AmericanMineralogist, Volwne66,pages 1050-1053,I98l
Alforsite, a new memberof the apatite grouD:
the barium analogueof chloiapatit6t
NnNcy
G. NEwBERRv, ERIC J. EssENE, AND DoNALD R. peecon
Department of Geological Sciences
University of Michigan
Ann Arbor, Michigan 48109
Abstract
Alforsite, ideally Bar(POo)3CI,
is a new member of the apatite group occurring in contact
metamorphosedevaporitic rocks from Fresno and Mariposa counties,California, associated
with fluorapatite and many other rare barium minerals previously described by Alfors et a/.
(1965).It is optically uniaxial, negative,with c.r,e= 1.70(l). The density is calculated to be
4.83Q) gm/cm3 for the end member composition.It is hexagonal,spacegroup p63,/m,with
a: 10.25(l) and c = 7.64(DL. The strongestlines in the powder pattern are 3.06(100),
2.95(30),2.13(30),2.03(30),and 1.928(30).The associatedassemblagiof witherite, quarti,
and sanbornite buffers the CO, fugacity at metamorphic pressuresand temperatures.The
name is in honor of Dr. John T. Alfors of the California Division of Mines and Geology in
recognition of his contributions to the study of barium minerals.
Introduction
The sanbornite deposits of eastern Fresno and
Mariposa counties, California, contain many rare
barium minerals,principally silicates(Rogers,1932).
The metamorphic sanbornite-quartz rock and foliated quartzite, both of which contain the new mineral, are describedby Matthews and Alfors (1962),
Stinson and Alfors (1963,1964),Alfors and putman
(1964),Alfors and Stinson(1965),and by Alfors et al.
(1965).The sanborniterocks appear to be metasedimentary rocks, presumablymeta-evaporites.The regional geology of the area is describedby MacDonald (l94l) and by Krauskopf (1953). All the
sanbornite localities are within a few hundred meters
of granodiorite intrusions and it will be assumedbelow that the rocks last equilibratedin a contact metamorphic eventproducing hornblende-and pyroxenehornfels faciesconditions at pressuresof l-3 kbar.
Alforsite was first discoveredin samplesfrom the
Big Creek locality during qualitative electron microprobe analysisof what appearedin thin-sectionto be
ordinary apatite. A distinctive reddish-violet cathodoluminescenceand EDA observations showing
major levels of barium, chlorine, and phosphorus
were noticed in several grains selectedfor further
I Contribution No. 366 from
the Mineralogical Laboratory, Department of Geological Sciences,the University of Michigan, Ann
Arbor, Michigan 48109.
0nn3-40/.X/8r /09 I 0- I 050$02.00
study. The phase was found to have the structural
and physical characteristicsof apatite, and determined to be the barium analogueof chlorapatite.As
the natural phase is rare and very fine-grained even
at the type locality, studies were supplementedby
work on a syntheticend-memberbarium chlorapatite
supplied by Dr. Charles Prewitt of the State University of New York at Stony Brook. Alforsite was subsequently found in samplesfrom the Incline locality
where it is, as at Big Creek, associatedwith fluorapatite and many barium minerals.
We have named this mineral for Dr. John T. Alfors of the California Qivision of Mines and Geology
in recognition of his extensivestudiesof the type locatty of the mineral and his descriptions'ofthe other
associatedrare and new minerals. The name and
mineral have been approved by the IMA Commission on New Minerals and New Mineral Names.
Type material, designatedto be that of the Big Creek
locality, is preservedin the mineral collectionsof the
University of Michigan and the National Museum of
Natural History, SmithsonianInstitution under cataloguenumber NMNH l475ll.
X-ray diffraction
Single crystalsof alforsite were separatedby meticulously scratching exceptionally small (approximately 0.02mm dianeter) crystalsfrom thin sections
105|
NEWBERRY ET AL.: ALFORSITE
where it had been identified and differentiatedfrom Table l. Powder diffraction data for synthetic Bas(POa):Cl. CuKc
radiation, I14.6 mm diameter camera
the more common fluorapatiteusing optical and electron microprobe methods. Therefore, space group
1/lo
d(obs)
I/Io
hkl
d(calc)
d(obs)
and unit cell parameterswere determined by stan'I
dard Weissenbergand precessionX-ray diflraction
20
.615
5
l0l
5.79
5.68
20
1
5
photomaterial.
