Phys Chem Minerals (1992) 18:453-459 PHYSICS CHEMISTRY []]I]MIHERALS �9 Springer-Verlag 1992

Fracture-Induced Emission of Alkali Atoms from Feldspar J .T . D i c k i n s o n 1, L . C . J e n s e n 1, S . C . L angford 1, and P . E . R o s e n b e r g 2

1 Physics Department, Washington State University, Pullman, WA 99164-2814, USA 2 Geology Department, Washington State University, Pullman, WA 99164-2812, USA

Received August 6, 1991 / Accepted October 28, 1991

Abstract . Emiss ion o f neu t ra l a t o m s (K and Na) and molecules ( H 2 0 and K O H ) obse rved dur ing f rac ture o f K- f e ld spa r have been accoun ted for by two i n d e p e n d e n t mechan i sms . H 2 0 and K O H emiss ions are a t t r i bu t ed to the vent ing o f f luid-f i l led inclusions, while emiss ion o f a tomic K is due to surface effects a c c o m p a n y i n g c leavage o f crys ta l l ine fe ldspar . The in tens i ty o f emi t t ed po tas s ium, at least 6 x 1014 a t o m s / c m 2 o f surface area, is sufficient to affect K act ivi t ies in so lu t ion dur ing mi- c rob recc i a t ion in the presence o f r o c k - d o m i n a t e d fluids.

Introduct ion

The f rac ture o f m a n y sol id ma te r i a l s in v a c u u m is ac- c o m p a n i e d by the emiss ion o f p h o t o n s and par t ic les , inc luding cha rged and neu t ra l a t o m i c and mo lecu l a r spe- cies (D ick inson et al. 1991 a). Col lect ively, these emis- s ions are k n o w n as fracto-emission. I n a s m u c h as Na- bea r ing ma te r i a l s have been shown to be cop ious emit- ters o f a tomic N a (e.g. s o d i u m sil icate glasses, D i c k i n s o n et al. 1988; NaC1, D i c k i n s o n et al. 1991a), an invest iga- t ion was in i t i a ted to explore the poss ib i l i ty tha t a com- m o n r o c k - f o r m i n g minera l , K- fe ld spa r , w o u l d behave in a s imi lar manne r . I f so, f r ac to -emiss ion o f a lkal i a t o m s f rom fe ldspar migh t be suff icient to affect the c o m p o s i t i o n o f r o c k - d o m i n a t e d fluids du r ing mic robrec - c ia t ion.

E x p e r i m e n t

The emission of atomic and molecular species was measured in ultrahigh vacuum using quadrupole mass filters, which permit the passage of ions within a narrow range of charge-to-mass ratios (q/m). Neutral atom densities are measured by positioning an ion- izer at the entrance to the quadrupole mass filter. A special arrange- ment of quadrupole filters allowed the simultaneous measurement of transient emissions at two masses. Two UTI Model 100C mass filters were positioned on either side of a single electron bombard- ment ionizer (see Fig. 1), so that ions created in the ionizer were directed by electric fields to the nearest mass filter. By monitoring

two masses simultaneously, the relationships between the various emitted species can be determined unambiguously. For example, one can generally distinguish between signals due to the emission of different species and signals resulting from the fragmentation (cracking) of a single species in the ionizer. The relationships be- tween emitted species often provide valuable clues as to the emis- sion mechanisms.

A large single crystal of K-feldspar (microcline, var. amazonite from Colorado), selected for this study, was characterized by opti- cal microscopy and chemical analysis. Thin-section examination showed that two mineral phases are present: microcline (~90%) and exsolved albite (~10%). A bulk X-ray fluorescence (XRF) analysis and an electron microprobe analysis (EMPA) of each phase are given in Table 1. The crystal was cut (parallel to cleavages)

Quadrupole sample Quadrupole n~a~ filterl NE . . . . ~l,,~r'~

[ 1 . . . . . 1 ionizer . . . . . 2

Fig. 1. Diagram of the two-quadrupole apparatus used for the si- multaneous measurement of emissions at two different masses. NE signifies "Neutral Emission"

Table 1. Chemical Analyses of K-feldspar (microcline var. amazon- ite) (wt. %)

XRFa EMPA b

Microcline Albite

Na20 2.52 0.62 11.37 FeO 0.03 0.05 0.04 K20 12.98 15.78 0.11 S i O 2 65.05 64.56 67.46 CaO 0.09 0.00 0.01 AlzO3 18.57 18.30 19.21 TiOz 0.00 0.00 0.01 MgO 0.00 0.02 0.02 MnO 0.00 0.01 0.01

Total 99.27 99.34 98.24

a X-ray fluorescence b Electron microprobe

454

g b~ 00

r

7 '

6

5

4

3,

2'

1'

Mass 18 and 39 Signals During Fracture of Feldspar

(a) Mass lg

" ~ ~-'r- ~, .,vl~q,-T... ~,---,.~- ,'

J 1'0 1'5 20 2'5 30 Time (ms)

700.

