M I N E R A L O G I C A L MAGAZINE, SEPTEMBER 1983, VOL. 47, PP. 393-6

Tristramite, a new calcium uranium phosphateof the rhabdophane group
D. ATKIN, I. R. BASHAM, AND J. F. W. BOWLESGeochemistry and Petrology Division, Institute of Geological Sciences, 64 Gray's Inn Road,London WC1X 8NG

ABSTRACT. Tristramite, a new mineral of the rhabdophane group, has the composition(Cao.s,Og.~gFeao.~7)zl.oo[(PO,)0.79(SO,)o.12(CO3)o.ov]z0.98"l.77H20. It occurs
as a late-stage replacement or matrix to brecciateduraninite (var. pitchblende) associated with sulphides inhydrothermal veins related to Hercynian granites insouth-west England. It is hexagonal, space group P6222,
with a 6.913+ 0.003A and c 6.422+ 0.006A. The strongest
lines of the indexed powder pattern are 2.99 (100), 2.83(100), 2.14 (50), 1.850(50), 5.99(40),4.37 (40), 3.46(30). For
comparison, new indexed powder data for rhabdophanefrom Fowey Consols, Cornwall, are included.The mineralis pale yellow to greenish yellow, uniaxial positive withco 1.644and e 1.664,and does not fluorescein either shortor long-wave ultraviolet light. The habit is acicular orfibrous and no cleavage has been observed. Density(g/cm3) 3.8-4.2 (meas.), 4.18 (calc.).
TRISTRAMITE was first discovered in the courseof X-ray diffraction examination of radioactive
material collected from south-west England by staffof the former Atomic Energy Division of theGeological Survey and Museum of Great Britain(now Institute of Geological Sciences). With thecollection of additional material, and the advent ofelectron microprobe analysis, a full description ofthe mineral has now proved possible. Althoughtristramite has been identified in samples fromdumps at several old copper and/or tin mines inCornwall, including Trequeen, Wheal Alice, Wheal
Buller, Wheal Damsel, and Wheal Providence, thebest specimens found to date come from Wheal
Trewavas. Here the mineral occurs both as afine-grained aggregate and as coarser-grained
infillings occupying fine fractures and cavities.
The data that follow have been obtained frommaterial from this occurrence, which is therefore
considered to be the type locality. Specimens arelodged at the Geochemistry and Petrology Division of the Institute of Geological Sciences and atthe British Museum (Natural History). The mineralis named after the medieval figure associated withthe Arthurian legend. Both the name and descrip-t~ Copyright the MineralogicalSocietytion of the mineral have been approved by theCommission on New Minerals and Mineral Namesof the International Mineralogical Association.
Occurrence. Tristramite is found in uraninite (var.
pitchblende)-bearing quartz veins in which commonly associated sulphides include chalcopyrite,pyrite, sphalerite, galena, and marcasite. At most ofthe localities studied, tristramite occurs as finegrained aggregates of acicular to fibrous crystals
with an overall greenish body colour due to theintimate association with pitchblende. Direct replacement of vein pitchblende is sometimes found,
as at Trequeen, but most commonly tristramite
forms a matrix to brecciated pitchblende, often
in close association with marcasite and chalcopyrite or digenite (fig. 1A and B). Secondary uranium
minerals are also frequently present.
The dumps from which the type material fortristramite was obtained are sited on the coast ofCornwall at Trewavas Head (NGR SW597 267),
the southernmost exposed area of the small
Tregonning-Godolphin granite intrusion. Wheal
Trewavas was active as a small copper producer
about the middle of the last century (Dines, 1956)
but pitchblende-bearing ore containing tristramite
can still be collected from the old dumps. The
locality is unique among those examined in that thecommon fine-grained matrix or replacive tristramite grades into coarser material in patches. More
importantly, well-crystallized material occurs as aninftlling to fine fractures or in cavities lined withchalcopyrite and marcasite (fig. 1C and D). The
following description of the properties of tristramite has been made from this vein-type material, withthe exception of density measurements, which weremade on bulked samples of the replacement/matrix
type.
Physical and optical properties. Tristramite hasan acicular to fibrous habit. In the fine fracturesand cavities crystals range in length from 5 to80 pm and have an estimated width in the range 2 to5 /~m. The width of the tristramite veinlets is394

