Davisite, ideally CaScAlSiO_6, is a new member of the Ca clinopyroxene group, where Sc^(3+) is dominant in the M1 site. It occurs as micro-sized crystals along with perovskite and spinel in an ultra-refractory inclusion from the Allende meteorite. The mean chemical composition determined by electron microprobe analysis is (wt%) SiO_2 26.24, CaO 23.55, Al_2O_3 21.05, Sc_2O_3 14.70, TiO_2 (total) 8.66, MgO 2.82, ZrO_2 2.00, Y_2O_3 0.56, V_2O_3 0.55, FeO 0.30, Dy_2O_3 0.27, Gd_2O_3 0.13, Er_2O_3 0.08, sum 100.91. Its empirical formula calculated on the basis of 6 O atoms is Ca_(0.99)(Sc_(0.50)Ti^(3+)0.16^(Mg)0.16Ti^(4+)0.10 Zr_(0.04)V^(3+)_(0.02)Fe^(2+)_(0.01)Y_(0.01))_(∑1.00)(Si_(1.03)Al_(0.97))_(∑2).00O_6. Davisite is monoclinic, C2/c; a = 9.884 Å, b = 8.988 Å, c = 5.446 Å, β =105.86°, V = 465.39 Å^3, and Z = 4. Its electron back-scattered diffraction pattern is an excellent match to that of synthetic CaScAlSiO6 with the C2/c structure. The strongest calculated X-ray powder diffraction lines are [d spacing in Å (I) (hkl)]: 3.039 (100) (221), 2.989 (31) (310), 2.943 (18) (311), 2.619 (40) (002), 2.600 (26) (131), 2.564 (47) (221), 2.159 (18) (331), 2.137 (15) (421), 1.676 (20) (223), and 1.444 (18) (531). The name is for Andrew M. Davis, a cosmochemist at the University of Chicago, Illinois

Ma, Chi

Rossman, George R.