Precession
the
synthetic
l
l
0
methodson
s.l3
5.17
'I . 5 0 0
30
5
200
4.44
4 .41
' I. 566
graphs of the natural material, although yielding
.511
l0
lll
4.26
4.25
1. 4 7 7
s
201
3.8s
very weak reflections,duplicated those of the synI r .a +
'I
.453
002
L3.82
thetic material, confirming that alforsite has the apaI
.
5
102
3.51
3.52
'I 4 2 3
tite structure. Due to the rarity of alforsite grains,
l0
. 398
120
'I2 0
?0
| . JOO
00
powder diffraction data were also obtained using the
3.06
[ 3.07
30
I .336
112
13.06
syntheticbarium chlorapatite.Thesedata were used
2
1.286
30
300
z-vo
z -a?
to refine unit cell parametersby least-squaresmeth1. 2 6 2
2
t3o
?.45
lz.+a
ods. Powder diffraction data for the synthetic phase
1.242
I03
L2.45
1. 2 2 7
2
1
3
'
l
2.36
I z.34
are listed in Table l. Alforsite is hexagonal, with
1.207
302
12.34
I' I. t 8 l
40
4ol
spacegroup P6r/m, analogousto apatite. Unit cell
2.13
|z.] 3
.l44
2?2
L2 . 1 3
parametersate a: 10.25(l)and c : 7.64Q)4.
'I
'I
.125
0
132
2.07
Extremely long X-ray exposuresof Weissenberg 2 . 0 7
2
0
,107
30
230
2.03
| 2.04
and precessionphotographsof the synthetic apatite
30
. 0 77
1?3
L2.03
3
.067
20
231
1. 9 6 8
1.97r
'I 0
failed to reveal the superstructurereflectionswhich
0
30
.042
303
I .931
1.928
occur in X-ray photographsof pure or nearly pure
'I
'I
l0
. 026
sol
I r .730
.734
chlorapatites and other members of the apatite
?0
1.005
t4z
lt.tze
5
403
1.672
group. [f present, these reflections would indicate
f 1.674
3
3
1
L r .657
that the true structure is monoclinic, pseudo- 1 . 6 3 8
a
241
1.639
hexagonal,with one a-translationdoubled relative to
an hexagonalunit cell (Mackie et al.,1972). The synrrSro,oPbo
or)(Pt
thetic alforsite shows no evidence of monoclinic Sroo,)O,,,u(CLnnFo
or) and (BaoorCao
The F + Cl in
respectively.
character.but we cannot be absolutelvsureabout the *Sroo,)O,,nr(CLrrFo,o),
excessof 1.00and the slight deficiencyof Si + P is
natural phase.
consistentwith a minor CO''F substitutionfor POo
Chemicalanalysis
similar to some carbonate-fluorapatites(McConnell,
Analysesof alforsite were made with the Univer- 1973) although deviation from the ideal formula is
sity of Michigan ARL-EMxelectron microprobe with within the precisionof the analysisfor P and Cl. The
three wavelength-dispersivecrystal spectrometers ideal formula is Bar(POo)rCland alforsite is the barand an Ortex current digitizer to compensatefor ium analogueof chlorapatite.
beam current fluctuationsduring data collection.The
Physicalproperties
analyseswere taken at an excitation potential of 12
Alforsite occursas isolatedsmall subhedralgrains,
kV and a specimencurrent of 0.02pa with LiF, PET
and TAP crystal spectrometers.Standardsused were generallylessthan 0.05mm in diameterbut rarely up
fluorapatitefor F and Ca; syntheticbarium chlorapa- to 0.2 mm. These colorlessgrains resembletypical
tite for Ba, Cl, and P; and strontianitefor Sr. Analy- fluorapatite, exhibiting low birefringence and high
sesfor Mn, Si, Pb, and S using synthetic MnrSiOn, relief. This makes it difficult to distinguish alforsite
SiOr, and PbS standardsshowed these appearedin from fluorapatite and from many of the associated
minor amounts. Careful tests were made to insure high-relief barium minerals exceptby using the electhat no damageto or volatilization of the sampleoc- tron microprobe. Alforsite is identifiable through an
curred during analysis. Drift, atomic number, ab- intense violet fluorescencein the 10-15 kV electron
sorption,and fluorescencecorrectionswere made us- beam of a luminoscopeor electron microprobe.This
ing the program EMPADRVII (Rucklidge and fluorescenceis more intenseand redder than the vioGasparrini, 1969).Analyses of the apatite from the let color exhibited by other apatites,and is very disIncline and Big Creek localitiesare listed in Table 2. tinctive. No fluorescenceappears under short- or
These analysesresult in formulae, calculated on the long-wavelength ultraviolet light. Alforsite is unibasisof O + Cl + F : 13,of (Banu.Sro,rCa.,.)(Pr*axial, negativewith extremely low birefringenceand
J.JI
J.JO
1tl
c
NEWBERRY ET AL.: ALFORSITE
Table 2. Electron microprobe analysesof alforsite
oxi de
8ig Creek,
I ncli ne,
wt. %
F r e s n o ,C o .
si 02
l.,lariposa,Co.