D. A T K I N E T A L .

FIG. t. Photomicrographs showing the mode of occurrence of tristramite in specimens from Trewavas Head: A(reflectedlight); brecciated pitchblende (light grey) oxidized along margins or bordering cracks (darker grey); matrix composed offine-grained tristramite with scattered grains of pyrite. B as A but in transmitted light; lighter tones in the tristramite
matrix correspond to areas of coarser grain size. c (transmitted light); well-crystallized tristramite as a cross-fibre infill toa cavity bordered by sulphides. D (reflected light, crossed polars); cavity with tristramite infill and surrounded byintergrowth of marcasite and pitchblende. Scale bar = 0.1 mm.
generally less than 200 pm and the crystals occur
either in the form of a cross-fibre infiU or as radial
aggregates--a habit typical of rhabdophane. The
colour is lustrous yellow to greenish yellow but
colour banding is frequently shown due to variations in the content of ubiquitous minute blebs orplates of goethite, approximately 2 #m across. The
fine grain size of tristramite and the presence ofintergrown goethite preclude any accurate density
determinations although repeated separations weremade by centrifuging in Clerici's solution in an
attempt to remove impurities. On the basis of theseexperiments, the density was found to lie in therange 3.8-4.2 g/cm 3. The fibrous nature of themineral did not permit any reliable hardness
determinations and no cleavage could be confirmed.
In transmitted light the colour is very pale yellow
to greenish yellow although the colour appears
stronger when a polished block is observed underincident light. There is no fluorescence under either
long or short-wave ultraviolet light. Tristramite isuniaxial positive with extinction parallel to thepositive elongation or fibre-length. Refractive
indices were determined as co 1.6444-0.002 and1.664 4- 0.002. Pleochroism has not been observed
but under crossed polars there is a tendency toshow an anomalous blue birefringence colour.
Chemical composition. Electron microprobe
analyses of the tristramite are given in Table I. The
methods and the apatite and pyrite standards
described by Bowles (1975) were used in conjunction with an artificially produced uranium-bearing
glass (Smellie et al., 1978) and the quantitative
correction programme of Mason et al. (1969).
Qualitative analysis using an energy-dispersive
system showed that only Ca, Fe, U, P, and S arepresent in concentrations over 0.1 70 within therange of elements from N a to U, and no further
elements were detected by optical spectroscopy.
Because of the fine grain size and textural
relationships of the tristramite it was not possible
to obtain pure material for separate determinationof H 2 0 content. However, partial wet chemical

TRISTRAMITE, A NEW MINERAL

395

TABLE I. Electron-microprobe analysis ofT A n L E II. X-ray powder data for tristramite andtristramite

rhabdophane

I2

Tristramite

CaO
UO2*

13.74
35.08

C a 2+U *+0.544 /
0.289 ~ 1.00

Fe203~

5.99

F e 3+0,167 )

P2Os
SO 3

25.09
4.31

PO 3SO~-0.786 I
0.120 |

(Total84.21)