English

Caltech Authors - Main

American Mineralogist, Volume 94, pages 845–848, 2009
0003-004X/09/0506–845$05.00/DOI: 10.2138/am.2009.3209      845
Letter
Davisite, CaScAlSiO6, a new pyroxene from the Allende meteorite
Chi Ma* and GeorGe r. rossMan
Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, Califonia 91125, U.S.A.
abstraCt
Davisite, ideally CaScAlSiO6, is a new member of the Ca clinopyroxene group, where Sc3+ is 
dominant in the M1 site. It occurs as micro-sized crystals along with perovskite and spinel in an 
ultra-refractory inclusion from the Allende meteorite. The mean chemical composition determined by 
electron microprobe analysis is (wt%) SiO2 26.24, CaO 23.55, Al2O3 21.05, Sc2O3 14.70, TiO2 (total) 
8.66, MgO 2.82, ZrO2 2.00, Y2O3 0.56, V2O3 0.55, FeO 0.30, Dy2O3 0.27, Gd2O3 0.13, Er2O3 0.08, 
sum 100.91. Its empirical formula calculated on the basis of 6 O atoms is Ca0.99(Sc0.50Ti3+0.16Mg0.16Ti4+0.10 
Zr0.04V3+0.02Fe2+0.01Y0.01)Σ1.00(Si1.03Al0.97) Σ2.00O6. Davisite is monoclinic, C2/c; a = 9.884 Å, b = 8.988 Å, c 
= 5.446 Å, β =105.86°, V = 465.39 Å3, and Z = 4. Its electron back-scattered diffraction pattern is 
an excellent match to that of synthetic CaScAlSiO6 with the C2/c structure. The strongest calculated 
X-ray powder diffraction lines are [d spacing in Å (I) (hkl)]: 3.039 (100) (221), 2.989 (31) (310), 
2.943 (18) (311), 2.619 (40) (002), 2.600 (26) (131), 2.564 (47) (221), 2.159 (18) (331), 2.137 (15) 
(421), 1.676 (20) (223), and 1.444 (18) (531). The name is for Andrew M. Davis, a cosmochemist at 
the University of Chicago, Illinois.
Keywords: Davisite, CaScAlSiO6, new mineral, Sc-rich pyroxene, refractory phase, ultra-refractory 
inclusion, Allende meteorite
introduCtion
During a nano-mineralogy investigation of the Allende meteor-
ite, a Sc-rich pyroxene was observed in an ultra-refractory inclusion. 
Electron-microprobe, high-resolution SEM, electron-backscatter 
diffraction (EBSD), and Raman analyses have been used to char-
acterize its composition and structure. Synthetic CaScAlSiO6 is 
known (Ohashi and Ii 1978). Highly Sc-enriched pyroxenes (13–16 
wt% Sc2O3) have been found in four Ca-,Al-rich inclusions from the 
Ornans, Murchison, Efremovka, and Ningqiang meteorites based 
on chemical analyses (Davis 1984; Davis and Hinton 1985; Simon 
et al. 1996; El Goresy et al. 2002; Lin et al. 2003). We report here the 
occurrence, composition and crystal structure of a clinopyroxene 
from the Allende meteorite, CaScAlSiO6, where scandium is the 
dominant trivalent component in the M1 site. 
MineraL naMe and type MateriaL
The new mineral and its name have been approved by the 
Commission on New Minerals, Nomenclature and Classification 
(IMA 2008-30). The name is for Andrew M. Davis, Professor 
of Cosmochemistry at the University of Chicago, born in 1950, 
in honor of his outstanding contributions to meteorite research. 
He observed a highly Sc-rich pyroxene (~15 wt% Sc2O3) from 
a meteorite in 1984 (Davis 1984). Holotype material (Caltech 
Section Allende12 MC2M) has been deposited in the Smithso-
nian Institution’s National Museum of Natural History and is 
cataloged under USNM 7555.
oCCurrenCe, assoCiated MineraLs, and oriGin
A fine-grained aggregate of davisite occurs with REE-
rich perovskite and spinel in a single Ca-,Al-rich refractory 
inclusion (CAI) from the Allende meteorite (Figs. 1–2). The 
fractured inclusion is exposed in one polished thick section, 
prepared from a 1 cm Allende specimen at Caltech. The mineral 
occupies most of the area in this CAI, which is about 130 µm 
wide in the section plane, surrounded by a matrix of mostly 
olivine and troilite. Nearby are fragments of a likely type-II 
chondrule. The Allende meteorite, which fell at Pueblito de 
Allende, Chihuahua, Mexico, on February 8, 1969, is a CV3 
carbonaceous chondrite.
appearanCe, physiCaL, and optiCaL properties
The type material occurs as an aggregate. Electron back-
scatter diffraction mapping reveals that the aggregate consists 