600'

500.

.~ 400.

300.

z 200"

100.

0~

35

30

25

20

15

10

5

0:

(b) Mass 39

h qt . j . i

i0 25 30 iS 2b Time (ms)

(c) Load

1'0 l'S 2'0 2'5 3'0 Time (ms)

Fig. 2. Typical (a) mass 18 signal (H20), (b) mass 39 signal (atomic K), and (e) load (in Newtons) accompanying the fracture of feld- spar. Note that the unit of current in (a) is gA, while the unit of current in (b) is nA. The water signal is much stronger than the potassium signal

into 2.5 x 6 x 13 mm 3 platelets which were later polished. The plate- lets were then mounted in a sample cassette which fed samples one at a time into position for fracture in three point bend across a support span of 6.3 mm. The entire apparatus was mounted in an ultrahigh vacuum system and pumped down to a base pressure of 10-7 Pa. The output of the quadrupole ion detectors was ampli- fied with identical electrometers and digitized at 10-20 ~ts intervals. Simultaneous load measurements were made with a quartz piezo- electric force transducer mounted behind the loading nose which provided time of fracture to within _+ 10 laS.

Results

Typica l measu remen t s a t 18 a m u ( H 2 0 § and 39 a m u (K +) are shown in Fig. 2. The a c c o m p a n y i n g load curve

4

3

2

1'

0 '

0.0

Mass 18 and 39 Signals During Fracture of Feldspar

(a)Mass 18

- - T - - T I I

0.2 0.4 0.6 0.8 1.0 Time (s)

600

~, 500'

--~ 400, t~ g~

300, g 2001 I

0.0 0.2 0.4 0.6 Time (s)

35.

30'

25,

20'

x5, I0'

5,

O. 0.0

(b) Mass 39

1" 0.8 1.0

(c)Load

0'.2 0~4 0'.6 o'.s 1.0 Time (s)

Fig. 3. The (a) mass 18 signal (H20), (b) mass 39 signal (atomic K), and (e) load of Fig. 2 shown on a much longer time scale. The reduced peak amplitudes in (a) and (b) are an artifact of the smoothing routine used to reduce the number of plotted points to a manageable number

shows smal l l oad d rops p r io r to the m a i n f rac ture event. These l oad d rops are mos t l ikely due to local ized f rac ture events. As shown in Fig. 2, each l oad d r o p p r io r to ul t i - ma te fai lure is a c c o m p a n i e d by a long- l ived burs t a t mass 18 and a very smal l emiss ion at mass 39. These burs ts are cons is ten t wi th the release o f wa te r f rom fluid- filled inclusions. Effervescence o f d isso lved gases and wa te r v a p o r in i t ia l ly yields intense wa te r emission. How- ever, evapora t ive cool ing unde r high v a c u u m cond i t ions soon freezes the r ema in ing mater ia l , which subsequen t ly subl imates as a much reduced rate. These processes are ref lected in the m o r e or less p e a k e d emiss ions a c c o m p a - nying f racture , fo l lowed by a long l ived (hundreds o f mi l l i seconds) decay ing c o m p o n e n t . In add i t ion , the vac-

7

g6 "~ 5

4

3

2

1

0

Mass 18 and 56 Signals During Fracture of Feldspar

30

25

Z~ 20,

15

10

(a) Mass 18

1'0 1'5 2'0 2'5 30 Time (ms)

120

~, loo

80 m er e .

60 ~D L q

4o

2O

0 0 5 10 15 20 25 30

Time (ms)

351 (c) Load

5

0 0 5 1'0 15 20 2'5 30

Time (ms) Fig. 4. Typical (a) mass 18 signal (H20), (b) mass 56 signal (KOH), and (c) load accompanying fracture of feldspar. The triangles in (a) and (b) are idealizations of the bursts at fracture used to esti- mate the total electrometer output associated these bursts

Mass 18 and 23 Signals During Fracture of Feldspar

455

J t~ e~

7 .

6.

5~

4.

3.

2.

0. o ~ 4'o 6b sb

Time (ms)

16

12

0 20 40 60 80 Time (ms)

100 1~0

(b) Mass 23

100 120

30. (c) Load

25, "=-~ ------~--- -_- J _ _ . ~-:- __ , _ , 2 0 '

15,

.I 10,

5,

0, 0 2'0 4'0 6'0 80 100 120

Time (ms) Fig. 5. The (a) mass 18 signal (HzO), (b) mass 23 signal (atomic Na), and (c) load accompanying the fracture of feldspar

uum system has a measured 200 ms time constant for pumping out a transient increase in partial pressure of HzO, which also contributes to the tail.

