AmericanMineralogist, Volume 64, pages 1059-1062, 1979 Mineral inclusionsin four Arkansas diamonds:their nature and significance NnNrBr.r.B S. PeNrareo. M. Geny NBwroN. Susnennyuoe V. GocINENI, CsnRr-ns E. MELTON Department of Chemistry, University of Georgia eNo A. A. GnnoINI Department of Geology, University of Georgia Athens,Georgia 30602 Abstract Totally-enclosed inclusions recovered from four Arkansas diamonds by burning in air at 820" C are identified by XRD and Eoex analysesas: (l) enstatite,olivine, pyrrhotite + pent- landite, (2) eclogitic garnet, (3) enstatite+ peridotitic garnet + chromite, olivine + pyrrho- tite, pyrrhotite, pentlandite,pyrite + pyrrhotite, pentlandite * a nickel sulfide,(4) diamond, enstatite,magnetite. This is the first report of nickel sulfide in diamond. A review of in- clusionsfound in Arkansasdiamonds showsa similarity with those from world-wide local- ities, and indicatesa global consistencyin diamond-formingenvironments. Introduction opaque and transparent totally-enclosedinclusions were apparentunder binocular microscopeexamina- impermeable, Since diamonds are relatively inert, tion. The 0.45 ct. stone was colorlessand of elon- pre- and hard, inclusions therein are about as well gated rounded shapewith a highly polished surface. pro- served as they could possibly be. Their study Four inclusions were detected.The 0.50 ct. crystal wherein vides entghtment about the environment was a colorlessrounded tetrahexahedronwith a diamondsformed. cleavageplane on one sideand containedtransparent A considerable literature exists on diamond in- and opaque inclusions. The 0.62 ct. diamond was Afri- clusions,but most of it dealswith diamondsof also colorlessand of rounded tetrahexahedralform. can, Siberian, and South American origin (e.9., Severalrelatively large flat opaque inclusions ("car- Meyer and Tsai, 1976;Mitchell and Giardtni,1977; bon spots") were observed.All the diamonds were Orlov, 1977).ln all, about thirty minerals have been free ofdetectable surfacecleavage cracks. identified. But for a few notable exceptions, e.8., qaartz in Brazilian diamonds, they can be classedas Experimental being derived from either eclogitic or ultramafic The diamonds were cleaned and burned by the rock. There have been,however, relatively few stud- same proceduresdescribed by Newton et al- (1977). ies on inclusions in North American diamonds.The The furnace temperature was 820"C. After com- purposeof this work was to investigatecrystalline in- bustion, residual matter from each diamond was re- clusions in four Arkansas diamonds. The diamonds coveredfrom the bottom of its platinum boat with a of 0.43, 0.45, 0.50, and 0.62 carat weight were ob- sharply-pointedwood stick moistenedwith distilled tained from Mr. Dicke Hughes,a former operator of water, transferredto a glassslide, examinedby opti- known the Arkansaskimberlite property that is now cal microscopy, then mounted on sharply-pointed as the Crater of Diamonds StatePark. The diatreme wood sticks with mucilage for XRD analysis. Sub- is located about 4 km SSE of Murfreesboro, Pike sequentto X-ray diffraction analyses,samples were County, Arkansas(Gogineni et al.,1978). recoveredand transferredto Sptr't/EDAxmounts for chemicalanalyses. The latter were conductedin con- Descriptionof the diamonds junction with co-mounted referencestandards pre- The 0.43 ct. diamond was a pale yellow rounded, viously analyzed with a MAC-400 electron beam flattened, apparently twinned hexoctahedron.Both probe. wo3 oo4x/79/09 lo-1059$o2.oo r059 1060 PANTALEO ET AL.: INCLUSIONS IN DIAM1NDS Analytical data mensions(Table l) and chemistry(Table 2) are simi- 0.43ct. diamond lar. Their end-membercomposition is almandine40, andradite 10, grossular 12, pyrope 38. They corre- Recovered inclusions are one enstatite crystal, spond to Group 3 garnet (common eclogite garnet) three olivine crystals, and one compositecrystal of according to the classificationmethod of Dawson pyrrhotite and pentlandite. The enstatitewas color_ and Stephens (1975). SEM examination revealed less and tabular-prismaticin form. Unit-cell dimen_ hillock-like irregularities on their rounded surfaces. sionsare given in Table l. The crystal was lost in at_ The latter may reflect a partial solution prior to their tempting transfer from XRD to EDAx mounts. Thus, inclusion in the diamond (Fig. l). chemical data are not available. A fourth crystal, tan to pale yellow, tabular and The three olivine crystalswere all castsof octahe_ flexible, was lost during recovery from the platinum dral form (Fig. l). Although initially colorless,all be_ boat. In appearanceit resembleda mica crystal. came orange-redas a result of the diamond burning process.All had similar unit-cell dimensions(Table 0.50ct. diamond 1) and chemistry(Fon,-Table 