of crystals that are about 2–12 µm in width with different 
orientations. In thin section, it is transparent with a light-gray 
color, which may be caused by adjacent and underlying phases. 
Streak, luster, hardness, tenacity, cleavage, fracture, and den-
sity were not determined because of the small grain size. The 
density, calculated from the empirical formula, is 3.38 g/cm3. It 
is non-fluorescent under the beams of the electron microprobe 
and SEM. Optical properties were not determined because of 
the small grain size; n (calc) = 1.736. In the section, the crystal 
grains are irregular to subhedral. No forms or twinning were 
observed. The a:b:c ratio calculated from the unit-cell parameters 
is 1.0997:1:0.6059.* E-mail: chi@gps.caltech.edu
MA AND ROSSMAN: DAVISITE, CaScAlSiO6, A NEW PYROXENE846
CheMiCaL CoMposition
Quantitative elemental microanalyses were conducted with 
the JEOL 8200 electron microprobe operated at 15 kV and 25 
nA in a focused beam mode. Standards for the analysis were 
anorthite (CaKα, AlKα, SiKα), ScPO4 (ScKα), TiO2 (TiKα), 
zircon (ZrLα), YPO4 (YLα), fayalite (FeKα), V2O3 (VKα), 
forsterite (MgKα), DyPO4 (DyLα), GdPO4 (GdLα), and ErPO4 
(ErLα). Analyses were processed with the CITZAF correction 
procedure (Armstrong 1995). Six individual analyses reported 
in Table 1 were carried out using WDS mode. No other elements 
with atomic number >4 were detected by WDS scans. 
The empirical formula, based on 6 O atoms, is 
Ca0.989(Sc0.502Mg0.165Ti3+0.152Ti4+0.105Zr0.038V0.017Y0.012Fe2+0.010Dy0.003 
Gd0.002Er0.001)Σ1.007(Si1.028Al0.972)Σ2.000O6
where Ti3+ is calculated based on stoichiometry. The ideal, end-
member formula is CaScAlSiO6, which requires: CaO 23.75, 
Sc2O3 29.21, Al2O3 21.59, SiO2 25.45, Total 100.00 wt%.
CrystaLLoGraphy
Crystallography by EBSD at a sub-micrometer scale was 
carried out using the methods described in Ma and Rossman 
(2008, 2009) with an HKL EBSD system on the ZEISS 1550VP 
scanning electron microscope, operated at 20 kV and 8 nA in a fo-
cused beam with a 70° tilted stage. The structure was determined 
and cell constants were obtained by matching the experimental 
EBSD pattern (Fig. 3) with the structures of synthetic synthetic 
CaScAlSiO6 (Ohashi and Ii 1978), esseneite (Cosca and Peacor 
1987), and orthorhombic pyroxenes (Molin 1989). 
The EBSD patterns can be indexed only by the monoclinic 
C2/c structure to give a best fit based on unit-cell data from 
CaScAlSiO6 (Ohashi and Ii 1978) (Fig. 3), showing a monoclinic 
structure, space group: C2/c, a = 9.884 Å, b = 8.988 Å, c = 5.446 
Å, β =105.86°, V = 465.39 Å3, and Z = 4 with the mean angular 
deviations as low as 0.18. No errors are stated because the cell 
parameters are taken directly from the data of the matching 
CaScAlSiO6 phase in Ohashi and Ii (1978).
X-ray powder-diffraction data (CuKα1) are calculated from 
Table 1. The mean analytical results of davisite
Constituent* wt% Range Stand. Dev. EPMA Standard
SiO2 26.24 25.11–28.58 1.25 anorthite
CaO 23.55 23.37–23.65 0.10 anorthite
Al2O3 21.05 19.39–22.05 0.92 anorthite
Sc2O3 14.70 13.05–15.49 0.88 ScPO4
TiO2† 8.66 8.40–8.89 0.19 TiO2
MgO 2.82 2.20–4.03 0.65 forsterite
ZrO2 2.00 1.66–2.15 0.18 zircon
Y2O3 0.56 0.51–0.60 0.03 YPO4
V2O3 0.55 0.49–0.64 0.05 V2O5
FeO 0.30 0.27–0.33 0.02 fayalite
Dy2O3 0.27 0.16–0.33 0.07 DyPO4
Gd2O3 0.13 0.08–0.21 0.05 GdPO4
Er2O3 0.08 0.03–0.14 0.04 ErPO4
 Total 100.91     
* All listed constituents are above their detection limits at 99% confidence
† Total TiO2 wt% recalculated to 4.64 wt% Ti2O3 + 3.57 wt% TiO2 to obtain stoichi-
ometry and valency balance, gives a new analytical total = 100.46 wt%.
FiGure 1. Backscatter electron image of the davisite-containing 
refractory inclusion in a polished section (USNM 7555) of the Allende 
meteorite.
FiGure 2. Enlarged BSE image showing davisite with perovskite 
and spinel (Fig. 1). The cross marks where the EBSD pattern (shown 
in Fig. 3) was collected.