<0.05
]t1n0
0.'l
<0.,l
Ca0
+.o
Sr0
2.7
Ba0
6 7. 7
<0 . 1
0.7
2,0
>t.J
Pb0
0.8
< 0 .1
Pzo5
22.7
2l.0
cl
3.6
F
0.7
< 0 .I
sum
0=F,Cl
102.6
-nq
9 8 .9
-0.8
sum
'l0l
.7
3.5
9 8 .I
m o br a t i o s *
P
2.94
si
l,tr
0.01
<0.01
<0.01
Ca
0.75
0.13
Sr
0.24
0.19
Ba
4.05
4.58
<0.01
Pb
0.03
cl
F
0.93
0
S B a r ( P O o ) r ( ,cF] )
%sr5(P04)3(ct,F)
C c a s ( P 0 4 ) 3 (, cFl)
c P b s ( P 0 4 ) 3 (, cFt)
2.98
<0.01
0 .9 9
0.05
0 .1 4
ll.93
1l.96
79
93
5
'15
4
J
I
<l
Occunence
Alforsite occursas an accessorymineral in samples
from both the Incline, Mariposa County, and Big
Creek, Fresno County, localities as describedby Alfors, e/ al. (1965). Big Creek samplesare from the
sites in Section 27, Township I I S, Range 25 E on
their sketch map and were provided by Alfors. The
samplesfrom the Incline locality were obtained from
the University of Michigan Mineralogical Collection
and more speci-ficinformation on sample sites was
not available.
Alforsite is, in both cases,.05 mm or smaller grains
disseminatedin fine-grained metamorphic sanbornite-quartz rock. The alforsite-bearingsarrples are
massivegrey rocks with gneissicbanding due primarily to segregationsof quartz. In samplesfrom Incline,
prominent bandsof pink gillespiteoccur and here alforsite is associatedwith sanbornite, witherite, and
celsianin quartz-rich and gillespite-richbands.Fresnoite, walstromite,tourmaline,and pyrite though less
abundant are disseminatedthroughout the rock. The
lighter grey samplesfrom Big Creek, which contain
alforsite, are quartz-rich and contain no gillespite.
The alforsite and fluorapatite are scattered within
different fine-grained bands which contain a high
proportion of quartz and alternate with bands bearing quartz in lesser amounts. Both bands contain
abundant celsian,witherite, and sanbornitewith accessory,disseminatedfresnoite,walstromite,tourmaline, and pyrite.
Nearly pure fluorapatite is also common in these
iatoms norrralized to 0 + Cl + 5 = 13
Pf=Ps
3 kbor
high relief. The refractive indices were determined as
carefully as possible on small grains removed from a
thin section of the Big Creek sample and are @,€ :
1.70(l). It was possibleto determinethe refractiveindices to a higher degree of accuracy on the larger
grains of the synthetic analogue using crushed grain
mounts,and theseare€: 1.694(3),
c.r: 1.696(3).The
density of alforsite was calculatedto be 4.73(2) gm/
cm3 and 4.80(2) gm/cm'for Big Creek and Incline
localities, respectively,assuming cell contents of 2
formula units and the formulae given above. The
density calculated for the end-membercomposition
is a.83(2)gmlcm3. Sincethe grains of both the natural and syntheticsamplesare very small, surfacetension effectsprohibited measurementof actual densitv
for comparison.
coz
2 kbor
600
T( . C )
500
4oo6
I kbor
WI
o.5
Xcoz/(Xcoz* Xuzo)
10
Fig. l. Calculated decarbonation equilibria for the reaction:
Witherite (Wt) + Quartz (Qz) = Sanbomite(Sb) + CO2.
NEWBERRY ET AL.: ALFORSITE
samples, perhaps suggesting a solvus gap between
Ba-Cl and Ca-F end-member apatites, but more
data would be necessaryto evaluatethis possibility.