2). Six inclusions were recovered.The largest was a Due to the burning process,sulfide inclusionswere tricrystal composedof a colorlesstabular-prismatic convertedto oxides.Sutfrde identification is basedon the texture, chemistry, and crystal structuresof the residual oxides standardized against those derived from similar burnings of known referencesulfides. All sutflde residueswere crustaceousin texture. The residual shapeof the pyrrhotite * pentlandite resid_ uum was near-spherical,indicating a globular shape for the original sulfide. XRD analysisgave parrerns for magnetite and bunsenite.Eoax analysisihowed a composition of 95-l00%oFd., 5-OVoNi for pyrrho_ tite,and 25-50VoFe,75-5OVo Ni for pentlandite. 0.45ct. diamond Identified inclusions consist of three rounded. elongated,orange-brown crystals of iron-rich garnet, one of which is shown in Figure l. Their unit-cell di_ Table l. Single-crystal XRD data on mineral inclusions from Arkansas diamonds Inc 1us ion Source dianond Unit ce11 dinensions (A) Fe-rich garnet 0.45 carat a = 11.552(6)* Cr-lich garnet 0 50 carat a = 1I.522(L) O1ivine 0.43 carat a = 4-756(5) b = 10 r9s(8) Fig. l. (a) Octahedral cast ofFon, olivine from the 0.43 ct. dia- = c 5.965(8) mond. The inclusion is about 0.1 mm in length. (b) One of the Enstatite 0.43 carat a = 18.230(5) three almandine-rich garnets from the 0.45 ct. diamond. Its length b = 8 821(4) is about 0.06 mm. Their surfaces contained multitudes of hillock- c = 5 187(2) like irregularities. A 10,000x view of the latter on the garnet Enstatite 0 50 carat a = rB 248(2) shown is illustrated in (d). (c) 4000x view of rhe general octahe- b = 8.807(8) dral topography ofthe c = 5.I77(5) outward-facing chromite layer on the chro- mium-rich garnet from the 0.5 ct. diamond trirnineralic inclusion. (e) A restricted 20,000x view of the chromire. (f) The bimineralic inclusion composed of an octahedral cast of Foe2 olivine (left) and It Enraf-Nonium CAD-4 conputerized diffractmeter, CuKd radiatioD a crystal of pyrrhotite (attached **Bracketed nunbers are the standard deviations. homogeneous crustaceous residue of Fe and Ni oxides with a hexagonal shape on the right) from the 0.5 ct. diamond. The width of this inclusion is about 0.06 mm. PANTALEO ET AL.: INCLUSIONS IN DIAMONDS l06l crystal ofenstatite (Enn,)attached on one corner to a Four sulfide inclusions (pyrrhotite, pyrrhotite + larger central pink rounded chromian pyrope garnet, pyrite, pentlandite, a nickel sulfide + pentlandite) which in turn was cappedon a flat side oppositethe were similarly identified on the basis of residual enstatiteby a thin black layer of chromite (Fig. l). oxides relative to similarly-derived reference stan- The rounded garnet surfacealso showedan etch-like dards.The rectangularform ofthe oxide residuum of hillock texture (Fig. l). After initial single-crystal the pentlandite inclusion indicated an initial crystal XRD analysis,the enstatiteand garnet were broken form. The others showedrounded shapes,suggesting apart and re-analyzed separately.Unit-cell dimen- initial globular forms. XRD analysisof the residual sions for the enstatiteand garnet are given in Table oxide from the nickel sulfide * pentlandite inclusion l. Chemical analysesare given in Table 2. The gar- gaveonly a bunsenitepattern. Eoex analysisshowed net end-member composition is pyrope 32, knor- that the inclusion consistedof about 907onickel sul- ringite 30, uvarovite 5, khoharite 2, alrnandine31. It fide (95-1007oNi, 5-0Vo Fe) and lOVopentlandite correspondsto a Group l0 garnet of Dawson and (25VoFe,75VoNi).There is not sufficientevidence to Stephens(1975). Surfacesof the chromite layer make a mineral identification of the nickel sulfrde. showed mainly an octehedral topography (Fig. l), The pyrite showeda nickel content ranging from 5 to but dodecahedraland cubic topographieswere also l1%o. observed.EDAx analysesshowed a l:2 Fe:Cr cationic composition. 0.62ct. diamond An olivine * pyrrhotite bicrystal was about half This diamond was not completely burned. It was silicate and half sutfide (Fig. l). The olivine (Fon,, possible to eject from the remaining diamond two with a trace of Ni) has octahedralform. The hexago- small tabular colorlessinclusions that XRD analyses nal shapeofthe oxide residueofthe sulfide suggests showed to be diamond. A third small finely poly- an initial crystal form. The cationic chemistryof the crystalline inclusion, fluffy in appearance,was sub- sulfideis98Vo Fe. 27oNi. jected to powder X-ray diffraction analysis and found to contain enstatite and magnetite. "Carbon Table 2. Energy-dispersiveX-ray analysesof some mineral in- spots" observedin the diamond were apparentlycon- clusionsfrom Arkansasdiamonds sumed in the burning processand presumablywere carbon. Mineral