MINERALOGICALTIIEAMERICAN

MINERALOGIST,NOTES

VOL. 48, NOVEMBER-DECEMBER,

1963

THE CRYSTALLOGRAPHY
OF META-AUTUNITE (I)1
M.lr.cor.rt Ross, [/. S. GeologicalSuraey, Washington 25, D. C.
INrnolucrroN
The exact unit-cell size and space-groupsymmetry of meta-autunite(I), Ca(UOrPOn)r.nH2O, have been in dispute for some time, forexample, see Takano (1961), Donnay and Donnay (1955), Makarov
and Ivanov (1960) and Volborth (1959). In this paper data are presentedin an attempt to give a more completeanswer to these crystallographic questions.
There appear to be three hydration statesfor autunite. Autunite is thefully hydrated phase, meta-autunite (I) is in the next lower hydration
state, and meta-autunite (II) (not found occurring naturally) is in thelowest hydration state. The exact number and positions of the watermolecuiesin the autunites are not known.
The material examinedhere is from the Daybreak mine, Mt. Spokane,
Washington and is similar to that studied by Leo (1960). The autuniteand meta-autunite (I) crystals from Mt. Spackanevary in color fromIight to very dark green,almost black. Leo found the Ua+content of thedarker crystals to be higher than that of the lighter crystals. The darker
phasealso had higher indicesof refraction and a higher density than thelighter phase.Leo gives the formula(Ca, Sr, U'+)o nr(UO.)r 16-1s6(POr)z.oo9.77HzO

for the darker autunite and the formula( C a , S r , U a + ) os g ( U O r ) rs u ( P O n )6, s . 1 0 . 6 4 H 2 O

for the lighter autunite. Although an attempt was made to maintain theautunite crystals in the fully hydrated state prior chemical analysis
there was probably some dehydration to meta-autunite (I). For otherpertinent data on the chemical and physical properties of autunite andmeta-autunite (I) from Mt. Spokane the reader is referred to Leo'spaper.
Expn,trl.elttAl

WoRK

Both light and dark green,nearly black meta-autunite (I) crystalswere
examined by Buerger precessiontechniques.Many small crystals were
photographedand only those which gave unusualiy sharp Bragg reflections were selectedfor a completestudy. Zirconium-filteredmolybdenum
radiation (Ka:0.7107 A) was used for most photographs.Buerger preI Publication authorized by the Director, U. S. Geological Survey.

1390

MINERALOGICALNOTESTaBLn 1 UNrr-CBr,r.Dere ron MBra-AuruNrrr (I), Ca(UOsPOr)z.nHrO
Present studlrl

Unit cello (A), (A)Laue group
Space group

V (A)Pseudo-unit cell

Tetragonalt 9. 7 8 + 0 . 0 2
16.92+0.03
4fmmm
P4122
6620

Donnay and Donnal'(1ess)
I'etragonal
19.82
849
4/mmm
P4f mrnmz
3335

Tetragonal
6.99
8.46Tetragonal

P4/nmm

P4/nmmDensity (calc.)

3 . 5 3 g / c m 33

3 503

Density (obs.)

3 .45-3 . 5.5g/cm3 a

3.48

Z
9:t

l6
1 . 5 7 9 - 1. 5 8 6 1

8

Forms

11101,10011

Locality

Mt. Spokane, Washington

a'(A)
c'(A)
Pseudo-spacegroup

70r
8.49

Lauter, Saxony

I Size and color of crystals examined are: a) black, 0.05 by 0.20 by 0 35 mm, b) lightgreen, 0.03 by 0 20 by 0.42 mm. Both crystals gave identical unit-cell data with a:19.78
and c:16.92 A.
2 or P422, P4mm, P42m, P4m2.
3 for 6HzO.
I Leo (1960)c e s s i o np h o t o g r a p h so f t h e h k 0 , h k 1 , h k 2 , h k 4 , h k 6 , h h l , h ( h f 4 ) 1 , a n dh (h+8)l reciprocallattice nets were nade. Exposuretimes were approximately 80 hours. The unit-celi data found in the present study are corrpared in Table 1 to thosegiven by Donnay and Donnay (1955)for metaautunite (I) from Lauter, Saxony (U. S. National Museum Nos. 1342,5675). Prior to the study by Donnay and Donnay most workers recogn i z e d o n l y t h e p s e u d o - c e lwl i t h a : 7 . O A a n d c : 8 . 5 A . | t t e p s e u d o - c e l lis rotated t[5o about the c-axis with respect to the true cell. All r-ray
photographs show 4f mmm. Laue symmetry. The following condition
limiting the possiblereflectionswas observed:
0 0 1 : :l 2 n1391

MINERALOGICAL NOTESlxl-')
Uri-1*ID/*a 5 -<-Frc. 1. &ftOBuerger precession photograph taken with zirconium-fi.ltered molybdenum
radiation of meta-autunite (I) from Mt. Spokane, Washington. showing weak supe-rlattice reflections. The subscript (s) refers to the axes of the pseudo-celiwith a: b:6.99 A.

The space group is thrs P4222.The pseudo-cellwith a:6.99 A andc:8.46 A showsthe following condition limiting the possiblereflections:hk0:h*k:2n.
The space group for the pseudo-cell is P4/nmm. Figure l shows aBuerger precessionphotograph of the hkO reciprocal lattice net of metaautunite (I). The weak super-lattice reflections are apparent. The factthat Donnay and Donnay also observed these weak reflections on asample from a difierent locality indicates that the large o-dimension of
19.8 A is typical of this mineral. The only disagreement with the dataof Donnay and Donnay is that the presentstudy indicatesa doubling of
the c-dimension. This doubling is indicated by extremely weak spots
appearing in the hk1 photographs. Films made with copper radiations h o wt h e p r e s e n c oe f t h e 2 2 1 , 4 4 I , 8 8 1 , 0 4 1 , 0 8 1 ,O . t 2 . 1 , 2 6 I , 4 8 1 ,a n d
4.12.1 reflections. Films made with molybdenum radiation show thep r e s e n c eo f t h e 4 4 1 , 4 8 1 ,4 . 2 0 . 1 ,4 . 2 8 . 1 , 8 . 1 6 . 18, . 2 4 . L ,1 2 . 2 0 . t ,1 2 . 2 4 . 1 ,and 16.20.1reflections.