The two apatiteshave not been found touching one
another in thin sectionand are generallyin different
bands in the rocks. The coexistenceof theseapatites
probably reflects a difference in bulk sediment layer
composition, and may not indicate a solvus gap in
the system.Apatitestypically form completesolid solutions between end-members,but a solvus is possible on crystal-chemical grounds as the difference in
ionic radii betweenCa and Ba atoms is quite large
(Shannonand Prewitt, 1969).
Alforsite occursin a univariant assemblageof sanbornite-quartz-witherite. Energy dispersiveanalysis
of thesephasesindicates that they are >95Voof the
end-membercomponents.CO, fugacity is buffered at
a given P-T by the reaction:
witherite * quartz : sanbornite+ CO,
BaCO,+ 2SiOr: BaSirO,+ CO,
Although thermodynamic data are incomplete for
sanbornite, Kelley (1962) estimated AG? data for
sanbornite allowing calculation of the reaction using
standard thermodynamic data for the other phases
(Robie et al.,1978). The resultsare given in Figure I
for 1,2, and 3 kbar at variable X.o,/(X.o, * &,").
No quantitative P-T information are available for
the contact metamorphismlimiting the usefulnessof
the calculationfor determhing X.o,. However,it can
be seen from Figure I that temperaturesof 500600oC are required for the witherite-sanbornitequartz assemblageover a wide range of X.o,. If the
thermodynanic data for sanbornite are correct, adjacent wollastonite rocks (Alfors et al., 1965) require
higher T or lower X.o, (Greenwood, 1967).A more
preciseinterpretation of the P-T-X data dependson
more accurategeothermometryand on better thermodynamic data for sanbornite.
References
Alfors. John T. and Putman,G. W. (1965)Revisedchemicalanalysesoftraskite, verplanckite,and muirite from Fresno County'
California. American Mineralogist, 50' I 500-I 503'
Alfors, J. T. and Stinson,M. C. (1965)New mineralsfrom Fresno
County-Il. California Division of Mines and Geology, Mineral Information Service,18,27-30.
Alfors, J. T., Stinson,M. C., Matthews,R. A', and Pabst,A' (1965)
Sevennew barium mineralsfrom EastemFresnoCounty, California. American Mineralogist, 50' 3l+340'
Greenwood,H. J. (1967)wollastonite: stability in H2O-CO2mixtures and occuffenoe in a contact-metamorphic aureole near
Salmo, British Columbia. American Mineralogist, 52, 16691680.
Kelley, K. K. (1962)Heats and free energiesof formation of anhydrous silicates.United StatesBureau of Mines Report of Investigation,5901,l-12.
Kranskopf, K. B. (1953)Tungstendepositsof Madera,Fresnoand
Tulare Counties,California. California Division of Mines and
Geology SpecialReport, 35, l-120.
MacDonald, G. A. (1941)Geology of the westernSierra Nevada
betweenthe Kings and SanJoaquin Rivers,California' Univer'
sity of California Publications,Bulletin of the Department of
GeologicalSciences,26, 215-286.
McConnell, D. (1973)Apatite (Applied Mineralogy 5). SpringerVerlag, Wien.
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chlorapatite.Acta Crystalstnrcture of synthetic Ca5(POa)3C!,
lographic4828, 1840-1848.
Matthews, R. A. and Alfors, J. T' (1962) Sanbornitefrorn Rush
Creek,Fresno County, California Division of Mines and Geology, Mineral Information Service,15' l-3.
Robie, R. A., Hemingway, B. S., and Fisher, J. R. (1978) Thermodynamic proPerties of minerals and related substanc€sat
298.15K and I bar (l0s pascals)pressureand at higher t€mperatures.United StatesGeological Survey Bulletin, 1452,1456'
Rucklidge, J. C. and Gasparrini, E. L. (1969)Specificationsof a
complete Program for Processing Elecfion Microprobe Data:
EMIADRVII, Departmentof Geology, University of Toronto'
Shannon,R. D. and Prewitt, C. T' (1969)Efective ionic radii in
oxidcs and fluorides. Acta Crystallographica, B.25,925-946'
Stinson, M. C. and Alfors, J. T. (1963) Unusual minerals from
Fresno County, California. California Division of Mines and
Geology, Mineral Information Service,16' l0-ll.
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County-I. California Division of Mines and Geology, Mineral
Information Service,17, 23'l-238.
Acknowledgments
We are grateful to Prof. Charles Prewitt for providing the synand to Dr. John Alfors for providing the Big
thetic Ba5(PO1)3C1,
Creek sanbornite samPles.
Manuscript received,January 23, l98I;
acceptedfor publication, March 27' I98l-