Editing Lonsdaleite (section)

==Hardness==
According to the conventional interpretation of the results of examining the meagre samples collected from [[meteorite]]s or manufactured in the lab, lonsdaleite has a [[hexagonal crystal system|hexagonal]] [[unit cell]], related to the diamond unit cell in the same way that the hexagonal and cubic [[Close-packing|close packed]] [[crystal system]]s are related. Its diamond structure can be considered to be made up of interlocking rings of six carbon atoms, in the [[chair conformation]]. In lonsdaleite, some rings are in the [[boat conformation]] instead. At nanoscale dimensions, cubic diamond is represented by ''[[diamondoid]]s'' while hexagonal diamond is represented by ''[[Wurtzite (crystal structure)|wurtzoids]]''.<ref>
{{cite journal
 |last=Abdulsattar |first=M.
 |year=2015
 |title=Molecular approach to hexagonal and cubic diamond nanocrystals
 |journal=Carbon Letters
 |volume=16 |issue=3 |pages=192–197
|doi=10.5714/CL.2015.16.3.192
 |doi-access=free
 }}
</ref>

In diamond, all the carbon-to-carbon bonds, both within a layer of rings and between them, are in the [[staggered conformation]], thus causing all four cubic-diagonal directions to be equivalent; whereas in lonsdaleite the bonds between layers are in the [[eclipsed conformation]], which defines the axis of hexagonal symmetry.

Mineralogical ''simulation'' predicts lonsdaleite to be 58% harder than diamond on the [[Miller index|<100>]] face, and to resist indentation pressures of 152&nbsp;[[GPa]], whereas diamond would break at 97&nbsp;GPa.<ref name=theory>
{{cite journal
 |author1=Pan, Zicheng    |author2=Sun, Hong
 |author3=Zhang, Yi       |author4=Chen, Changfeng
 |name-list-style=amp
 |date=2009
 |title=Harder than diamond: Superior indentation strength of wurtzite BN and lonsdaleite
 |journal=Physical Review Letters
 |volume=102  |issue=5  |page=055503
 |doi=10.1103/PhysRevLett.102.055503
 |pmid=19257519  |bibcode=2009PhRvL.102e5503P
}}
*{{cite news |author=Lisa Zyga |date=12 February 2009 |title=Scientists Discover Material Harder Than Diamond |work=Phys.org |url=http://www.physorg.com/news153658987.html}}</ref> This is yet exceeded by [[diamond type|IIa]] [[material properties of diamond|diamond]]'s <111> tip hardness of 162&nbsp;GPa.

The extrapolated properties of lonsdaleite have been questioned, particularly its superior hardness, since specimens under [[crystallography|crystallographic]] inspection have not shown a bulk hexagonal lattice structure, but instead a conventional cubic diamond dominated by structural defects that include hexagonal sequences.<ref>
{{cite journal
 |first1=P. |last1=Nemeth         |first2=L.A.J. |last2=Garvie 
 |first3=T. |last3=Aoki             |first4=D.   |last4=Natalia
 |first5=L. |last5=Dubrovinsky      |first6=P.R. |last6=Buseck 
 |year=2014
 |title=Lonsdaleite is faulted and twinned cubic diamond and does not exist as a discrete material
 |journal=Nature Communications
 |volume=5 |pages=5447
 |pmid=25410324 |bibcode=2014NatCo...5.5447N
 |doi=10.1038/ncomms6447 |doi-access=free
|hdl=2286/R.I.28362|hdl-access=free}}
</ref> A quantitative analysis of the [[X-ray diffraction]] data of lonsdaleite has shown that about equal amounts of hexagonal and cubic stacking sequences are present. Consequently, it has been suggested that "stacking disordered diamond" is the most accurate structural description of lonsdaleite.<ref>
{{cite journal
 |author1=Salzmann, C.G.
 |author2=Murray, B.J.
 |author3=Shephard, J.J.
 |date=2015
 |title=Extent of stacking disorder in diamond
 |journal=Diamond and Related Materials
 |volume=59 |pages=69–72
 |arxiv=1505.02561 |bibcode=2015DRM....59...69S
 |s2cid=53416525 |doi=10.1016/j.diamond.2015.09.007
 |url=http://eprints.whiterose.ac.uk/93345/
}}
</ref> On the other hand, recent shock experiments with [[in situ]] X-ray diffraction show strong evidence for creation of relatively pure lonsdaleite in dynamic high-pressure environments comparable to meteorite impacts.<ref>
{{cite journal
 |author1 =Kraus, D.       |author2 =Ravasio, A.
 |author3 =Gauthier, M.    |author4 =Gericke, D.O.
 |author5 =Vorberger, J.   |author6 =Frydrych, S.
 |author7 =Helfrich, J.    |author8 =Fletcher, L.B.
 |author9 =Schaumann, G.   |author10=Nagler, B.
 |author11= Barbrel, B.    |author12=Bachmann, B.
 |author13=Gamboa, E.J.    |author14=Goede, S.
 |author15=Granados, E.    |author16=Gregori, G.
 |author17=Lee, H.J.       |author18=Neumayer, P.
 |author19=Schumaker, W.   |author20=Doeppner, T.
 |author21=Falcone, R.W.   |author22=Glenzer, S.H.
 |author23=Roth, M.
 |year=2016
 |title=Nanosecond formation of diamond and lonsdaleite by shock compression of graphite
 |journal=Nature Communications
 |volume=7   |pages=10970
 |pmid=26972122  |doi=10.1038/ncomms10970
 |bibcode=2016NatCo...710970K  |pmc=4793081
}}
</ref><ref>
{{cite journal
 |last1=Turneaure |first1=Stefan J.      |last2=Sharma  |first2=Surinder M.
 |last3=Volz      |first3=Travis J.      |last4=Winey   |first4=J.M.
 |last5=Gupta     |first5=Yogendra M.
 |date=2017-10-01
 |title=Transformation of shock-compressed graphite to hexagonal diamond in nanoseconds
 |journal=Science Advances
 |volume=3 |issue=10 |page=eaao3561
 |doi=10.1126/sciadv.aao3561 |pmid=29098183
 |issn=2375-2548 |pmc=5659656
|bibcode=2017SciA....3O3561T }}
</ref>