View author publications You can also search for this author inPubMed Google

Scholar * Agnes Kontny View author publications You can also search for this author inPubMed Google Scholar * Stefan Heissler View author publications You can also search for this author

inPubMed Google Scholar * Frank Schilling View author publications You can also search for this author inPubMed Google Scholar CONTRIBUTIONS A.A., B.R. and A.K. contributed in the

fabrication of the problem and overall scientific development. A.A. did the field work and polarized light microscopy, and assisted B.R. during SEM and TEM. B.R. interpreted the TEM results.

S.H. did Raman spectroscopy and helped in interpretation of the results. F.S. contributed in the development of the hypothesis. CORRESPONDING AUTHOR Correspondence to Boris Reznik. ETHICS

DECLARATIONS COMPETING INTERESTS The authors declare no competing financial interests. SUPPLEMENTARY INFORMATION SUPPLEMENTARY INFORMATION (DOC 1470 KB) RIGHTS AND PERMISSIONS This work is

licensed under a Creative Commons Attribution 4.0 International License. The images or other third party material in this article are included in the article’s Creative Commons license,

unless indicated otherwise in the credit line; if the material is not included under the Creative Commons license, users will need to obtain permission from the license holder to reproduce

the material. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/ Reprints and permissions ABOUT THIS ARTICLE CITE THIS ARTICLE Agarwal, A., Reznik, B., Kontny,

A. _et al._ Lingunite-a high-pressure plagioclase polymorph at mineral interfaces in doleritic rock of the Lockne impact structure (Sweden). _Sci Rep_ 6, 25991 (2016).

https://doi.org/10.1038/srep25991 Download citation * Received: 24 June 2015 * Accepted: 14 April 2016 * Published: 18 May 2016 * DOI: https://doi.org/10.1038/srep25991 SHARE THIS ARTICLE

Anyone you share the following link with will be able to read this content: Get shareable link Sorry, a shareable link is not currently available for this article. Copy to clipboard Provided

by the Springer Nature SharedIt content-sharing initiative