ATTENTION: FULGURITE! G.G.I(ochemasov,NatianfUr,ies,Projet "Rech~rchesMinieres - ~i1~/81/005",~epubl~~ueUnie du Cameroun,YaoundB, B.P. 836. * In searching for impact structures cn surface cf the Earth scienti- sts often give decisive cord t= ninerz1o;ical arguments not considering geological,st~ucturaland tcctcnic criteria as very important. This ep- proach is sometimes dangerous 2s the nineral forms like to those chnrac- teristic for supervelocity inpacts of neteozoids can be developcd under pure earth's conditions without cosmic factors. We have in view thc ful- gurites -rocks developing on the su~faccor near to the earth's surfzce in the depth of feu meters 2s resalt of lightning impact. The conditions of li~litningdischarges,their energies and tzr~~ts are badly known,but it bas however rencrkcd in some places that thry ?re attracted by certain ground electric anonalies. The natural electric ano- malies often form at contacts of basic and ultrabasic rocks with their country rocks of different compositions,in fault zones with water circu- lation,in zones of oxidation of su1phi.de mineralization (gossan),at out- croppings of ferruginous quartzites. High probability of finding of fulgurites exists naturally in places uhere storms are common like in the equtorial western Africa. In this area tropical storms during the rain season are characterized by high in- tensities and high frequencies of electric discharges. Some authors call, for examplesthe cameroonian downpours "the lightning rainsN. The isolated mountain Nabeba in the northern Congoyfor example,composed of ferruginous quartzites attracts lightnings that fall there very frequently giving in the course of certain storms alnost continuous light. It is practically impozsiblc to find traces of lightning impact in soil heavily covered with vsg~tation~butfragments of resistant meltcd rock-fulgurite during erosion can be concentrated like common quartz gra- vel in stream gravel beds. kfc have found indeed in few streams of the N!! part ~f the Congo craton (the SE of Cameroon) confined to geologically particular areas (contacts of serpentinites and dolerites with schists and quartzites,complicated by faults with traces of sulphidc mincraliza- tion) fragments of fulgurites. Usually they are rare,but sosetines form upto 20-30 $ of gravel fragnents. The fulgurite is presented by branch-like,~prig-like~icic1~-1ik~~ cylinder-like or irregular fragments of 1 cm to 10 cm long and 2-3 mn to 2-3 crn in width. Colour is light-greyyr;rey,yellow-grey,reddish,darl: broun- red. Sometimes in the middle of a cylinder-like body passes an "axial" cavity -trace of a burned plznt root which mas probably used by ramifying lightning for penetration in soil. Confusion of the fulgurite fragment with central cavity with possible cementation of soil around some roots is completely excluded by presence of traces of quartz melting. Composition and structure of the fulgurites is variable reflecting variable composition of "targett' (from lateritic soil at different levels to almost pure quartz alluvial) and different quantities of energy disper- sed at different depths of lightning penetration. A transverse section of the fulgurites sometimes is roughly concentrically zonal,

*New address: IGEM of the Academy of Sciences of the USSR,35, Staromonetny,Noscomy109O17,USSR.

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System ATTENTION: FULGURITE !

Kochemasov G.G.

Two components are easily distinguished in the fulgurite compositi- on : fragments and cement. Amount of fragments in different sections of the fulgurites varies between u~aktraces and approximately 50$,their size varies betueen few tens of microns and 2-3 mm. They are usually zngular and composed of quartz and quartz partly isotropic and vitraini- zed. Sometimes one observe in quartz linear structures of extinction and very complex extinction (result of high pressures?,an analogy to the planar structures?). Zones of quartz melting usually appear from one side of a fragment,but can zlso follow fractures. The zones of melting normal- ly arc characterized by ccnchoidal structures and sometimes are bordered by curved microfractures in still anisotropic quartz. The fragments sorne- tines consist of aggregates of quzrtz grains. One observe usually three types of cement : devitrified quartz - lechatelierite,siderite and red-brown iron oxides. Sometimes all three or tuo of them can be prescnt in one thin section. Siderite is normally nicro~ranularand often gives crosn skructures around quartz fragments. Very typical is presence in thc cenent 3f muscovite rnicroflakes. The ce- nrnt sometimes containc opaquc nicrospherules (0,02 - @,2 mm in diameter). Part of them is composed of iron cxiCcs. There is also one type of sphe- rules oith relatively high iizrlr~ss(their com?osition is still unknown), they czsily go auzy frcm ti? lzchzteli5rite cement under polishing. T45e lightning impact - FristLn"Lncous penetration of a powerful ele- ct-ic charge in soil - is pcorly studied. It is certainly accompanied by burning and evaporztion of a :,zrt of the substance (vegetation roots- CO,,noisturc -H,O,iron oxidcs -Fe,O for cxample,with precipitation of 3 ' ths fine graine2 siderite -FeCO,);or its melting (lechateli6rite,opaque sphcrulss),or its defornztion i6 solid state under very local high pres- surcs (deformation of quartz structure). Some of thkse results are usual- ly csnsidcred as proofs of the icpact of cosmic origin. One has to note :hat it is quite pozsible that thc opnquc sphsrules in the fulgurites fozncd in the ultrabasic environccnt uould be enriched ih Ni (the Sack- zrdunk in serpentinite soil is about 0,3 $ of Ni) and this is usually considered as one of tho sign8 of cosnic origin of spherules. Sanc of the products of lightning inpacts,as we have seen,can be found in streams and could be takzn for "proofs" of a meteorite impact in the area where one strongly wznts to find these "proofs". It is thou- ght that systematic geological and structural study of an area that could he an astroblem is not less inportant than findings of dispersed xinera- lo~icalevidences which sonetines are ambiguous. It has to be noted that old gigantic astroblems of shields cennot have these evidences even the- creticzlly as their rocks (prinarily impact melts) underuent the influ- ence of complex processes of the crystal differentiation,liquation,~eta- morphism and metasomatisn. In conclusion onc can pass a remark that the fulgurite formation is in sense a natural model of an inpact process. It is interesting to note that the numerous spherules found in the plzcz of the Tunguska event of 1933 (1,2) could be partly related to fulgurite formation that could have accompanied this event. REFERENCES: (1) Nazarov M.A. et a1 (1983) LPS XIV,p.548-549; (2) Zbik M. (1983) LPS XIV,p.877-878.

O Lunar and Planetary Institute Provided by the NASA Astrophysics Data System