High-Pressure Crystal Chemistry of Andradite and Pyrope: Revised
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American Mineralogist, Volume 74, pages 352-359, 1989 High-pressurecrystal chemistry of andradite and pyrope: Revisedprocedures for high-pressurediffraction experiments RonBnrM. HaznN,Llnnv W. FrNcrn GeophysicalLaboratory, CarnegieInstitution of Washington, 2801 Upton Street,N.W., Washington, D.C. 20008, U.S.A. Ansrucr Modified single-crystalX-ray diffraction techniqueshave been adopted in the study of the crystal structure of andradite garnet at several pressuresto 19.0 GPa (190 kbar) and the compressionof a mantle pyrope to 6.0 GPa. Bulk modulus, K, of end-member natural andradite from Val Malenco, Italy, is 159 + 2 GPa (assumingdK/dP: 4), whereasthat of pyrope from Kakanui, New Z.ealand,is 179 + 3 GPa. In andradite, the Ca-containing 8-coordinated polyhedron, Fe3+-containingoctahedron, and Si-containing tetrahedron all display significantcompression. The larger Ca-containingpolyhedron, which is more com- pressible than Fe- or Si-containing polyhedra, has the same bulk modulus as the bulk crystal. Below l0 GPa, bond compressionis accompaniedby a significant decreasein Si- O-Fe angle. This behavior is similar to that of framework silicates, in which a corner- linked network of smaller, more rigid polyhedra (Si and Fe) collapsesabout a larger, more compressible polyhedron (Ca). At higher pressure,however, polyhedral distortion may play a significant role in garnet compression. We use an optimized procedurefor collecting X-ray diffraction data from single crystals for which peak-to-backgroundratios are small, and the ratio of accessiblereflections to structural parametersis large. A subsetof reflections-those most influenced by variable atom positional parameters-is scannedat relatively slow rates.By counting a selectsubset of reflectionsfor longer times, we greatly increasedthe percentageof observedreflections, improved precision of refined positional parameters and interatomic distances,and re- duced the total diffractometer time for the data-collection experiment. maximum pressure.A third difficulty is the reduction in '.rrnonucrroN peak-to-backgroundratios resulting from the use of small Garnet is one of about half a dozen silicate structure crystalsat higher target pressures.A crystal must be thin- types thought to constitute most of the Earth's upper ner than about 25 pm if it is to be preserveduncrushed mantle and transition zone. Pyrope (MgrAlrSirO,r) and at pressuresabove l0 GPa. Futhermore, thicker Be seats grossular (CarAlrSirO,r) were thus among the first sili- are required; thus peak-to-background ratios are often cates to be examined with high-pressure single-crystal small in high-pressuresingle-crystal experiments. X-ray diffraction techniques (Hazen and Finger, 1978). Improvements in diamond-cell design and operation Those studies, which employed the triangular Merrill- have greatly reducedthe first two problems. The rangeof Bassettdiamond anvil cell, were limited to about 6 GPa availablehydrostatic pressure has beenincreased to above (60 kbar). It was not possible to resolve some important 30 GPa through the use of solidified gas as the pressure details of the structure response(the compressionof Si- medium (Mills et al., 1980; Jephcoatet al., 1987). Jeph- O bonds, for example) from those data. coat et al. (1988), for example, have obtained Raman Until recently, researcherswere limited to an upper spectroscopicmeasurements on a singlecrystal of BeO to pressurerange of about l0 GPa for single-crystalexper- pressuresapproaching 50 GPa using a lever-arm dia- iments in the laboratory. At least three factors contrib- mond-anvil cell and the gas-mediaprocedures. In these uted to this limit. First, all of the standard hydrostatic experiments, the open cell is placed in a steel bomb, which pressurefluids, mixtures of alcohols and water, become is pressurizedto 0.2 GPa with the appropriate gas. The solid and stiff at pressuresbetween l0 and 14 GPa (Fu- diamond cell is closed by a remote-controlled motorized jishiro et al., l98l). Single crystals strain and become gear assembly. H, He, Ne, and Ar have been used suc- uselessfor diffraction experiments under these condi- cessfully as media in single-crystal experiments, with tions. The secondlimitation is the difficulty of operating higher hydrostatic pressuresresulting from the lighter the standard Merrill-Bassett diamond cell (see,e.g., Ha- gases. zen and Finger, 1982) above l0 GPa. Diamond align- The design of the X-ray cell has also been improved ment is not well controlled in the three-screw,triangular with strongerBe support disks and better diamond align- pressurecell,andsogasketdeformationusuallylimitsthe ment (see,e.g., Mao and Bell, 1980).Single crystals of 0003-o04x/89/0304-0352$02.00 352 HAZEN AND FINGER: ANDRADITE STRUCTURE REFINEMENT AT 19 GPa 353 wiistite have been studied to 22 GPa in a cell designed obtained from the Smithsonian collections fNMNH for X-ray diffraction experiments (Hazen et al., l98l). number ll934l). The mantle-derivedspecimen, a frag- The principal objective of this study was to employ our ment of a monomineralic megacryst,was collected from improved single-crystalpressure cell and gas-mediatech- an eroded volcanic neck. The composition of the un- niques to determine the crystal structure and compress- zoned crystal, as reported by Mason (1966) and Jarosew- ibility of andradite garnet to above 15 GPa. Andradite ich (1972), is (Mg, ooCa. rnMno o, Fefr .