CEMENT and CONCRETE RESEARCH. Vol. 9, pp. 259-268, 1979. Printed in the U.S.A. 0008-8846/79/020259-10502.00/0 Copyright (c) 1979 Pergamon Press.

PHASE STUDIES IN THE SYSTEMS

CaO-AI203-CaCrO 4 and SrO-Al203-SrCr04*

Dennis Peters + and F. A. Hummel Ceramic Science and Engineering Section

Materials Science and Engineering Department The Pennsylvania State University

University Park, PA 16802

(Refereed) (Received Aug. 18, 1978; in f inal form Jan. 5, 1979)

ABSTRACT

Exploratory studies in the two systems have established the joins which exist between the 3:3:1 compounds and the binary compounds in the respective CaO-AI203 and SrO-AI203 systems. More detailed studies showed that the compound 3CaO.3AI203.CaCrO 4 takes CaO-AI203, CaO.2AI203 and AI203 into solid solution to the extent of 30, 20, and 25 mole percent, respectively. The compound 3SrO-3A1203-SrCrO 4 takes SrO'AI203, SrO'2AI203, SrO-6AI203 and AI203 into solid solution to the extent of 35, 30, i0 and 50 mole percent, respectively. The ha[[ynite solid solutions have'potential as yellow or yellow-green pigments in applications where hexavalent chromium can be tolerated.

Des 4tudes exploratoires des deux syst~mes ont permit d'4tablir les joints existant entre les compos4s 3:3:1 et les compos4s binaires dans les syst~mes respectifs CaO-AI203 et SrO-AI203. Des 4tudes plus d~taill~es ont montr~ que le compos4 3CaO-3AI203-CaCrO 4 prend CaO'AI203, CaO.2AI203 et AI203 en solution solide jusqu'~ une teneur de 30, 20 et 25 mole% respectivement. Le compos4 3SrO-3AI203-SrCrO4 prend SrO'AI203, SrO.2A1203, SrO.6AI203 et A1203 en solution solide jusqu'A une teneur de 35, 30, i0 et 50 mole% respectivement. Les solutions solides d' " hauynite sont potentiellement utilisables comme pigments jaunes ou jaune-verd~tres dans des applications o~ le chrome hexavalent peut ~tre tol~r~.

*Abstracted from an M.S. ~"nesis in Ceramic Science by Dennis Peters, June 1974.

%Dennis Peters is now with C-E Refractories, Combustion Engineering, Valley Forge, PA 19481

259

260 Vol. 9, No. 2 DI Peters, F.A. Hummel

Introduction

The sodalite group of minerals is an interesting class from a structural and crystal chemical point of view. The following four are well known members of the group:

Sodalite Na8[AI6Si6024]CI 2

Nosean Nas[AI6Si6024]S04

Ha~yne (Na,Ca)4_s[AI6Si6024](SO4)I_2

Lazurite Na8[AI6Si6024]SI_ 4

Based on the work of Fukuda(l-2), Halsted and Moore(3), Kondo(4), Saalfield and Depmeir(5) and Vulkov, Boyadzhieva and Pamukov(6), it is now well known that many silica-free sulfate, chromate, molybdate and tungstate ha~ynites with the general formula M2+AII2024(M6+04)2 can be prepared.

It is the purpose of this paper to discuss two chromate " hauynes, 3CaO.3AI203.CaCr04 and 3SrO.3AI203.SrCrO 4 in terms of the two systems CaO-AI203-CaCrO 4 and SrO-AI203-SrCrO 4.

Literature

It is not appropriate to discuss the entire mass of sodalite mineral literature at this time. The early literature through 1961 is summarized by Deer, Howie and Zussman(7). More recent literature which is especially con- cerned with the ha~yne member of the sodalite family is by Saalfield(8), Loehn and Schultz(9) and Ponomarev, Kheiher, and Belov(10).

