Metamunirite, a new anhydrous sodium
metavanadate from San Miguel County,Colorado
HOWARD T. EVANS, JR.
U.S. Geological Survey, Reston, Virginia 22092, U.S.A.
Abstract
Metamunirite, [~-NaVO3, is found in cavities in sandstone in San Miguel County, Colorado, occurring
as fine, fibrous, colourless needles. X-ray powder and precession photographs show the crystals to beorthorhombic, space group Pnrna, with a = 14.134(7), b = 3.648(2), c = 5.357(2) A.. They are opticallybiaxial positive with n~ = 1.780(2) (llc),= 1.800(2) (lla), n~ > > 185 (lib, fibre axis; positiveelongation), 2Vz moderate. A crystal structure analysis, confirming the previously determined
structure of [~-NaVO3, shows the presence of (VO3)n chains made up of zig-zig VO5 square pyramids.
Metamunirite is probably formed by dehydration of munirite, NaVO3.2H20.
KEVWORDS: metamunirite, vanadates, San Miguel County, Colorado, munirite.
Physical properties of metamunirite

Introduction
THE most common vanadates occurring among

the efflorescences within the weathered vanadium-bearing sandstones, such as those of theColorado Plateau, are the orange-coloured decavanadates pascoite, hummerite, heumulite, andother unnamed decavanadate salts. In morealkaline environments the colourless metavanadates rossite and metarossite are more rarely
found. Patrick E. Haynes, a mineral collector inCortez, Colorado, has recently found excellent
examples of rossite in the Burro mine, near Slick
Rock, San Miguel County, Colorado. The
mineral transforms to metarossite through loss ofwater (CaV206.4H20 --~ CaVzO6.2H20 ) in a few
weeks' time in the open air. Mr. Haynes has also
found in this setting, at the Burro mine and also at
the nearby Deremo-Snyder mine, specimens
showing clusters of radiating colourless needles
up to 0.2 mm in length in cavities in sandstone,
which have proved to be the orthorhombic formof sodium metavanadate, [3-NaVO3. This newmineral is described below, and named metamunirite in allusion to its probable relationship tomunirite, NaVO3.2HzO. The name has been
approved by the International Commission on
New Minerals and New Mineral Names. The typematerial is so far extremely rare, and has been
deposited with the Smithsonian Institution,
Washington, D.C.

The needle-like crystals, which sometimes arebroadened into narrow laths, are extremely soft,friable and fibrous in character. They are scattered on the surface of a fracture cavity insandstone in which some previously depositedclay filling is evident. The crystals are opticallybirefringent, showing positive elongation andparallel extinction. The indices of refraction of
the natural fibres, measured in immersion oils,
are about 1.78 normal to the fibre direction. The
indices measured on a synthetic crystal (0.4 mmlong) using a spindle stage are n~ = 1.780, nf~ =1.800 (+0.002); n v is much higher and was notmeasured. Optic angle 2Vz is estimated to be 30 ~to 40 ~ The orientation is X = a, Y = c, Z = b,with respect to the orthorhombic unit cell (seebelow). The average n calculated from thecomposition and density by the Gladstone-Dalelaw (Mandarino, 1976) is 1.930. The density ofmetamunirite calculated from the unit cell volumecontaining 4NaVO3 is 2.926 g/cm3; Perraud(1974) reported the density measured on hissynthetic ~-NaVO3 to be 2.877 g/cm 3. The
dominant crystal habit is {101}, with no clearly
developed terminal faces. The easy splitting ofthe crystals into fibres probably occurs by perfectcleavage on this form, and on {001}. The crystals
are readily soluble in water.

Mineralogical Magazine, December 1991, Vol. 55, pp. 509-513(~ Copyright the Mineralogical Society

510

H. T. EVANS, JR.

X-ray crystallography
X-ray powder data for metamunirite wereobtained from natural fibres randomly oriented ina ball mount, using a Gandolfi camera and both
Cu-K0~ and Cr-Kc~ radiations. After eliminatinglines due to quartz, the d-spacings were found to
conform precisely to those recorded for [A-NaVO3by Morris et al. (1981) (PDF No. 32-1198), as
shown in Table 1. A fibre pattern was obtained ona bundle of natural crystals, which completelyconfirmed the indexing of hOl reflections as given
in Table 1. Powder data have also been published
for this phase by Lukfics and Strusievici (1962),
Feigelson et al. (1972), and Perraud (1974), all in
substantial agreement with those given in Table 1.
The similar data originally offered by Hanawalt
et al. (1938) (PDF No. 1-246) were erroneously
ascribed to NaVO3.H20 (see description ofdehydration phenomena below). Least-squares
analysis of our Cr-Kc~ powder data for the natural
crystals yield the following unit cell parameters,
shown below in comparison with the best published data:

This worka,•b,Ac, A.
V,~ 3

Morris et al.
(1981)

Kato andTakayama(1984)

