Ameilcan Mineralogist,

Volume 66, pages 1068-1072, IgBt

Sodianpotassianhydroxonianmeta-autunite:fi rst natural occurrenceof an intennediatememberof a predictedsolid solution series
C.R.M. Burr exp J. Gnasnnr
Divisionof Mineraloglt, CSIRO
PrivateBag, P.O., Wembley,Vl/estern
Australia,6014

Abstract.UOr.pO4. 3.25HrO,
Sodianpotassianhydroxonianmeta-autunite,(NaoroKo,rr(HrO)o.,u)
is a novel member of the meta-autunitegroup. It occurs in a playa 105 km NNE of Kalgoorlie, Western Australia as void and fracture infillings in near-surfacegypsiferousmuds.
Mineral aggregatesare bright greenish yellow with a strong yellow fluorescenceunder UVlight and consistof micaceouscrystallitesusually 2G-50prn in diameter.The mineral has a
R.I. of 1.575-1.580and is probably uniaxial, optically negative.Reversibledehydration occurs readily on heating or under vacuum;dehydration is completeabove 300'C but remains
reversibleeven after partial melting has occurredon heating to l200oC. Principal X-ray diffraction lines are d : a.gatrc), 3.49(200),2,47(rt3) and 220), 1.559(420).By the Guinier
powder technique,theselines have intensitios75, 100,75 and 50 respectively.These corresPondto a-meta-autuniteI structurewith tetragonalpseudo-celldimensiona' : 6.971L and,c' : 8.545A.Heating did not produce a phaseequivalent to the meta-autuniteII structure.
The sodianpotassianhydroxonian meta-autuniteis the first reported occurrenceofan intermediate member of a predicted solid solution serieswith potassium,sodium and hydroxonium end members.It differs in origin from other naturally-occurring autunite and metaautunite minerals in being a direct precipitate from groundwater, rather than an alteration
product.

Introduction
Severalmembers of the autunite and meta-autunite groupswith monovalentcationsin interlayer positions are known as naturally occurring minerals
znd/or as synthetic compounds. Ross and Evans(1965) suggestthat there may exist a solid solution
series,the torbernite group, with the generalformula:A(UOr ' POo)'3HrO where A : Li*, Na*, K*, Rb*
Cs* NHn* and/or HrO*. This seriesmay also be isomorphous with arsenate minerals with the forrrula
A(UOr'AsOo)'3HrO.
The known naturally-occurring end members ofthe monovalent uranyl phosphateseriesrelevant tothis paper are meta-ankoleite,(IQrrBa.rXUOr. PO.)
3H,O (Gallagher and Atkin, 1966), sodium metaautunite (Chernikov et al.,1958) and hydrogen autunite (Camargo,l97l), all of which occur rather sparingly as alteration products. We have examined
meta-ankoleitefrom the type locality by electronmicroprobeanalysisand have not found barium, so themineral is probably a true end member of the series.
w03 -004x/ 8| / 09l 0- I 068$02.00

A new phase, sodian potassianhydroxonian metaautunite, describedhere, is the first reported natural
occurrence of an intermediate member of such a series and differs from them in being a direct precipitate in a playa, rather than an alteration product.
This occurrence also is significant in that all othersimilar enrichmentsof uranium in drainagesin Western Australia are due either to carnotite or to uranium in chalcedonic or opaline silica (Butt el al.,1977). The discovery of this mineral was made by
EssoMinerals Ltd. in 1977.
Location
The sodian potassianhydroxonian meta-autunite
occursin BoomerangLake, 6 km SW of Menangina
homestead,about 105km NNE of Kalgoorlie, Western Australia, at latitude 29"52'lO"S and longitude121"52'25"E(Fig. l). BoomerangLake is a playa, situated in a semi-arid region with hot summers andmild winters. At Kalgoorlie, averageannual rainfall
is 263 mm, and mean daily minimum and maximum
temperatures
are l8.3oC and 33.6oCin January,and1068

BI]TT AND GMHAM:

e:3
€-!)
r

Pliiys late.
Radiomotric anonaly
Autunilg locally05
Km

30.00's L
r2r.30c

Fig. 1. Map showing site of occurrenc€ of sodian potassianhydroxonian meta-autunite, Menangina, Westcrn Australia.