Gilioli, Massazza, and Pazzuoli(ll) verified the existence of 3CaO-3AI203.CaSO 4 and SrO-3AI203"Sr$04 in a study of the quarternary system CaO-SrO-AI203-SO 3 and found complete solid solution between the ternary com- pounds, as reported earlier by Kondo(4). Kondo(4) was the first to synthesize 3CaO-3AI203-CaCrO 4 and 3SrO-3AI203"SrCrO 4 and later Peters and Hummel(12) found complete solid solubility between 3CaO-3AI203"CaCrO 4 and 3SrO.3AI203-

SrCrO 4 .

Ford and Rees(13) found that tetragonal calcium chromate (CaCr04) loses oxygen at 800°C and the compound 9CaO-4CrO3-Cr203 is produced at the composi- tion corresponding to a CaO:Cr203 ratio of 3:1.

Later, Ford and White(14) investigated the binary system CaO-Cr203. They found that calcium chromate (CaCrO 4) begins to dissociate at 750°C to form 9CaO.4CrO3"Cr203 and 3CaO.2CrOq.2Cr203. Further increase in temperature above 750°C slowly reduced Cr 6+ -3+ to Cr . Complete dissociation will occur at I125°C forming CaO and CaO.Cr203. Ei-Rafei(15) confirmed the existence of 9CaO-4CrO 3- Cr203 and its subsequent phase transformation to monocalcium chromite and free CaO.

Experimental Procedure

Compositions were prepared from reagent grade CaCO3, SrCO3, CrO3 and Alcoa (-325 mesh) tabular alumina. Ten gram batches were weighed to 0.01 gram accuracy on an analytical balance and mixed in distilled water in glass mortars for 5-10 minutes. The consistency of the wet batch was very carefully con- trolled to insure intimate dispersion of the water soluble CrO 3 and to pre- vent segregation during drying at IIO°C. Batches were dried further at 120°C for 8 hrs., dry mixed, and dried again at 350°C for 2-3 hrs., calcined at 900°C for 4 hrs., to insure decomposition of all carbonates, dry mixed and fired be- tween 1200-1400°C for 15-48 hrs., depending on composition.

Vol. 9, No. 2 261 PHASE STUDIES, CaO, A1203, CaCrO 4

Identification was done using a Norelco X-ray diffractometer with a Cu tube at 40 KV and 15 ma or an Fe tube at 40 KV and I0 ma. Routine diffraction patterns were run at 1 ° per minute in a 26 range between 60 ° to 17 ° . Pre- cision diffraction data were obtained by using Pt or Si as an internal standard and running at 1/4 ° 26 between 60 ° and i0 °. This technique was especially necessary for limiting solid solutions as shown in Table I. A polarizing microscope was frequently used to examine compositions for secondary phases.

Results and Discussion

A. Exploratory Study of the Systems CaO-AI203-CaCrO 4 and SrO-AI203-SrCrO4 in air

I. Behavior of the Simple Chromates

The decomposition of CaCrO 4 in air at temperatures over 800°C has been well documented(13-15). Formation of ternary phases such as 9CaO.4CrO3-Cr203 and 3CaO.2CrO3-2Cr203 has been observed as well as CaCr204.

It is interesting to note that, in this study, yellow CaCrO 4 remained stable to 1200°C, providing it coexisted on the join with the 3:3:1 compound (composition 22)(16). Compositions not lying on the join behave as expected, forming the chromite, CaCr204, at 1200-1300°C (compositions 19 and 20).

Also, in this study, yellow SrCrO 4 was melted around 1300°C on a platinum foil in a Globar furnace and recrystallized during cooling. X-ray diffraction showed that only SrCrO 4 was present, which suggested that SrCrO 4 melted con- gruently. However, the development of a green color after any heat treatment above 1000°C also suggested that a lower oxide of chromium (3 + or 5 + ) was forming, at least in small amounts. It should be noted that SrCrO 4 remained stable at 1300°C when it coexisted on the join with the 3:3:1 compound (composition 38)(16). Composition 37, which was considerably removed from the 3:3:l-SrCrO 4 join, did not contain a Cr 3+ phase after heating to 1400°C according to X-ray analysis, but it was green, indicating at least some small amount of reduction to lower oxides of chromium.