the cell parameters from Ohashi and Ii (1978) with the empirical 
formula from this study using Powder Cell version 2.4 (2000). 
The strongest X-ray powder diffraction lines are [d spacings 
in Å (I) (hkl)]: 3.039 (100) (221), 2.989 (31) (310), 2.943 (18) 
(311), 2.619 (40) (002), 2.600 (26) (131), 2.564 (47) (221), 
2.159 (18) (331), 2.137 (15) (421), 1.676 (20) (223), and 1.444 
(18) (531).
speCtrosCopiC properties
Raman microanalysis was carried out using the methods 
described in Ma and Rossman (2008, 2009). The Raman spec-
trum gave no indication of either H2O or CO2 in davisite. Raman 
microanalyses show that the spectrum of davisite (using a 514.5 
nm laser) has intense fluorescent features and is not comparable 
to that of synthetic CaScAlSiO6 (Sekita et al. 1988), which 
only covers a limited range, as shown in Figure 4. The intense 
fluorescence features are due to the significant amount of REE 
elements. Consequently, the identification of davisite is based 
on EBSD and electron-probe results only.
disCussion
Davisite is a new member of the Ca clinopyroxenes (diop-
side group) with space group C2/c (Morimoto et al. 1988), the 
Sc-dominant analog of esseneite (CaFe3+AlSiO6). Scandium is 
MA AND ROSSMAN: DAVISITE, CaScAlSiO6, A NEW PYROXENE 847
including oxides, pyroxenes, and perovskites in Ca-,Al-rich 
refractory inclusions. The newly discovered mineral allendeite 
(Sc4Zr3O12, IMA 2007-027) and its associated Sc-stabilized ta-
zheranite [cubic zirconia, (Zr,Sc,Ca)O1.75] from Allende are the 
most Sc-rich oxides found to date (Ma et al. 2009). Davisite is 
one of the more Sc-rich phases ever reported currently surpassed 
only by pretulite (ScPO4), thortveitite [(Sc,Y)2Si2O7], kolbeckite 
(ScPO4·2H2O), and the newly approved mineral allendeite.
Refractory inclusions are the first known solids formed in 
the solar system. Davisite is a new refractory mineral from 
Allende, associated with REE-rich perovskite with a formula 
(Ca0.81Y0.07Dy0.03Gd0.02Sc0.02Na0.01)(Ti4+0.94Al0.03V0.01Fe0.01)O3 and 
Mg-Al spinel in an ultra-refractory inclusion. This fine-grained 
inclusion shows an irregular shape without a rim. Its mineral-
ogy is characterized by high refractory and rare earth elements 
enrichment in perovskite and davisite. The inclusion is likely 
formed through high-temperature condensation in the solar 
nebula, followed by partial melting and crystallization. A more 
detailed geochemical investigation of refractory elements and 
REE in this ultra-refractory inclusion is in progress (Ma et al. in 
prep). It is unlikely that this CAI is associated with the nearby 
Type-II ferromagnesian chondrule fragments.
Other pyroxenes from CAIs in the Ornans, Murchison, Efre-
movka, and Ningqiang chondrites where Sc3+ dominates the M1 
site with 44 to 56% occupancy (Davis 1984; Davis and Hinton 
1985; Simon et al. 1996; El Goresy et al. 2002; Lin et al. 2003) 
would be davisite as well if their structure can be verified to 
be C2/c by EBSD or other diffraction methods. Ca-pyroxenes 
containing up to several weight percent of Sc2O3 are not rare in 
CAIs (e.g., Simon et al. 1996; El Goresy et al. 2002; this study). 
It is apparent that a solid solution exists between Al-,Ti-rich 
diopside and davisite.
aCknowLedGMents
The Caltech Analytical Facility at the Division of Geological and Planetary 
Sciences is supported, in part, by grant NSF EAR-0318518 and the MRSEC Pro-
gram of the NSF under DMR-0080065. Funding from NSF grant EAR-0337816 
is also acknowledged. We thank Alan Rubin and Kimberly Tait for reviewing 
this manuscript.
reFerenCes Cited
Anders, E. and Grevesse, N. (1989) Abundances of the elements: Meteoritic and 
solar. Geochimica et Cosmochimica Acta, 53, 197–214.
Armstrong, J.T. (1995) CITZAF: A package of correction programs for the quan-
titative electron microbeam X-ray analysis of thick polished materials, thin 
films, and particles. Microbeam Analysis, 4, 177–200. 
Cosca, M.A. and Peacor, D.R. (1987) Chemistry and structure of esseneite 