META.AUTUNITE

tron micrographs (Fig. 2) illustrate this habit veryclearly. The mineral has a strong,bright yellow fluorescenoeunder both long- and short-waveultra-violet light. Optical characteristicsare difficult to determine, due to the small size of the crystallitesand, inany case,vary with water content. The mineral appearsto be uniaxial (or, ifbiaxial, 2V is closeto zero)and optically negative. The refractive indices are between 1.575-1.580.An attempt to measurethe specific gravity by sink-float methods failed becauseof
reaction of the sample with the heavy liquid (Clerici
solution). There was insufficient material for classicaldensity measurement,but attempts to use a micropycnomet€rmethod result in valuesbetween3.7 and4.7.

Chemical comPosition
4.8oC and l6.5oC in July. The playa forms part of achain of lakes which are the present expressionof a
Handpicked samples were dissolved in concennow alluvium-choked,ancient trunk valley dlxining trated hydrochloric acid, diluted and analyzed bysoutheastwardsacross the.deeply weathered Ar- atomic absorption spectrometry for sodium and pochaeanYilgarn Block of southwesternAustralia. The
lakes are usually dry, and there is surface water flow
only after exceptional rains; however, subsurfac€
flow follows the course of the ancient valleys.
The mineral has been found in a small atea, approximately 50 x 100 m, close to the northern shoreof the lake. It occuni in saline, gypsiferous muds inthe saturated zone, mostly at depths of 30-100 cm. It
appears as prominent" bright greenish yellow (Munsell 2.5 GY 8/8) disseminations, fracture coatingsand veinlet infillings in the red-brown (Munsell 2.5
YR 4.5/7') muds. Colors were detcrmined on wet
fracture surfaces.The veinlets, up to 1.5 mm x 20mm, usually have no depth and the mineral appea$
to be a late-stageinfilling of voids, shrinkagecracks
and, occasionally, of cylindrical channels up to 2 mm
in diameter, interpreted as root casts and/or insect
burrows. Viewed under ultra-violet light, the mineralis finely disseminated throughout the muds. Carefully hand-picked material can still contain small
amounts of quartz, gypsum and halite. A uraniumoxide phase is also present electron diffraction data
indicate a cubic fluorite type cell, a: 5.46L c'onesponding to uraninite. This has probably formed by
reduction ofthe precipitated autunite phase.
Physical properties
Aggregates of the sodian potassian hydroxonian
meta-autunite are soft and consist of small crystallites up to 1.5 pm thick and 100 pm diameter (butmostly in the range 2G-50 pm), with a micaceous
Fig. 2. Scanning electron micrographs of sodian potassian
habit and a somewhatresinousluster. Scanning elec- hydroxonian meta-autuniteshowingplaty habit.