These qualitative data indicate that SrCrO 4 is more thermally stable than CaCrO4, but even CaCrO 4 remains stable to 1200°C if it coexists on the join with the 3:3:1 compound.

2. The Ternary Systems

The system CaO-AI203-CaCr04 is a true ternary system only below a temperature of about 800°C and at any temperature above 800°C, oxygen would be a component of the system and it would have to be studied under controlled oxygen pressure. The end members of the system would be Ca-AI-Cr-O2, due to the presence of both Cr 3+ and Cr 6+ (or other valance states such as Cr 4+ or Cr 5+) at various temperatures between 800-2000°C. The same analysis applies to the system SrO-AI203-SrCrO4, but apparently SrCrO 4 will withstand con- siderably more temperature than CaCrO 4 before undergoing reduction to phases containing Cr 3+ (or Cr 4+, Cr5+).

The real interest in both systems lies in the regions bounded by the refractory compounds CaO'AI203-3CaO'3AI203"CaCrO4-AI203 and SrO-AI203- 3SrO.3AI203-SrCrO4-AI203. The melting or decomposition point of each member of these subsystems is well above 1500°C and they can be studied in air without danger of decomposition at temperatures above 1000°C.

The compositions used for the exploratory study are shown in Figures i and 2 and Ref. 16 which also contains the phase analyses.

TABLE I

X-ray Diffraction Data for the

Limiting Solid Solutions in the System

SrO-AI203-SrCrO4

fx)

UO

hkl

400

422

440

620

444

642

800

822;644

743;840

664

844

862;10,2,0

10,4,2

880

10,6,0

12,0,0;884

"6513SrO'3AI203"SrCrO4]

+.35SRA1204

d(~)

i/

1

--

O

4.71

39

3.84

i00

3.33

77

2.98

i0

2.72

71

2.52

16

2.36

43

2.22

41

2.11

34

1.92

34

1.85

ii

1.67

23

1.61

I0

"70[3SrO-3AI203.SrCr04]

+'~0SrAI407

o

d(A

) I/

I -

-0

4.69

41

3.85

i00

3.33

88

2.98

9

2.72

i00

2.52

17

2.36

61

2.22

38

2.11

43

1.93

48

1.85

12

1.72

9

1.67

34

1.58

8

• 90[3SrO'3AI203-SrCrO 4 ]

+'IOSrAII2019

d(~)

i/~

-

-O

4.70

32

3.84

100

3.33

65

2.97

9

2.71

90

2.51

16

2.35

38

2.22

37

2.10

29

1.92

24

1.85

ii

1.72

7

1.66

19

1.61

7

"50[3SrO-3AI203-SrCrO4]

+.50A1203

0

d(A

) 4

69

3 84

3 33

2.9

8

2.7

2

2 52

2.3

5

2.2

2

2.1

1

2.0

1

I .9

2

1.85

1.72

1.6

7

1.6

1

1.5

8

----

X~)

41

10

0

81

14

81

21

49

39

35

11

38

10

10

24

10

13

CD

fD

rD

6o

-11 ¼ -C

c-

3

O)

< O ~O

Z O t~O

Vol. 9, No. 2 PHASE STUDIES, CaO, A1203, CaCrO 4

CoCr04 ~ 2 4

263

~23

CoO

0 I 02

~22

~21 20 19 0 0

O17 I O

3:1 12:7 I:1 1:2 I:-6

FIG. 1

Compositions and Compatible Phases in the System CaO-AI203-CaCr04 (Mole Percent)

AIzO 3

SrCrO 4 ~4o

,39

3 8 0 3 7

SrO

27 0

28 o 3 ~

36,l.3=3,1

35

5:1 3=1 I.'1 I.'2 I;6

FIG. 2

Compositions and Compatible Phases in the System SrO-A1203-SrCrO 4 (Mole Percent)

AIzO 3

264 Vol. 9, No. 2 O. Peters, F.A. Hummel

For the system CaO-AI203-CaCrO 4 the following data were established:

(a) Under the firing conditions employed, in air, the only ternary compound which exists below 1300=C is 3CaO-3AI203"CaCrO 4.

