T H E D E C O M P O S I T I O N P R O D U C T S OF A N O R T H I T E A T T A C K E D BY P U R E W A T E R AT E L E V A T E D T E M P E R A -

T U R E S AND P R E S S U R E

By

A. F . FREDERICKSON AND J. E . C o x , JR.,

Washington University, St. Louis, Mo.

A B S T R A C T

The behavior of anorthite under the test conditions is similar to that of other feldspars and quartz. The solubility of anorthite in pure water was measured at 300 bars pressure between 200 ~ and 350~ Under these conditions, anorthite not only partially dissolves, but also disintegrates by shedding small particles of crystalline material which forms, with matter in true solution, a nonhomogeneous suspension. Two zeolites (afwillite and xonotlite) develop at the higher temperatures. Their stability is determined by the CaO :SiO2 ratio of the solution.

The anorthite fragments react with the solution. They maintain their identity although their composition and structure gradually change over a wide temperature range. Possibly the zeolites inherit their structure from the altered anorthite. I t is almost certain that xonotlite inherits its structure from afwillite.

I N T R O D U C T I O N

For better understanding of the mechanism of the weathering process and the manner in which clays and other minerals are made from mineral debris, it would be helpful if the geologist had a clearer concept of the physical and chemical nature of these decomposition products. This paper is concerned only with the first part of this large problem: the physical nature of the disintegration products of some common rock-forming silicates attacked with pure water. The chemical behavior of this material will be discussed separately at a later date. The solubility of the silicates in water is relatively small; consequently, the experiments were run at elevated temperatures to take advantage of the fact that the hydrogen ion concentration of water increases with temperature. With the consequent increased activity of the solution, the minerals are attacked more vigorously and yield a larger amount of disintegration products.

E X P E R I M E N T A L C O N D I T I O N S

The volume of fluid in the bomb at a given temperature determines the pressure that will develop. By removing appropriate amounts of fluid during each sampling, it was possible to obtain a complete solubility curve

III

112 SECOND NATIONAL CONFERENCE ON CLAYS AND CLAY MINERALS

0,16

0.14 - -

0.12 u o'

O.lO %

0.08

0 _o

y - i!~ e o ~,I ~. �9

E 0.04 / - ~ ~ ,a " { ....."

. . . . . . . .

0.02 ,.. "" ~ ' ~ q " " " ~ t

200 250 300 350

Temperature, ~

FIGURE 1 . - - C o m p a r i s o n of the "sohibility" of anorthite in water at elevated tem- peratures at 300 bars pressure to the "solubilities" of albite and quartz.

over a range of t empera tures at constant pressure , wi thout opening the bomb or otherwise d i s tu rb ing it.

The apparen t "solubi l i ty" of some common silicates was de termined as fol lows: A suitable crystal was suspended in a 195 ml-capaci ty bomb filled with enough distil led water to give 300 bars p ressure at the des i red tempera ture . W h e n the solution had been held at the des i red t empera tu re for a p rede te rmined t ime (usua l ly 100 hou r s ) , a por t ion was s iphoned f rom the bomb chamber into a known length of evacuated tubing. The sample was then dra ined into ta red a luminum cups and evaporated. The solubi l i ty was recorded as the amount of solids contained in 100 par ts of water. In this manner , the solut ion sample was isolated f rom the bomb chamber and none of the mater ia l had an oppor tun i ty to g row back onto

DECOMPOSITION PRODUCTS OF ANORTHITE 113

the nutrient crystal during cooling. Two bomb heads were employed. One contains a long tube that extends to within one-half inch of the bottom of the bomb; the other has a tube that is used to sample the solution 6 ~ inches from the bomb bottom. Solubility determinations were made f rom these different places in the bomb to test the homogeneity of the solution.

The disintegration products contained in the isolated samples were examined by optical, x-ray, and electron microscope techniques. Some of the samples were electrodialized.

R E S U L T S Solubility

The apparent solubility of anorthite in pure water is shown in Figure 1 by the dotted lines. For purposes of comparison, the solubility curves for quartz (Frederickson and Cox, 1954a) and albite (Frederickson and Cox, 1954b) are included. For each mineral, the upper curve represents the amount of material contained in 100 nfl of solution siphoned f rom the bottom of the bomb whereas the lower curve represents the solution siphoned from near the bomb top. The data for each point on the curve are given in Table 1.