CO2~H20w

1.46

CO~-

14.33

H20

0.98

d~l ~Ado~ Ahkild.lc A

do~ A

10i0

5.99

5.99

I

401010

6.03

6.03

I

55

10il
11~01121
2020

4.38
3.46
3.04
2.99

4.37
3.46
3.04
2.99

4030
2100

lOT111201121
20~04.38
3.48
3.05
3.01

4.38
3.48
3.03
3.01

55
50
390

1012

2.83

2.83

100

1012

2.82

2.82

100

2.71
2.35

2.70
2.35

125

20~11122

2.72
2.35

2.73
2.35

120

0.074 J

2130

2.26

2.26

6

2130

2.28

2.28

8

1.77

2022
0003
2131
10i3
3030
3031
2132
1123
20'~32240
3032
22~I1

2.19
2.14 }
2.13
2.02
2.00
1.906
1.850
1.820
1.741
1.728
1.695
1.669
1.660

2.19
2.14

250

2.01
2.00
1.906
1.850
1.821
1.740
1.728
1.696

4120
50
310
925-112022
2131
0003
3030
1013
3031
2132
1123
22~0
2023
3032
22~I1
31~-0

2.19
2.15
2.12
2.01 ~
2.00 j
1.916
1.853
1.813
1.740 ~
1.736
1.699
1.678
1.672

2.19
2.15
2.13
2.01

450
104

1.916
1.853
1.815
1.740

13
50
5
20

1.699
1.681
1.672

20
115

31211

1.617

w H 2 0 added by difference.
2. Atomic proportions.
analysis of a concentrate, s h o w n by X-ray p o w d e r
p h o t o g r a p h y to comprise only tristramite, chalcopyrite, goethite, a n d pitchblende, c o n f i r m e d t h e
Ca:Pratioo b t a i n e d by m i c r o p r o b e analysis.
A p p o r t i o n i n g the results of the wet chemical
analysis so t h a t S a n d Fe form chalcopyrite a n d Fe
a n d H 2 0 + form goethite leaves excess H 2 0 . W h e nallocated to a tristramite c o n t e n t calculated o n thebasis of the C a a n d P values, this indicates a n H 2 0c o n t e n t in tristramite of the order of 15%. A n
interpreted analysis is therefore made, b y analogywith r h a b d o p h a n e a n d brockite (Fisher a n dMeyrowitz, 1962), by first a d d i n g C O 2 (2.46%) t oachieve valency balance a n d t h e n H 2 0 (14.33 %) tob r i n g the total to 100 %. T h e presence of a smalla m o u n t of CO2 was confirmed u n d e r the microscope from the o b s e r v a t i o n of the evolution of gas
bubbles w h e n the mineral is i m m e r s e d in HC1 acid.
T h e a m o u n t of H 2 0 added is greater t h a n f o u n d inr h a b d o p h a n e b u t it is considered acceptable in viewof the confirmatory evidence of the wet chemical
data. Limited parallel heating experiments in therange 150 to 2 0 0 ~ also d e m o n s t r a t e d approximutely twice the weight loss from tristramite t h a nfrom r h a b d o p h a n e a n d suggested the water to bezeolitic in nature.
T h e f o r m u l a c a l c u l a t e d i s therefore:( C a o . 5 4 U o 4. 2+9 F e o .31+7 1 g l . O O[ ( P O 4 ) o 7 9 ( S O 4 ) 0 1 2 ( C O 3 1 0 07]Igo 98" 1 - 7 7 H 2 0.
.
.
.
with the ideal formula+ , F e 3 +) [ ( P O 4 ) , ( S O 4 ) , ( C O 3 ) ] "1.5_2H20

where

C a > U 4 + , F e 3 +, a n dhkil

2021
1122

1. Weight p e r cent.
* Total U expressed as UO2.
t T o t a l F e e x p r e s s e d a s F e 2 0 3.
CO2 a d d e d t o b a l a n c e valency.

(Ca,U,Rhabdophane

PO 4 > SO4,CO

331~o31211

2133
1014
222[240~0

31~2
3033
40~1
1124
2024
3250

40~2
22~3

3251
3193
2134
4130

4131
3232
1015
3034
40~34132
11~55050
2025
5031

1.662

1.608

1.609

1.555
1.551
1.522
1.497
1.475

-1.551
1.521
1.497
1.476

1.46o -~ 1.456
1.456 J--402118

1.415--1.357
1.3561 . 3 4 5 ~ 1.343

1.373

1.343j

-46

1.466
1.460
1.448

2024
3250
40~23251

1.408
1.383
1.362
1.351

224.3

-1.309
1.281
1.263

1.256
1.251
1.227
1.210
1.204

-1.211-540~34132

--

--

1.251

1.197
1.180

1.177

-20
19
--

7

---

~ 1.176313,41.154 [

33~03381
42~05032

1.152J
1.134 } 1.132
1.131
1.122
1.122
1.117 ~
1.115
1.114 I
1.095
1.084
1.080
1.075
1.070
1.067
1.061
1.054
1.045 [
1.044 J

3_

1.152

7
2

1

1.1154--1.075
1.067

--

--5

1.043

-4
5

1.553
1.540
1.527
1.507
1.480

3033
11;~4

1.312
1.309
1.306
1.280
1.263

1.1561176)

5181
10i6~53
3234

2133
1014
2252
40710
31~2

1.458

22~43253

2135
4133
42/;14094
3382
3035
51~00006
42~2

6
10
2
38

1.346

1.619

-1.540
1.527
1.506
1.481

5-8
2
16

1.467
--

1
--

1.448
--1.3611.352-1.315

6
--3
4-5

1.306
LE88
1.268
1.249

7
166

4150
3193
2134
4131
3232
3034

1.315 ~
1.314J
1.305
1.288
1.268
1,248 ~

10i5

1.24711.229
1.216-1.216-3

5050

1.205

--

--

1.815

-1

11~55051

1.197
1.184

22214

1.175

--

--

20~53380

1.1741.160[-1.160-3

3233

1.159 J

31;~.4

1.153-1.139-13381
42~05052

1.141 ~
1,139 )

1.127

1.128

14281
4133
2135
40~
3382
51~03035
42~2
51~1

1.121
1,118
1,112
1,095
1.090
1.083
1.076
1.073
1.067

1.121
---

1
--

1.088
1.082
-1.073120006
5033
1016
3254

1.062
L048
1.046 ~
1.044 J--L044--33

The measurements were made on photographs taken in a 114.6 mm
diameter camera with Cu-K~x radiation (2 = 1.5418A).