(b) The 3:3:1 compound is compatible with AI203 and all of the binary calcium aluminate compounds, except CaO.6AI203, at the temperatures used for the exploratory work.

(c) At 1200°C or above, compositions 19 and 20 yielded the 3:3:1 com- pound and calcium chromite, CaCr204, as might be expected from previous literature. Compositions 16 and 17 contained no CaCr204 according to X-ray diffraction patterns, suggesting that, if present, it was below the limit of detection by X-rays. However, it is also possible that little or no CaCr204 formed due to the influence of the large amount of 3:3:1 present and the tendency for the chromates to persist under these circumstances.

(d) Compositions 25 and 26 showed that no basic calcium chromates form in this system, as they do in the case of lead chromates, where the 4:1, 2:1, and i:i basic chromates are known to exist. It is interesting to note that pentavalent chromium is developed in compositions 25 and 26 at temperatures around I000-1200°C, as identified in diffraction patterns for Ca3(CrO4) 2.

(e) The colors of the compositions are good indicators of the oxidation state of the chromium. Compositions in the triangle 3:3:1-I:I-A1203 yield yellows or yellow-greens after firing at 1300°C. In other sections of the system, high temperature firings yield green colors, indicating the ~resence of a lower oxidation state of chromium (3 + or 5+).

For the system SrO-AI203-SrCr04, the following observations were made:

(a) Under the firing conditions employed, in air, the only ternary compound found below 1400°C was 3SrO.3AI203.SrCrO 4.

(b) The 3:3:1 compound is compatible with AI203 and binary compounds of the following SrO:AI203 ratio: 3:1, i:i, 1:2, and 1:6. No strontium analogue of the 12CaO'7AI203 compound exists, but compounds of the 5:1 and 4:1 ratios have been claimed.

(c) Composition 38 indicates that SrCrO 4 is stable to 1300°C in the presence of the 3:3:1 compound.

(d) Compositions in the triangle 3:3:1-I:I-A1203 yield yellow and yellow-green colors after firing at 1400°C. In other sections of the system high temperature firing yields green colors indicating the presence of a lower oxidation state of chromium.

(e) The compositions 4SrO'AI203"SrS04, reported earlier by Vulkov, et al.(6), was fired at 1300°C for 16 hours in air. X-ray diffraction analysis showed 5SrO.AI203, SrSO 4, 3:3:1, and SrO to be the phases present indicating that the compound 4SrO'AI203"SrSO 4 does not exist under these conditions. Hence, the existence of 4SrO.AI203.SrCrO 4 is also doubtful.

B. Phase Relations in the Subsystems CaO.AI203-3CaO'3AI203"CaCrO4-AI203 and SrO.AI203-3SrO'3AI203"SrCrO4-AI203

i. CaO'AI203-3CaO'3AI203"CaCrO4-AI203

X-ray diffraction and optical microscope studies showed that 30% CaO.AI203, 20% CaO'2AI203, and 35% AI203 were soluble in the 3:3:1 phase after heat treatment at 1300°C for 16 hours as shown in Figure 3. Although there were only very minor changes in the positions and intensities of the more intense diffraction peaks for the 3:3:1 compound, the solid solution limits could