1392

MINERALOGICALNOTESTna AuruNrro SrnucrunB Pnosr,BN[The identity of the unit-cell parameters of the light- and dark-green
crystals indicates that Ua+ does not enter the meta-autunite (I) structure. The strongestline of uraninite was observedin r-ray powder patterns of the black autunite crystals (Leo, 1960,p. 115). It is probable,as
Leo suggestsin his paper, that the UOr in the chemicalanalysesis due tofinely disseminated particles of uraninite. These particles probably
causethe darkening of the autunite crystals.
Takano (1961)has suggestedthat the large cell found by Donnay andDonay (1955) is incorrect and that a pseudo-cellwith a--6.972 A, andc:8.17 A is actually the correct one. Takano obtained his data with adiffractometer (and Weissenbergcamera?)using copper radiation. Suchtechniquescannot be expectedto pick up the very weak reflectionsthat
indicate a larger unit cell. Makarov and Ivanov (1960) have attempted
to solve the structure of meta-autunite (I) on the basis of the sub-cell.
These workers collectedOkl and hkO intensity data with rotation techniques using copperradiation. The choiceof an incorrect spacegroup by
Makarov and Ivanov forced them to proposea structural model in which
one calcium atom must be distributed over two positions (2c) and six
water moleculesover eight positions (87) of space group P4f nmm. Theirstructure, based on an electron density map projected on (100) confirms Beintema's (1938) proposedstructure for the (UO2PO+)""-Iayers,
but their proposal for the positions of the interlayer calcium and watermoleculescannot be consideredcorrect. By calculating Fourier projections in the incorrect spacegroup, they have forced a falsestatistical distribution of these atoms.
The difficulty encountered by various workers in the study of theminerals of the autunite group is due to the fact that the large contribution of the heavy uranium atoms to the observed intensities tends toobscurethe contributions of the light oxygen atoms and cations. Also,
for compounds which contain heavy elements such as uranium, c-ray
absorption errors will generallybe large, particularly if copper radiation
is used. The absorption of r-rays will tend to further obscurethe contributions of the light atoms to the reflections.
The uranium atoms in the autunite type structures usually occupy
specialpositions,and consequently,will contribute scattering intensity
only to certain special classesof reflectionscorrespondingto a pseudocell. In such casesthe true cell and symmetry can be deducedonly bytaking into account the other classesof very weak reflections.Becauseit
is the light oxygen atoms that determine the principles of coordination,
bonding, and packing in these structures, these weak reflectionsmay

MINERALOGICAL NOTES

1393

play a crucial role in the crystal structure analysis. An attempt to proposea structure basedon the pseudo-cellon the assumptionthat the truestructure is a minor distortion of it will probably lead to anomalousresults. The need for careful attention to the weak classesof reflections has been demonstrated by Ross and Evans (1962), and Ross(1962) in their study of the crystal structures of the autunite-likec o m p o u n d sK ( L I O z A s O + ) . S U r O( a b e r n a t h y i t e ) ,N H n ( U O 2 A s O a ) . 3 H 2 O ,K(HaO) (UOzAsOa)r.
6H2O, and Cu(UOzPOa)2.
8HrO (meta-torbernite).
In order to ensurethe determination of the correct crystallographyby
recording the very weak ;r-ray reflections,the following procedureshave
beenfound to be most essential:(1) very small, well crystallized single crystals are studied,(2) photographs are taken with molyebdnum radiation using longexposuretimes (50 to 200 hours),(3) the Buerger precessioncamera is used for it gives the clearestandmost easily interpretable photographs, and(4) precessionphotographsare taken especiallyof the reciprocallattice
net planesparallel to the flattened {001} form of the thin crystal plates(for example, hk0, hk1, etc.) so as to minimize the absorption errors(Donnayand Donnay, 1955).
RelnnBncrs
BnrNrnue, J. (1938) On the composition and the crystallography of autunite and themeta-autunites Rec. trataur ch,im.Pays-Bas 57, 155-175.
DorlNlv, G.rlnrnr.r.n auo J. D. H. Douwav (1955) Contribution to the crystallography ofuranium minerals. U. S. Geol. Su.rley Rept. TEI-507.
Lno, G. W. (1960) Autunite from Mt. Spokane, Washington. Am. Mineral. 45,99-728.
Mer,tnov,E. S eNo V. I Ive.Nov (1960) Crystal structure of meta-autunite,Ca(UOi)z(POr)z'6HzO. Dohlad.yAhad. Nauk S,S,SR,132, 673-676 (in Russian).
Rcss, Mnrcolu(1962) The crystallography, crystal structure, and crystal chemistry ofvarious minerals and compounds belonging to the torbernite mineral group. Ph D.
Thesis, Harvard University, Cambridge, Mass.
--AUDH. T. Evaxs, Jn. (1962) The crystal structures and crystal chemistry of various
members of the metatorbernite group (a"bs.).Am. Mineral. 47, p. 202.
'l'.rxano,Yu<ro (1961) X-ray study of autunite. Am. Mi.nerol. 46, 812-822.
Vor.lontrr, A. (1959) Strontian meta-autunite from the Daybreak mine, Mt. Spokane,Washington. Am. Mineral. 4, 702-7 71.