BUTT AND GRAHAM: META-AUTUNITE

tassium, by colorimetry for phosphate (Murphy and electron microprobe analysesof this group of minerRiley, 1962) and, by polarography for uranium als.
(Deutscherand Mann, 1977).Sampleswere also anaStructural considerations
lyzed for water soluble sodium by atomic absorption
spectrometry. The mineral was analyzed by the AusThe variable water content mentioned above is a
tralian Microanalytical Service for hydrogen using a well-known feature of the autunite group of minslPregl combustion train technique and for carbon and als, but its consequencesfor electron probe analysis
nitrogen using a Perkin-Elmer 204 CHN analyz,er and for X-ray diffraction do not appear to have been
(MacDonald,1974\. No other alkali or alkaline earth fully appreciated.Our natural material gave very difmetals, nor any nitrogen, were detected.
ferent difraction patterns by the Debye-Scherrer
The analytical results are shown on Table l. Anal- and Guinier ls6hniqu€s,becausethe Guinier cam€raysis A gives the mean analytical data, which have was evacuated and caused dehydration to a second
been recalculatedin B, assumingI gomFositionof intermediatephase.
the type (Na,K,HrO).UOr.POo.xHrO and omitThe name autunite is usually reserved for theting, as impurities, NaCl" COr and insolubles. Re- fully-hydrated tetragonal phase; meta-autunites are
versiblelossesof water on heatingvary from 6.6 *.Vo more or lessdehydrated, with roman numerals desigat 33"C to 12.3 wl.Voat 300oC. The analysiscorre- nating progressivelylower dehydration states. The
spondsto a formula (Naor"K.rr(HrO)0,.). UO, . POo partially dehydrated tetragonal phase (of which our.3.25It2O.
air-dried material is an example) is called meta-autuElectron microprobe analyses of these samples nite I; and the irrcversibly dehydrated orthorhombic
proved unreliable, even for qualitative use, owing to phase (not found naturally) is called m€ta-autunite II
the mobility of sodium and potassium under the elec- (Rossand Evans, 1965).The mineral describedheretron beam. Virtually all the sodium is lost after 3 sec- does not fit into this scheme,becausethere are two
onds' exposure to the beam, and all the potassium distinct but related phasesof the meta-autunite I typewithin 60 seconds(Butt and Vigers, in prep.) Con- both showing reversible hydration. In addition thissequently, caution must be exercised in assessing minsl4l does not form an irrevenibly dehydrated
phase equivalent to meta-autunite II.
Debye-Scherrer,Guinier and powder diffractomeTable l. Chemical composition of sodian potassian hydroxonian
ter X-ray data indicate that the air-dried material iswell crystallized, with the lattice spacings shown in
Table 2. The pattern can be indexed on the basis of ameta-autunite I structure with tetragonal pseudo-cellU
4 9. 2 7( 3 )52.L8
dinensions a' : 6.9714, c' - 8.545A. By analogy
P
6 . 6 7( 2 )7. 0 6
with the known structures, the true cell is probably aNa
2.70(2)2.86= l9.7lA, c : 17.094.The mineral has a marked
K
2 . 3 7( 2 )
2.5L
layer
structure and diffractometer patterns haveH
L . 4 7( r )L.45stronglyenhanced (00I) reflections. These vary inc0.17(1)withvarying water content, the moit obvious
spacing
NaCl
3 . 1 6( r )step-wise; two different spacingsmay
changesbeing
Insoluble
1.60(1)+
together,and the pattern of air dried material
occur
H'
0 ,10
above
indicates the presence of small
described
O: tn UO,
6,62
7. 0 1ofafully
hydrated phase,corresponding toamounts
tn PO4L5. t I
1 4. 5 8with a basalspacingof 19.4A.It is assumed
autunite,ln CO,
0 .68
that the observed basal spacing should be doubled,ln HrO
LI.76
1 1. 6 3
although the (001) reflection is not visible in thistrr ttro*
0 .56
phase.Even when wet, the majority of material does
Total
roo.2499.95
not hydratc to this phase, suggestingthat only a reA = Mean eompositian in ueight percent; nutnbers instricted range of compositions will hydrate fully, andpuentheeea = nwnber of determitntiona.
NaCL is uatenthat
the sample is somewhat inhomogeneous, persoLuble Na calculated ae NaCL.
B = RecaLeulated aseuning atotric ptoportions
haps on a micro-scale.
Ll.:P:(Na+N+Hi1+).=1:7:.1.., onttting
NaCL, COg andInterestingly, there is a variation of basal spacing
Lnao LUDLee d8 LntDurfLDLea.
with relative hunidity in the meta-autunite I phase,

BUI:T AND GMHAM:
Table 2. Observedand calculateddiffraction pattems for air-driedsodianpotassianhydroxonian meta-autunite
Index

dob"

dc.lc

IntensLEy

001
011
110

8.75
5.45
4.94
4.30
3.70
3.49
3.36
3.23
2.94

8.55
5.40
4.93
4.27
3.64
3,49

w
tr
MS
!J

3.24
2.93

2,68

2.70

v
vt.l
vt.r
vw

2.54

2.52

111

LO2
200

27L
003
202
2L2

rrrl ''tt
zzol

22t 2.37
30r 2.24
zo3\ , ,^310J
004 2.L4
222\
t'tt"
3L]-l

w
S

vl,|

z . tt \
z.ttl

MS

2.37
2,24
z.zr\
z.zol

!1I
vl.r
w

i,ii:tL

302
321
z r al

2.049 2 . O 4 r
1.887 1.886

VW
Vw

lrr I

l. /oo

vw

400\

1 '/l

t.ttzl
1.7431t.tqf

3L3too5\ , ?^o
401J
105\
r 'o)Y
411/
330
r.642

r .zorlH

i:133)w

I .660'\

r.osg
I

Index

dob"

d""1"

rorl

t.576

420

1.559 r.559

r.)/J!
L . 5 t 2Jl,t3221 1 , 5 3 4 1 . 5 3 4!
1 . 5 3 |4
I .s33j
422 1 . 4 6 1 r . 4 6 4413 t.446 1.453

!'I,I
Vll

zosl

Intensity\,'l,l

Jr4 |
42Ll

rosl
:or/ L . 3 7 6 r . 3 76 J
rrol
r. :oa]sr0i t . 3 6 7 r . 3 6 7i
423J
Jr) I

sui
4o4)
4141

r . 3 6 7)

'r ' -r c-r-)|

1.350 1.3s0
I

1.350J
1.326\

sozl 1 . 3 2 8 r . 3 2 5 j
316\

Vw

1 .1 9 6 \r .196r . r 9 5Jr- _r r ll
r .rasl
3 3 5i t . 1 8 4 r . r 8 s l
s31J
1.184J504 1 . 1 6 3 L . t 6 7
2o7lr.r)zlI,'I{Ivw
Wv