Vol. 9, No. 2 265 PHASE STUDIES, CaO, AI203, CaCrO 4

CaCr04

CoO 3=1 12,7 I:1 1,2 I,6 AIz03

FIG. 3

Subsolidus Phase Relations in the System CaO.AI203-3CaO-3AI203.CaCrO4-AI203 at 1300°C (Mole Percent)

SrCrO 4

t,3'1

SrO 5:1 3=I I,I 1:2 1,6 AIzO 3

FIG. 4

Subsolidus Phase Relations in the System SrO.AI203-3SrO.3AI203.SrCrO4-AI203 at 1400°C (Mole Percent)

266 Vol. 9, No. 2 D. Peters, F.A. Hummel

easily be detected by the presence of a second phase in the X-ray pattern, and further confirmed by its presence under the microscope. There was no solubility of the 3:3:1 compound in AI203 or the binary aluminates.

2. SrO.AI203-3SrO. 3A1203. SrCrO4-AI203

A solubility of 35% SrO.AI203, 30% SrO.2A1203, 10% SrO'6AI203, and 50% AI203 in the 3:3:1 compound was found by X-ray and microscopic examination of samples fired at 1400°C for 16 hours, as shown in Figure 4. Although there were no major changes in the position of the diffracted peaks in the X-ray patterns, the intensities of the peaks underwent major changes as the four compounds were taken into solid solution (Table I).

As is the case of the calcium system, it was possible to establish the limit of solid solution within 4 to 8% by the detection of a second phase in the X-ray pattern or by microscopical examination, depending on which join was involved. The fact that all of the 3:3:1 solid solutions were intense yellows helped immensely in the microscopic examination, because secondary phases had little or no color and were easily detected. There was no solu- bility of the 3:3:1 compound in AI203 or the binary aluminates.

II .

C. The Relationship Between Sodalite, Nosean, and Hauynzte Structures

Barth(17,18,19) claimed that a tetrahedral network of [(AI,Si)12024] n- is characteristic of all minerals of the sodalite and ultramarine families. Subsequent work by several persons, especially Kondo(4), has shown that all of the Si can be replaced by A1 in synthetic hauynite type compounds, inferring that many intermediate compositions could be made by partially substituting [AI04] 5- tetrahedra for [Si0414- tetrahedra and appropriately charge compensa- ting in other sites in the structure.

A series of logical steps in the transition from sodalite to silica-free ,!

hauynites would be as follows:

Sodalite: Na8[AI6Si6024]CI 2 (i)

Nosean: Nas[AI6Si6024]SO 4 (2)

Na8[AI6Si6024]CrO 4 (3)

~4Ca4[AI6Si6024]SO 4 (4)

In the first step, SO4 = or CrO4 = would charge compensate the two chlorines but leave one vacancy on the "halide" site. In the second step, replacing all the Na charge with Ca charge would lead to half filled sites in the Na and halide positions, creating a very open structure and probably leading to its breakdown. Barrer(20) and Barrer and Baynham(21) have made the nosean

synthesis as in formula(2).

If one wished to make a synthetic hauynite with all the Na and halide sites filled, a possible formula is:

Na6Ca2[AI6Si6024](S04) 2 (5)

Na6Ca2[AI6Si6024](Cr04) 2 (6) f~

Kondo(4) has made both of these hauynites and the existence of the chromate analogue was again confirmed in this study.

It has been suggested(7) that a possible formula for an all-calcium

hauynite might be:

~3Ca5[AI6Si6024](SO4) 2 (7)

Vol. 9, No. 2 267 PHASE STUDIES, CaO, A1203, CaCrO 4

where all the halide sites are now filled, but three vacancies remain in the sodium site. If such a structure were possible, it would suggest the follow- in~ intermediates between the aluminosilicate ha~ynite and a silica-free hauynite as follows:

~2NaCa5[AI7Si5C24](S04)2

~Na2Ca5[AI8Si4024](S04)2

Na3Ca5[AI9Si3024](S04)2

~Ca7[AIIoSi2024](S04) 2

NaCa7[AlllSi1024](S04)2

Ca8[AII2Si0024](S04)2

(8)

(9)

(10)

(ll)

(12)

(13)

If one did not wish to use sodium in the sodium site, alternative formulas using only Ca and vacancies would be possible such as:

F~2Ca6[AI8Si4024](SO4) 2 (14)

or

~l.5Ca6.5[A19Si3024](S04) 2 (15)

(Compare with formulas 9 and i0).