Appearance of Products

Some of the material siphoned from the bomb was evaporated onto suit- able mounts and examined with an electron microscope. The different types of material are shown in Plate 1. Two distinctly different crystalline phases are identified on the basis of their morphology. The background of all samples examined consists of a gelatinous material that forms thin films which crack and curl after prolonged exposure to the electron beam.

TABLE 1. - -"SoLuBILITY" OF ANORTHITE IN WATER AT ELEVATED TEMPERATURES AT 300 BARS PRESSURE

Temperature Solubility ~ parts solid per 100 parts water

Solution Extracted from Bottom of the Bomb 200 0.029, 0.025 260 0.047, 0.041, 0.032* 300 0.041, 0.037, 0.032, 0.038

0.034, 0.038, 0.036 360 0.059, 0.063, 0.065, 0.058

Solution Extracted from Top of the Bomb 200 0.013, 0.016, 0.018 260 0.034, 0.038, 0.035 300 0.025, 0.032, 0.027, 0.030,

0.023, 0.018, 0.027 360 0.046, 0.041, 0.037, 0.030*

* Values discarded owing to partial loss of sample during extraction.

I14 SECOND NATIONAL CONFERENCE ON CLAYS AND CI.AY MINERALS

PLATE 1 . - -E lec t ron micrographs of the zeolites in bomb "solution" siphoned at 350 ~ C.

(A) Mag.: 24,000 X. The long, fibrous, needlelike crystals occur in felted masses in a finer-grained, mottled background material. This material is xonotlite (5CaSiO:~~ The identification is verified by x-ray examination.

By varying the printing technique for various portions of the negative, the tube- shaped character of some of the long, thin crystals can be discerned. The long crystal extending from the top of this micrograph is an example.

(]3) Mag. : 18,000 X. ']'he large, flat, thin, equant crystals with prismatic to hexa- gonal habit are identified as afwilIite ( 3 C a O - 2 S i O 2 . 3 H ~ O ) . Some of the small,

D E C O M P O S I T I O N P R O D U C T S OF A N O R T H 1 T E 115

very black crystals near tile middle of tile micrograph actually are well-formed prismatic crystals. Note the fine-grained background film in both micrographs. It cracks and curls under the heat of the electron beam. The film probably consists of the unused aluminum hydroxide gels mixed with the calcium ions and silica gels not used by the growing crystals.

TABLE 2 . - X-RAY DIFFRACTION PATTERNS OF ANORTHITE SOLUTION PRODUCTS

(Filtered copper Ka radiation)

Product at Anorthite Product at Afwillite Product at Xortotlite Product at 200~ A S T M 260~ A S T M 300~ A S T M 350~

d I d I d I d I d I d I d I

12.19(4)

8.23(2)

6.94(10)

6.30(5) 5.70(3)

3.82(5)

6.92(10)

6.28(3)

4.08(2) 3.92(9)

-o3.80(2)

3.63(1) 3.53(5)

3,34(4) -o3.37(1) 3.34(5)

3,19(7) -o3.20(10) 3.15(1) 3.16(8)

3.03(4) 3.04(1) 2.95(3)

2,88(3) -o2.83(3) ,-2.82(3)

2.64(5) -o2.65(1) 2.64(3)

2.48(3) .-o2.51(5)

2.27(7)

2.14(5)

--3,2.26(1) ~--2.26(8)

-o2.14(5) <---2.16(4) 2.10(2) 2.02(2)

2.00(3)

1.93(2) 1.88(1) 1.84(4) *-1.82(2)

8.59(3)

7.19(10) 7.06(7)

6.53(7)

5.79(3) 5.05 (6) 4.69(4)

4.12(5) 3.89(1)

6.30(3)

4.24(9)

3.89(9)

3.70(5) 3.62(3)

-o3.39(3) 3.28(5)

--~3.16(9) 3.02(4)

3.44(1)

3.23(9) <---3.17(7) *-3.02(2) 3.06(10)

2.82(10) 2.71(9)

~2.64(3) 2.57(5) 2.49(1) 2.42(3) 2.33(7) 2.30(4)

2.14(7)

2.05 (2) 2.00(2)

1.94(7) 1.91 (4) 1.85(6)

,-2.83(7)

,--2.65(5)

<--2.47(1)