14.134(7) 14.155(6)14.147(2)3.648(2)3.6499(11) 3.6496(6)5.357(20
5.3625(15)5.364(1)276.3(3)277.1
276.96(9)This nonconventional cell orientation is retained
here rather than the conventional celt of Morris
et al. (1981) in order to facilitate the comparisonof the b (fibre) axis with that of the monoclinic
structure of munirite. The transformation from
Morris et al. (1981) to this setting is (010/001/100).
A synthetic preparation of sodium metavanadate was crystallised from water solution at room
temperature, yielding fibrous, radiating crystals
entirely similar to the natural ones. A single
crystal was selected from this preparation andused for precession photography and singlecrystal intensity measurement by automatic diffractometry. The precession patterns showed aclearly resolved primitive orthorhombic lattice,
but the spots were generally diffuse. The patterns
confirm the unit cell and space group Prima first
defined by Lukfics and Strusievici (1962) and later
redetermined by Kato and Takayama (1984).
Using a modified Picker automatic singlecrystal diffractometer with MoK0~ radiation, 595
reflections with 20 < 40 ~ were measured in thehemisphere. These were averaged to 154

independent observations, of which 71 had
I > 1.5o(I). Weighted least-squares refinement of
the structure parameters of Kato and Takayama(1984) led to the conventional reliability index Rw= 0.070 (R = 0.18) in isotropic thermal mode.
The structure parameters obtained are in good
agreement with those of Kato and Takayama(1984), and did not provide any improvement
over their result.

Chemical composition of metamunirite
An energy-dispersive X-ray test in the scanningelectron microscope showed the presence of only
sodium and vanadium in approximately equalamounts. Joseph Nelen of the Smithsonian Institution has kindly made a microprobe analysis offibres from the Deremo-Snyder mine, mountedwith epoxy resin on a glass slide. For this analysis
an A R L SEMQ instrument was used with wavelength-dispersive technique: excitation voltage 15
kV; sample current 0.025 ~a on brass; beam
defocussed to 25 ~m to reduced specimen heating; data processed with the M A G I C IV program. For 31 point determinations the following
results were obtained (weight percent): Na20,24.8; VaOs, 75.2. The theoretical values forNaVO3 are 25.4 and 74.6 respectively. The
standard deviation for both oxides is 0.4%. The
sums were consistently deficient by 8 to 10%. No
visible change in the specimen (boiling or poring)
was observed such as would be expected from lossof water. The deficiency in the totals is attributed
to the presence in the beam, in every test, ofsignificant amounts of mounting medium, which
could not be avoided because of the finely fibroustexture of the specimen. The slightly low value for
Na may be caused by the volatility of Na relative
to V. Nevertheless, the chemical composition isfirmly established by the probe analysis and theX-ray diffraction studies described above.

Crystal chemistry of metamunirite
The crystal structure analysis of metamunirite
shows that the mineral phase is anhydrous,
corresponding to the well-established polymorphof sodium metavanadate [3-NaVO3 (Fig. 1). Fromaqueous solutions, which presumably are thesource of the natural metavanadates, Perraud(1974) has shown that the monoclinic phase
NaVO3.2HaO is stable below 34.6~
but canexist metastably to higher temperatures. This
phase constitutes the mineral munirite (Butt andMahmood, 1983; Evans, 1988). Above 34.6~
according to Perraud, the anhydrous orthorhom-

METAMUNIRITE,
Table

h2

ki.

1X-raypowderdata511

A NEW MINERAL
formetamunirite

and~-NaVO 3d(calc)d(obs)d(obs)l(rel)d(obs)