{) (Al, r* Fel.d.Ti"orr)- (CarFerSi.O,r)was selectedbecause of its relatively high (Si, e,5Al0 'ss)O,r. This composition, representedas frac- averageelectron density compared to other common gar- tions of end-member garnets,is as follows: 0.670 pyrope, net species.At the same time, we undertook compres- 0. 197 almandine, 0. 105 grossular,0.025 andradite, and sional studies to 6 GPa in the standard Merrill-Bassett 0.006 spessartine.This optically isotropic sample is free diamond cell of a natural, mantle-derived pyrope garnet ofvisual defectsand possessesa deep red color. Studies from Kakanui, New Zealand. We wished to determine of the cation ordering of diopside megacrystsfrom the whether the natural sample-with composition approxi- same locality (McCallister et al., 1975) suggestequilibra- mately 670/opyrope,2Oolo almandine, 10.50/ogrossular, and tion above 1300'C and subsequentrapid eruption and 2.5o/oandradite-has a compressibility significantly dif- cooling of the Kakanui samples. ferent from that of end-member pyrope. Both the andradite and pyrope experiments were af- Room-pressuredata collection and structure refinements fected by the problem of collecting X-ray intensity data Room-pressure crystal-structure analysis was per- on small crystals with poor peak-to-backgroundratios. formed on eachgarnet prior to the high-pressurestudies. Andradite data collectionswere limited by the small size Equant 100-pm fragments were mounted on glasscapil- of crystals that can be preserved at pressuresabove l0 laries and centered on an automated four-circle difrac- GPa, whereas pyrope data suffer from the intrinsically tometer with graphite-monochromatizedMo radiation (tr weak X-ray scatteringby elements with Z less than 15. : 0.7093 A;. notn samplesdisplayed cubic unit-cell di- Conventional high-pressuredata-collection procedures, mensions within two estimated standard deviations. The by which all accessiblereflections are measured using room-pressurea cell edgesfor andradite and pyrope are moderate scan speeds,proved ineffective. The resulting 12.031+ 0.001 and I1.548 + 0.001,respectively, based refinements were of poor quality and lacked the resolu- on positions of centered reflections. We employed the tion necessaryto identify significant pressure shifts in procedure ofKing and Finger (1979), by which each re- atomic positions. flection is centeredin eight equivalent positions and there- A secondaryobjective of this garnet study, therefore, by correctedfor crystal centeringand diffractometer zero was to find an alternative data-collection algorithm. We EITOTS. decided to identifu an "optimal" subsetof accessiblere- Cubic andradite and pyrope garnetsadopt spacegroup flections that are most strongly affectedby atomic posi- Ia3d. Three cations-Si at (3/e,0,1/a),the octahedralcation tions and to collect those select reflections with slower at (0,0,0),andthe 8-coordinatedcation at (h,O,Vt)-are in scan rates. The garnet structure is ideally suited to test fixed special positions, whereasoxygen is in the general this new high-pressureX-ray procedure. Garnet's cubic position. Intensity data for three-dimensionalrefinements unit-cell edge of - 12 A yields a large unit cell with as were collected with omega step scans of 0.025" and 4.0 many as 1500accessible reflections corresponding to more secondsper step.We collectedan octant of data to sin d/tr than 100 symmetrically distinct reflectionsin a diamond- r 0.7 for andradite and a hemisphereof data for pyrope. cell experiment. Yet garnet has only three variable posi- Digitized data wereintegrated by the method of Lehmann tional parameters (the x, y, and z of oxygen). An opti- and Larsen (1974). Intensity data were correctedfor Lo- mized data-collectionalgorithm allows the difractometer rentz and polarization effectsand specimen absorption, to spend most of its time measuringan influential subset and the resulting structure factors were averagedto yield of reflections,rather than on unobservedreflections that 2 17 symmetrically independent reflections for andradite would be rejected during refinement. and 154 for pyrope.l Structurevariables, including