*-2.32(4)

2.26(5) 2.21(2)

*-2.16(2) 2.09(1)

*--1.99(1) 1.97(4)

<--1.94(9) 1.87(7)

<---1.83(7)

2.82(9) 2.69(7) 2.62(1)

2.50(7)

-o2.33(3)

-o2.25(4)

2.03(9)

1.95(4)

1.84(3)

<--3.86(10)

<---2.81(7) *--2.69(9)

2.59(8)

116 S E C O N D N A T I O N A L C O N F E R E N C E O N C L A Y S A N D C L A Y M I N E R A L S

Product at Anor th i tc Product at Afwilli te Product at Xonotl i tc Product at 200~ ASTM 260~ A S T M 300~ ASTM 350~

d I d I d I d I d [ d I d I

1.80(3) 1.80(7) 1.76(3) 1.76(7) +--1.75(4) 1.74(3) +--1.77(3)

1.72(3) 1.71 (1) 1.70 (6) 1~70(7)

1.65(3) 1.68(1) *--1.69(3) 1.63 (2) 1.63 (4)

1.59(4) 1.57(4)

1.53(1) 1.53(3) -+1.51(7) 1.48(3) 1.48(2) 1.45(2) 1.42(6) ,--1.45(2) 1.41(I)

1.38(6)

X-ray Examination of the Disintegration Products

To get a large enough sample which would yield a good x-ray diffraction pattern, separate bomb runs were made. The system was allowed to come to equilibrium at each temperature where solubility measurements were previously made, the bomb was quenched and the entire contents were poured into suitable containers and evaporated. The nutrient anorthite crystal had been wired onto the thermocouple tube in the bomb head before each run. It was not disturbed during the pouring operation so that no fragments from it would "contaminate" the material used for x- ray examination. The results of the x-ray examination are shown in Table 2. The diffraction pattern of the mineral considered to be the major con- stituent in each sample is placed in Table 2 between the products found in the bomb at each test temperature. Inspection of the patterns shows that although most of the lines for each product could be assigned to a definite mineral, an overlap frequently occurred indicating the presence of at least two minerals, one predominating over the other, and all four of the products assignable essentially to three minerals. Not all of the lines, however, have been identified. I t should be noted that anorthite, afwillite, and xonotlite have numerous spacings which are quite similar, and also that the bomb products have a number of lines in common; yet each bomb product can be predominantly assigned to one set of values, with a slighter similarity to another set of spacings, usually that on the right of it in Table 1.

D I S C U S S I O N

Under the test conditions used, anorthite not only dissolves in water but also disintegrates to yield tiny anorthite fragments that maintain their identity up to approximately 300~ These fragments, along with calcium ions in true solution and some colloidal material, constitute a non- homogeneous suspension as indicated by the different solubility values obtained f rom different parts of the bomb.

DECOMPOSITION PRODUCTS OF ANORTHITE 117

The x-ray data may be summarized as follows :

At 200~ Bomb product shows strong anorthite lines; evidence for afwillite and xonotlite is inconclusive. Small fragments of the nutrient anorthite crystal must be present.

At 260~ Anorthite is present but fewer lines are evident; otherwise the resemblance to the 200~ product is slight. Afwillite, a Ca-zeolite type mineral, appears for the first time.

At 300~ If any of these lines belong to anorthite (7.19, 6.30 A) they refer to a product that has undergone a great amount of alteration and expansion. The total pattern best fits afwillite. Some xonotlite may also be present. Afwillite is the phase suspected of causing the inflection in the solubility curves.

At 350~ The product is largely xonotlite, a mineral lower in Ca and water than afwillite. There is some possibility that afwillite may also be present. No anorthite.

The almost complete absence of anorthite lines from the bomb products above 300~ indicates that at higher temperatures the anorthite fragments removed from the parent crystal have been broken down or altered beyond recognition; the recrystallization of the solution products has been entirely into other structural forms.

It is impossible to say without a detailed structural comparison of the minerals involved whether or not the zeolites inherit part of their struc- tural pattern from either the anorthite or decomposed anorthite; these patterns, however, clearly suggest this possibility. The structural similarity of the two zeolites, however, makes it ahnost certain that during reaction with the fluid at increasing temperature, xonotlite inherits its structure from afwillite.