Attempts were made in this study to prepare the chromate analogues of (7) and (i0) above, but without success. After heating at 1300°C for 24 hours, composition (7) was a mixture of 12CaO-7AI203, AI203 and gehlenite, and (i0) was a mixture of ha~ynite and gehlenite, indicating that the suggested intermediates have no real existence and cannot be prepared (in air) as synthetic compounds.

Conclusions

The joins, compatibility triangles and the extent of solid solution on the joins were established for the subsystems CaO-AI203-3:3:I-AI203 and SrO-AI203-3:3:I-AI203. The refractoriness and solid solubility relations in these subsystems are of possible interest in refractories, cements and in- organic pigments for ceramic applications when high temperature stability is a dominant requirement during preparation or final application.

References

i. N. Fukuda, Kogyo Kagaku Zasshi, 64, 865-71 (1961).

2. N. Fukuda, J. Chem. Soc. Japan Ind. Chem. Soc., 34 (i), 138-9 (1961).

3. P. E. Halstead, A. E. Moore, J. Appl. Chem., 12, 413-17 (1962).

4. R. Kondo, J. Ceram. Assoc. Japan, 73 (i), 101-8 (1965).

5. H. Saalfeld, W. Depmeier, Krist. Tech., 7 (1-3), 229-33 (1972).

6. V. Vulkov, Kh. Boyadzhieva, A. Pamukov, Stroit. Mater. Silikai. Prom., 14 (2), 5-7 (1973).

7. W. A. Deer, R. A. Howie, J. Zussman, Rock Forming Minerals, Vol. 4, Longmans, Green, and Co. Ltd, 289-302 (1'963).

8. H. Saalfeld, Zeit. Krist., 115, 132 (1961).

268 Vol . 9, No. 2 D. Peters , F.A. Hummel

9. Johann Loehn and Heinz Schulz, Neues Jahrb. Mineral Abh., 109, (3), 201-10 (1968).

i0 . V. I . Ponomarev, D. M. Kheiher , N. V. Belov, 15 (5) , 981-1021 (1970).

i i . G. G i l i o l i , F. Massazza, M. Pezzuo l i , Cemento, 70 (9) , 137-48 (1973).

12. Dennis Peters and F. A. Hummel, J. Amer. Cer. Soc., 59 5-6, 270-271 (1976).

13. W. F. Ford, W. J. Rees, Trans. British Ceram. Soc., 47, 207-8 (1948).

14. W. F. Ford, J. White, Trans. British Ceram. Soc., 48, 417-27 (1949).

15. E. A. Ei-Rafaei, J. Appl. Chem. Biotechnol., 21 (9), 261-6 (1971).

16. Dennis D. Peters, Phase Study and Crystal Chemistry of Compounds Having 11 .

the Hauynzte Structure, M.S. thesis, 1974, Appendix B, Table IV, page 48.

17. T. F. W. Barth, Norsk, Geol. Tidssk., 9, 40 (1926).

18. T. F. W. Barth, I. Mat.-Nat., Oslo, No. 8, (1927).

19. T. F. W. Barth, Zeit. Krist., 83, 405 (1932).

20. R. M. Barrer, Contributions to Synthetic Mineral Chemistry, Proc. Internat. Symp. Reactivity of Solids, Gothenburg, Pt. i, 373 (1954).

21. R. M. Barrer, J. W. Baynahm, Journ. Chem. Soc., 2892 (1956).