396

D. A T K I N E T A L .

X-ray data. X-ray diffraction data for tristramite
were derived from 114.6 mm diameter DebyeScherrer powder cameras using Cu-K~t radiationand silicon as an internal standard. Intensities werevisually estimated in conjunction with a calibrated
intensity scale, the diffraction mount being prepared in the form of a sphere to avoid preferredorientation effects. Very minor amounts of goethitewere associated with the tristramite, and reflections
from this phase have been subtracted in the presentation of the tristramite data (Table II). The
diffraction pattern of tristramite is analogous withthat of rhabdophane and also brockite. For comparison, new powder data, derived under similar
experimental procedures, for rhabdophane fromthe mine dumps of Fowey Consols, Cornwall(a 6.960_ 0.003 A, c 6.372 + 0.006/~) are also listed
in Table II. Systematic absences in the tristramite
diffraction data are consistent with those of thespace group P6222 determined for the hexagonalrare-earth phosphates (Mooney, 1950). Utilizing
the method of least squares the cell dimensions oftristramite were determined as a 6.913 +0.003 A,
c 6.422_ 0.006/~, a : c = 1 : 0.929 and V 265.8 +0.5 A 3. On the basis of three formula units in theunit cell and the atomic ratios derived in Table I thecalculated density is 4.18 g/cm 3. It is to be notedthat samples of tristramite isolated from the replacement or matrix-type occurrences characteristically give multiphase X-ray powder patterns
and so are unsuitable for the derivation of satisfactory diffraction data.
Discussion. Several calcium uranium phosphateminerals have been described in addition totristramite, including autunite, meta-autunite,
phosphuranylite, phurcalite, ningyoite, and lermontovite. The status of lermontovite (Soboleva andPudovkina, 1957) is uncertain, the mineral being
inadequately described and lacking X-ray diffraction data, but an incomplete chemical analysis
showed a U : C a ratio of 10:1 (cfi tristramite
U : C a - I : 2 ) . Ningyoite is chemically similar totristramite, with a powder diffraction pattern
resembling that of rhabdophane, but it is pseudohexagonal and has been indexed on a primitive
orthorhombic lattice (Muto et al., 1959). Morerecently, ningyoites have been described from theUSSR (Belova et al., 1978) and Canada (Boyle et al.,1981). Both sets of authors derive cell parameters
closely similar to those of the type material from the
Ningyo-toge mine, Japan, but the diffraction datain both cases have been referred to a C-faced
orthorhombic cell. The axial ratios b: a from thesethree described ningyoites are 1.78 to 1.79 whichcontrast with the ratio 1.73 for the transformation

to orthorhombic axes of a true hexagonal cell.
Kucha (1979) has described unnamed monoclinic,hexagonal, and orthorhombic iron-thorium phosphates from Poland which are referred to themonazite, rhabdophane, and ningyoite groups ofminerals respectively. Kucha notes a progressive
increase in the water content from the monoclinic
to the orthorhombic form.
The widespread occurrence of tristramite insouth-west England indicates that late-stage phosphatization is an important part of the genetic
sequence in the uranium-bearing hydrothermalvein mineralization. The process does not always
result in the formation of tristramite, however, assome pitchblende is replaced by a phosphatebearing mineral with the cottinite structure. Preliminary chemical data on this mineral indicate that
arsenic and possibly other cations can substitute forphosphorus and that the phosphatization process
may be complex. A consideration of its paragenesis
and that of tristramite in relation to late-stage
phosphatization will be published at a later date.
Acknowledgements. The authors thank colleagues N.

Cogger for wet chemical analysis and A. Milodowsky for
Clerici separations. Dr M. H. Hey is also thanked for
helpful comments. The paper is published with theapproval of the Director, Institute of Geological Sciences(NERC).
REFERENCES
Belova, L. N., Gorshkov, A. I., and Ivanova, O. A. (1978)
Dokl. Akad. Nauk SSSR, Earth Sci. Sect. 238, 132 4.
Bowles, J. F. W. (1975) Rep. Inst. Geol. Sci. No. 75/9.
Boyle, D. R., Littlejohn, A. L, Roberts, A. C., andWatson, D. M. (1981) Can. Mineral. 19, 325-31.
Dines, H. G. (1956) The metalliferous mininff reffion ofsouth-west Enffland, 1, 240. Mem. Geol. Survey of Gt.
Britain, London.
Fisher, F~ G., and Meyrowitz, R. (1962) Am. Mineral. 47,1346-55.
Kucha, H. (1979) Mineral. Polonica, 10, 3-25.
Mason, P. K., Frost, M. T., and Reed, S. J. B. (1969)
B.M.-I.C.-N.P.L. computer profframs for calculatingcorrections in quantitative X-ray microanalysis. Nat.

Phys. Lab. (IMS), Rep. 2.
Mooney, R. C. L. (1950) Acta Crystallogr. 3, 337-40.
Muto, T., Meyrowitz, R., Pommer, A. M,, and Murano, T.
(1959) Am. Mineral. 44, 633-50.
Smellie, J. A. T., Cogger, N., and Herrington, J. (1978)
Chem. Geol. 22, 1 10.
Soboleva, M. V., and Pudovkina, I. A. (1957) [Uranium
Minerals Handbook, Moscow pp. 181-2]; abstr, in Am.
Mineral. 43, 379-80, 1958.

[Manuscript received 10 October 1982]