In the higher temperature tests (3000C and 350~ the larger particles which give the solubility values obtained for solution siphoned from near the bottom of the bomb are not anorthite fragments but predominantly a new precipitated crystal phase or one formed by additions and subtractions of material to the rearranged remnants of the original anorthite frag- ments. Apparently the first stage in the breakdown of the anorthite nutrient crystal is one in which small particles of various sizes break loose. These are undoubtedly accompanied by some material (the Ca ions) which goes into true solution. This appears to be the first step in the breakdown of all feldspars so far investigated (albite, anorthite and microcline 1) and also for quartz. The step-by-step process by which the hydrogen ions of the water attack the nutrient crystal to produce fragments and other material was described earlier (Frederickson, 1953). The larger feldspar fragments perhaps retain their structural identity but their composition must be different from the parent crystal. The gradual disappearance of

1 Unpublished

118 SECOND NATIONAL CONFERENCE ON CLAYS AND CLAY MINERALS

the anorthite lines with increasing temperature indicates that a gradual structural change follows the chemical change.

I t is important to note that the feldspar fragments are not just metastable relics. The solubility curves are reproducible; we have obtained as many as eleven points for one temperature, all of which fall within the range of those listed in Table 1. The duration of individual bomb runs was set at 100 hours which is greatly in excess of that necessary to get reproducible data. Although other interpretations are possible, we believe that gradual structual changes accompany the chemical alteration of the fragments until a point is reached where a different CaO:Ai~O3 :SiO2 ratio results in a product more stable than any that can be made from the expanded and altered anorthite lattice. At this point a new crystal phase, one of the zeolites, develops.

Some of the debris from the nutrient crystal and the ejected alteration products f rom the fragments exists in the solution as individual ions and colloidal aggregates. When the ratio of the components in this "solution" has reached the point mentioned above, a zeolite (afwillite, 3CaO '2S iO2" 3 H 2 0 ) begins to precipitate. The decrease in apparent solubility, indicated by the flexure in the solubility curve (Fig. 1), means that some of the solids previously in solution have been removed from the system or are now outside of the range of the siphon. A complete solubility curve is obtained without opening or otherwise disturbing the bomb so the only explanation we have to offer is that some of the newly developing afwillite grows on the side of the bomb, onto the nutrient crystal, or develops crystals so large that they settle toward the bottom of the bomb. Several runs were made to check this point without positive results. The bomb sides appear clean and the nutrient crystal is already so badly attacked that it is impossible to recognize any newly grown material if it occurs. X-ray examination of the tiny crystals on the nutrient anorthite proves the presence of afwillite but this probably was dumped onto the crystal when the bomb was quenched so does not help explain the reverse inflection of the solubility curve.

At higher temperatures (300~ the fragments of anorthite sloughing off the parent crystal and the afwillite react with the "solution" to produce a different zeolite (xonotlite, 5CaSiO2 �9 H 2 0 ) that incorporates more SiO2 in its structure. Its development here is perhaps due to a newly established C a O : S i O 2 : H 2 0 ratio in the system as the precipitated afwillite begins to be redigested with rising temperature and releases some of the calcium which it had tied up. The increase in solubility at 350~ indicates that neither afwillite nor xonotlite is a stable structure under these conditions.

No mention has been made of what has become of the alumina derived from the anorthite decomposition. The mineral forms so far identified in the residues do not incorporate aluminum as a major constituent, but several as yet unidentified x- ray lines suggest that further diffraction work may reveal the aluminous constituents or they may exist merely as colloidal

DECOMPOSITION PRODUCTS OF ANORTHITE 119

aluminum hydroxides in suspension. This material, along with some unused Ca ions and colloidal silica, constitutes the background films seen in the electron micrographs (P1. 1).

A C K N O W L E D G M E N T S

This work is part of Project No. 142B sponsored by the U. S. Army Signal Corps. Their financial assistance and encouragement is gratefully acknowledged.

R E F E R E N C E S CIT ED

Frederickson, A. F. (1951) Mechanism of Weathering: Geol. Soc. Amer. Bull., v. 62, p. 221-232.

Frederickson, A. F., and Cox, J. E., Jr. (1954a) Mechanism of "'solution" of quart~ in pure water at elevated pressure and temperature: Amer. Mineral. (in press).

- - (1954b) "Solubility" of albite in hydrothermal solutions: Amer. Mineral. (in press).