L . T 5 2 r . 1 5 2iazsl
r.152)
524
1.109 I .107
4o6l
r. ror'l
I .103

v

221

vw

)r4

l

r.Lo2)

1.094 1.094

vw

as well as small variations from sample to sample.
doo,has been observedto vary from l7.lA to 17.4A.
The hkl reflectionsremain sharpduring this swelling,
and there are correspoadingslight adjustmentsof theuranyl phosphatelayer itself, amounting to a changeof 0.2Voin the a-dimension.This must indicate partial occupancyof the water sitesin the meta-autunite
structure, and probably also points to the importanceof hydrogen bonding on the stability of the uranyl
phosphate layer.
A previously unreported phase is obtained bygently heating the meta-autunite I phase (to about40"C) or by dehydrating it in vacuo.The diffraction
pattern consistsmainly of broad peaks, although thebasal reflectionof 6.904 is sharp and the 001 reflection at 13.804 is now of reasonableintensity; themain lines of the pattern are given in Table 3. A
number of these lines can be indexed as hlfr reflections from a structure with essentially unchanged
uranyl phosphatelayers, and since the dehydration isreversible in a few minutes in air, this is probably
comect.On examination by €lectron nricroscopy, theentire ftkO net is easily obtained, although there is aslight discrepancyin spacing(c = 6.98+.02A by Xrays, 6.89*.02A by electron diffraction). The differenoe may be due to a higher vacuum and hence
greater dehydration in the electron microscope,or to

107I

META-AUT.UNITE

inhomogeneity in the sample. The electron diffraction patterns showed considerablevariations in intensitiesfrom crystal to crystal, especiallythose correspondingto centeringreflectionswith i + ft : odd;
there was alsoevidencefor the doubling of the a-axis
in one crystal, and of disorder along the c-axis. (ccould only be measuredby X-ray di-ffraction).
It is not possibleto index the diffraction pattern of
this dehydratedphaseon a tetragonalor monoclinic
unit cell without assumingan unreasonablylarge superlattice; it is therefore probable that the uranyl
phosphate layer is slightly distorted, and the truesymmetry may be triclinic.
Further heating to l20oc decreasesthe c-dimension slightly to 13.5A,but behaviorat higher temperatureshasnot beenfollowed as our heatingstagewill
not exceedthis temperature.However, heating themineral in a furnace and allowing it to cool in air
shows that dehydration remains reversible tol200oc, even in apparently melted material. The
only evidenceof this thermal treatmentis a moderatechange in relative intensities of diffraction rings.
There doesnot appearto be a phaseequivalentto thereported orthorhombic meta-autuniteII.
In order to clarify the rangesof stability of the partially dehydrated phases,the natural material andthree synthetic uranyl phosphateswere studied atseveraldifferent temperatures.The resultsare shownin Table 4. The predominant spacingat any temperature is underlined;variation of spacingwithin a particular phaseis seenby lesking down a vertical colunn. Note that the natural material is similar to thesynthetic sodium analoguein its behavior, and thatthe calcium analogue is the only one with appreciable amountsof a fully hydrated phase.There arealways at least two phasesin the region designatedmeta-autuniteI by Rossand Evans(1965).Thesedifferences,which occur very readily, togetherwith theTable 3. X-ray diffraction pattern (Guinier) of evacuatedsodian
potassianhydroxonianmeta-autunite,showingthe lrl<0reflections
h2+k2

dob"

dcalc

rntensity

r

6,90
5.46
5.19
4.94
4.77
4.58
4.30
3.69
3.49
3.44
3.40
2.88
2.469
2.214

6.98

M
!,I
VW
MS
M
MW
w
w
s
Mll
MW
vlJ
Ms
S

2

4

8
10

4.94

3.49

2.468
2.207

h2+k2

16

dob"

dcalc

1.894
1.839
1 . 74 8
r . 7 4 2 t . 74 5

20
26
32
34
36
40

r.646
1.628
1:565
L.372
L.238
1.193
1.167
1.108

vr,J
vt,r
Vr.r
lr
tv'lil

1.73I

18

rntensity

r.645
r.561
t.369
t.234
1.197
1.163
1.r04

M
Hr,I
l{s
W
vw
W
Vw
!r

BUTT AND GRAEAM: META-AUTUNITE
Table 4. d-spacingsof (002) reflections of natural sodian potassian hydroxonian meta-autunite, and synthetic sodium, potassium andcalcium autunitcs and meta-autunites under differcnt temperature conditions, A. Predominant spacings underlined.