I(rel)min I~in 2

syn 3

syn 3

std ~

std 4

hk

1

20 0

7.07

7.O7

7.O3

28

7.09

ii

00

1 0 1

5.01

5.05

5.03

I00

5.01

i00

1 0

1

20 1

4.269

4.283

62 01

40 0

3.534

25

40

3.530

3.535

57

3.532

0

1 0 2

3.242

3.241

3 241

44

3.248

18

1 0

20113.015

3.016

3.016

28

3.019

13O 1

11

1 12.949

2.957

2.955

33

2.954

35

4

0 1

2.951

0

0 22.679

10 22.632

3

1 12.540

4

1 0

2.538

2

0 2

2.685

2.689

2.538

2.505

ii

2,540

2.499

92.508

4

1 1

1

40

122.683

12

0 0

2.635

2

1 0 2

2.541

7

31

4

1 0

2

0 2

52.506

5

1

5 01

2.501

0

1

6 00

2.356

2.360

2

6 00

3 0 22.329

2.333

4

3 024

112.294

2.293

2.295

92.295

54

1 12

1 2

2.065

2.071

2.067

62.067

12

1 23

1 2

1.96281.96681.9642

13

1.965

5

3

1 2

7

0

11.8894

1.89381.8940

7

1.892

5

7

0

1

0 2

0

1.9242

1.82361.8242

191.824

6

0 2

0

1 0

1.77671.7698

4

1.771

3

1 0 3

1.732

12

0 3

1.716

55

1 2

1.679

2

8

0

1

4 203

1.77172 0 3

1.73135 1 21.71598011.67794

201.62090

1 3

1.60394 01.7161

1.71711.62044

1.62117

1.605191.60514O 1 3

31.5938

1 0

1.5902

1.58781.58925

1.5944

11.5515

1.55131.55002

0 2 2

1.50771.50258 0 2

1.47491.47559

1 1

1.39291.39378

0 3

1.25601.255434

2 31.20021.2002

3

4 2

4

1 4

1.1845

1.1844

2

48

4 2

4

0 3

78

1 0

1.5521142

141.509020 221.4703

11.474538

0 2

1.3926

5

1.3942

29

18

0 3

13

1 4

Notes:i.

Calculated

2.

Metamunirite:
quartz3.

synthetic

datalines

forStandard1981).

~-NaV03:

patternsample.
pattern,CrK~radiation;

severalomitted.

microdensitometer
4.

natural

Debye-Scherrer

Debye-Scherrer
peak
(PDF

pattern,CrK~radiation;

intensities.

32-1198)for ~-NaVO 3 (Morrisetal.,

512

H. T. EVANS, JR.

i~c~ .....
O~'

Q............ ;;:

.

.

.

.

.

o /.

/

Fro. 1. Crystal structures of munirite in P21/a according to Bj6rnberg and Hedman, 1977 (top), and metamunirite inPnma according to Kato and Takayama, 1984 (bottom). The end view of the (VO3)n chains shows the zig-zag
arrangement of the edge-shared VO5 square pyramids. The distorted octahedral Na-O contacts are also indicatedby fine lines.
bic phase [3-NaVO3 is formed stably from solution, but can form metastably below this temperature. Kato and Takayama (1984) have shown aclose structural relationship of [~-NaVO3 toNaVO3.2H20 (munirite; see Fig. 1). The lattertends to lose water in air, a process that cancontinue to the anhydrous state without serious
disruption of the crystal structure. Kato andTakayama found evidence for the existence of anintermediate state, which would correspond toNaVO3.H20. The metamurite crystals are oftenb o u n d e d by a broad (101) face (smeared opticalsignal) and a narrow (100) face (sharper signal).
These faces are probably pseudomorphs after theforms (100) and (001) of munirite, respectively.
The poor crystallinity of metamunirite (as shownby broadened Bragg reflections in both powder
and precession photographs) probably resultsfrom its formation as a pseudomorph after ahydrated form such as munirite.

Acknowledgements

Grateful thanks are extended to Patrick E. Haynes ofCortez, Colorado, who provided the mineral specimens
for this study; also to Dr. J. J. McGee of the U.S.
Geological Survey for his examination with the scanningelectron microscope. Special thanks are due to Joseph
Nelen of the Smithsonian Institution, who carried out
the microprobe analysis.

References

Bj6rnberg, A. and Hedman, B. (1977) The crystal
structure of NaVO3.1.89H20. Acta Chem. Scand.,A31,579-84.

METAMUNIRITE,Butt, K. A. and Mahmood, K. (1983) Munirite,naturally occurring sodium vanadium oxide hydrate,
a new mineral. Mineral. Mag., 47,391-2.
Evans, H. T., Jr. (1988) The crystallography ofmunirite, NaVO3.(2 - x)H20. Ibid. 52,716-7.
Feigelson, R. S., Martin, C. W., and Johnson, B. C.
(1972) Crystal growth and properties of some alkali
metal metavanadates. J. Cryst. Growth, 13/14,686-92.
Hanawalt, J. D., Rinn, H. W., and Frevel, L. W. (1938)
Chemical analysis by x-ray diffraction. Classification
and use of x-ray diffraction patterns, lndust. Eng.
Chem. Anal. Ed., 10, 457-512.
Kato, K. and Takayama, E. (1984) Das Entwasserungserhalten des Natriummetavanadats und die Kristallstruktur des Natriummetavanadats. Acta Cryst., B40,102-5.

A NEW MINERAL

513

Luk~cs, I. and Strusievici, C. (1962) Uber die Polymorphie yon Natriummetavanadate. Zeit. anorg, allgem.
Chem., 315,323-6.
Mandarino, J. A. (1976) The Gladstone-Dale relationship--Part I: Derivation of new constants. Canad.
Min., 14, 498-502.
Morris, M., McMurdie, H. F., Evans, E. H., Paretzkin,B., Parker, H. S., Panagiotopoulos, N. C., andHubbard, C. R. (1981) Standard x-ray diffraction
powder patterns. Nat. Bur. Stand. Monog. 25, Sect.
18, 68.
Perraud, J. (1974) Etudes sur la cristallisation dequelques metavanadates alcalins, simples ou doubles,
Na +, K +, et Cs +. Rev. Chim. min., 11,302-26.

[Manuscript received 13 August 1990;
revised 28 February 1991)