(CaFeAlSiO6), a new pyroxene produced by pyrometamorphism. American 
Mineralogist, 72, 148–156.
Davis, A.M. (1984) A scandalously refractory inclusion in Ornans. Meteoritics, 
19, 214.
Davis, A.M. and Hinton, R.W. (1985) Trace element abundances in OSCAR, a 
scandium-rich refractory inclusion from the Ornans meteorite. Meteoritics, 
20, 633–634.
El Goresy, A., Zinner, E., Matsunami, S., Palme, H., Spettel, B., Lin, Y., and 
Nazarov, M. (2002) Efremovka 101.1: A CAI with ultrarefractory REE pat-
terns and enormous enrichments of Sc, Zr, and Y in fassaite and perovskite. 
Geochimica et Cosmochimica Acta, 66, 1459–1491.
Jarosewich, E., Clarke Jr., R.S., and Barrows, J.N. (1987) The Allende meteorite 
reference sample. Smithsonian Contributions to the Earth Sciences, No.27.
Lin, Y., Kimura, M., and Wang, D. (2003) Fassaites in compact type A Ca-Al-rich 
inclusions in the Ningqiang carbonaceous chondrite: Evidence for partial melt-
ing in the nebula. Meteoritics and Planetary Science, 38, 407–417.
Ma, C. and Rossman, G.R. (2008) Barioperovskite, BaTiO3, a new mineral from 
the Benitoite Mine, California. American Mineralogist, 93, 154–157.
——— (2009) Tistarite, Ti2O3, a new refractory mineral from the Allende meteorite. 
FiGure 4. Raman spectra of davisite (this study) and synthetic 
CaScAlSiO6 (Sekita et al. 1988), collected under different conditions, 
showing relative intensities. The spectrum for the synthetic sample covers 
a very limited range, and is displaced vertically from the zero line to 
facilitate comparison with the davisite spectrum.
FiGure 3. (a) EBSD pattern of the labeled davisite crystal (marked 
with a cross) in Figure 2; (b) the pattern perfectly indexed with the C2/c 
CaScAlSiO6 structure.
a
b
a sparsely distributed element in the Earth’s crust, but is sig-
nificantly more abundant in the sun and primitive meteorites 
(Weaver and Tamey 1984; Anders and Grevesse 1989). The Al-
lende meteorite containing 11~12 ppm (Jarosewich et al. 1987) 
has provided a variety of phases with high scandium contents 
MA AND ROSSMAN: DAVISITE, CaScAlSiO6, A NEW PYROXENE848
American Mineralogist,94, 841–844.
Ma, C., Beckett, J.R., and Rossman, G.R. (2009) Allendeite and hexamolybdenum: 
Two new ultra-refractory minerals in Allende and two missing links. 40th Lunar 
and Planetary Science Conference, The Woodlands, Texas, Abstract 1402.
Molin, G.M. (1989) Crystal-chemical study of cation disordering in Al-rich and 
Al-poor orthopyroxenes from spinel lherzolite xenoliths. American Mineralo-
gist, 74, 593–598.
Morimoto, N., Fabries, J., Ferguson, A.K., Ginzburg, I.V., Ross, M., Seifert, F.A., 
Zussman, J., Aoki, K., and Gottardi, G. (1988) Nomenclature of pyroxenes. 
American Mineralogist, 73, 1123–1133.
Ohashi, H. and Ii, N. (1978) Structure of calcium scandium aluminum silicate 
(CaScAlSiO6)-pyroxene. Journal of the Japanese Association of Mineralogists, 
Petrologists and Economic Geologists, 73, 267–273.
Sekita, M., Ohasi, H., and Terada, S. (1988) Raman spectroscopic study of clinopy-
roxenes in the system calcium scandium aluminum silicate (CaScAlSiO6)—
calcium aluminum silicate (CaAl2SiO6). Physics and Chemistry of Minerals, 
15, 319–322.
Simon, S.B., Davis, A.M., and Grossman, L. (1996) A unique ultrarefractory 
inclusion from the Murchison meteorite. Meteoritics and Planetary Science, 
31, 106–115.
Weaver, B.L. and Tamey, J. (1984) Major and trace element composition of the 
continental lithosphere. In H.N. Pollack and V.R. Murthy, Eds., Physics and 
Chemistry of the Earth, 15, p. 39–68. Pergamon, Oxford.
Manuscript received February 4, 2009
Manuscript accepted February 18, 2009
Manuscript handled by bryan chakouMakos


Davisite, CaScAlSiO_6, a new pyroxene from the Allende meteorite

http://authors.library.caltech.edu/15855/1/Ma2009p4299Am_Mineral.pdf

Davisite, CaScAlSiO_6, a new pyroxene from the Allende meteorite

Abstract

Similar works

Full text

Available Versions

Caltech Authors - Main

Caltech Authors - Main