Wet,20"C
Alr dry, 20'C
40'c
60'c
90'c
120'C
120'c+20"c

Nat uralSodiumPotassium

9.9 8.89
9.9 8.80
8.80 6.90
6.90
6.90
6.74
8 .8 0

8.83
8.8
8.74
8.5s 6.93

8 . 9 2 7. 4
8.88 7.49
8.83 7.49
8 . 6 7, 8 5.58
6.57
6.57 6.r 5.62
8.88 7.61

D. /I

6. 7 08,72 7.3

Calclun10.37
1o.37 8.74
9.8 8.49
8.4r
7.50 6.54
7.27 6.50
8.22
6.55

difrculty with microprobe analysis already mentioned, stressthe need for caution ia dsaling withtheseminerals.

of retaining its structure and the property of reversible dehydration to its melting point of 1200"C.

Relationshipto otherautuniteand meta-autunitegroupminerals

The existenceof this mineral was brought to our attention by
Dr. M.K. Boots,EssoMinerals Ltd. Wet chemical analyseswere
done by Mr. C.E.S. Davis and Dr. R.L. Deutscher. Dr. R.L.
Deutscher also kindly made available cxamples of the synthetic
meta-autunitesand Dr. A.W.S. Johnson (CSIRO, Division ofChemical Physics) obtained thc electron diffraction pattems. Dr.
E.H. Nickel is thanked for valuable commentson the manuscript.

Acknowledgments

The sodian potassianhydroxonian meta-autunite
describedhere appearsto be the first known intermediate member of the meta-autunitesolid solution
seriespredicted by Ross and Evans (1965).Whether
the composition of the mineral from this locality reReferences
mains constantis uncertain.The interlayer cationsof
autunite and meta-autuniteminerals are exchange- Butt, C.R.M., Horwitz, R.C. and Mann, A.W. (1977)Uranium occurrencesin calcrete and associatedsedimentsin Western Ausable and this providesthe route by which many syntralia. Australia, CSIRO Division of Mineralogy Report No.
varieties(Fairchild,aremadethetic1929). ConFP. 16,67p.
sequently, the composition will depend upon the Camargo, W.G.R.de (1971) Minerais uraniferous de Perus, S.P.
activities of K, Na and other speciesin the
Boletim de Instituto de Geociencase Astronomia, Universidade
groundwater,the pH and the Eh, all of which may
de SaoPaulo,2, 85-201.
vary over the courseof a few months. In addition to Chcrnikov, A.A., Krutetskaya,O.V. and Organov,N.I. (1958)Sodium autunite. American Mineralogist, 43, p.303 (abstract
the diffraction evidencethat layer compositionsmayonly).
not be entirely uniform, evidence for a changing Deutscher,R.L. and Manq A.W. (1977) Determination of uracomposition is provided by the fact that the potasnium in natural waters by differential-pulsepolarographyof asium content for material collectedn 1977was 4.36
trioctyl-phosphineoxide extract.Analyst, 102,929-933.
wt.Vowhereasin 1978it was 2.37 wt.Vo(wet chemical Fairchild, J.G. (1929)Baseexchangein artificial autunities.American Mineralogist, 14, 265-275.
analyses).Only small amounts of the mineral were
Gallagher, M.J. and Atkin, D. (1966) Meta-ankoleile, hydrated
available for the earlier analysis and the results canpotassium uranyl phosphate. Bulletin of the Geological Suwey
not be confirmed.
of Great Britain, 25, 49-54.
This mineral differs somewhat from other mem- MacDonald, R.D. (1974)Developmentsin quantitativc elemcntal
microanalysis. Proccedings of thc Royal Australian Chernicalbers of the meta-autunitegroup in its mode of origin
Institute,41,3740.
and its dehydration behavior. Other autunite andMurphy, J. and Riley, J.P. (1962) A modified single solution
meta-autunite group minerals are usually found as
mcthod for the deterrnination of phosphate in natural watcrs.
alteration products of pre-existingminerals,whereas
Analytica Chemica Acta, 27, 3l-36.
this variety has been precipitated directly from saline Ross,M. and Evans,H.I. (1965)Studiesof the torbernitc minerals(III): Role of the interlayer oxonium, potassium and ammogroundwaters in the playa. On mild dehydration,nium ions, and water molecules.American Mineralogist,50, lthere are two phasesapparently equivalentto metat2.
autunite I, but there is no equivalent to the irreversibly dehydratedorthorhombic meta-autuniteII. InManuscript received,June 23, 1980;
deed the mineral has the remarkable characteristics
accepledfor publication, November 10, 1980.