Immediately after fracture, large peaks are observed in both signals, al though their time behaviors are mark- edly different. The signal at mass 39 rises more slowly and falls more quickly than the signal at mass 18. The peak mass 39 emission occurs about 1 ms after the peak mass 18 emission; delays between 0.5 and 2 ms are typi- cal. Although a small component of the mass 39 signal appears to be associated with water, by far the most intense port ion of the emission evolves independently. This is strong evidence that the species responsible for the intense mass 39 signal is not associated with occluded water. Although an exhaustive search for more massive

K-containing species was not made, no evidence was found in conflict with the assignment of the major por- tion of the mass 39 emission to neutral, atomic K.

The behavior of these emissions on a much slower time scale is shown in Fig. 3. The decay of the mass 18 signal requires several hundred milliseconds, while the mass 39 emission appears as a single, sharp burst after fracture. As discussed below, the amount of materi- al emitted at each mass may be estimated f rom the total charge measured by the two electrometers (current x time), given in Coulombs (C). The total mass 18 signal (Fig. 3) amounts to about 500 nC, while the total mass 39 signal (Fig. 2) amounts to about 2 nC.

To probe the emission of fluid-borne K, emission measurements were also made at 56 amu ( K O H +). Typi-

456

the fracture surface are largely crystallographic in orien- tation, confirming that fracture was for the most part confined to cleavage planes. The surface area produced in fracture is clearly greater that the nominal cross sec- tion of the sample. Figure 6 also shows angular voids which are common on the fracture surface; some of these voids are probably associated with fluid-filled inclusions.

Discussion

Fig. 6. An SEM micrograph of a typical feldspar fracture surface fractured in our apparatus

cal results are shown in Fig. 4. Again, intense bursts in the mass 18 signal are seen at load drops prior to fracture, with weak emissions at mass 56. Ultimate fail- ure is accompanied by bursts at both masses. In contrast to the mass 39, the mass 56 burst at failure rises quickly, in concert with the emission at mass 18. The mass 56 signal is also much broader, with a width comparable to what one might assign to the "effervescent" phase of water emission. This is consistent with the ejection of aqueous KOH from fluid filled inclusions, followed by evaporation of the ejected droplets. The absence of a long tail in the KOH signal is consistent with its low volatility and high sticking probability at the chamber walls. The very evident long, flat mass 18 (H20) emission seen so clearly here is consistent with sublimation from ice which is a zeroth order process. Thus, for relatively large surface areas, the rate of sublimation would be a constant.

A single measurement of emission at mass 23 (Na+), was also made. The results are shown in Fig. 5. This emission is rather weak, as might be expected on the basis of the relatively small amount of Na in this feld- spar. As in the case of K, little Na is observed prior to fracture despite substantial bursts of water accompa- nying small load drops. However, Fig. 5 also shows mul- tiple spikes, some long after fracture. Similar spikes were observed in Na emission from NaC1 (Dickinson et al. 1991a) and sodium silicate glasses (Langford etal. 1991), lending credibility to this observation.

A scanning electron micrograph of a typical portion of a fracture surface appears in Fig. 6. The surface shows heavily stepped regions, as in the upper left of the figure, and smoother regions, as in the lower right. Facets on

The sharp peak in mass 18 emission at failure is almost certainly due to "explosive" ejection of water from in- clusions suddenly exposed to the vacuum due to the rapid nucleation and expansion of bubbles associated with dissolved gases. Fluid inclusions were readily identi- fied in thin sections of the material by optical microsco- py. The vaporization/sublimation of these droplets in flight yields intense spikes at masses 18 and 56 (H20 and KOH), provided a direct path is available from the sample to the quadrupole ionizer. Such a path is pro- vided by the opening crack between sample fragments at ultimate failure. However, the path of particles emit- ted during prefailure fracture events is generally more convoluted. The ability of water molecules to "bounce" off the metal surfaces of the vacuum system accounts for the relatively intense mass 18 signals accompanying these same pre-failure events. However, even here the sharpness of the leading edge of these bursts is affected by the geometry of available paths from the crack to the ionizer. In contrast, the low volatility of KOH mole- cules prevents the detection of significant amounts of KOH in the absence of a direct, line-of-sight path to the ionizer. Thus, little emission at mass 56 is seen except at the main fracture event.

As noted above, the intense mass 39 burst is unrelated to the fluid-filled inclusions. The signal at mass 39 re- mains small hundreds of microseconds after failure, when the water-related emissions are quite intense. The narrowness of the mass 39 peak also contrasts markedly with the much broader water-related emissions. Since these emissions are not due to the aqueous component of the rock, solid state processes must be involved.

Similar emissions have been observed from a number of materials which lack fluid components. Bursts of alka- li and alkali-containing molecules have been observed following the fracture of single crystal LiF and NaC1 (Dickinson et al. 1991a). These emissions were shown to be associated with dislocations produced during frac- ture and annihilation of dislocations following fracture. Dislocations are often generated during crack growth, either by emission from the crack tip or by the activation of near-surface dislocation sources. In the stress field of the crack tip, they can be driven some microns into the bulk material. After fracture, these dislocations expe- rience an attractive force back to the surface. If they are not pinned by crystal defects, this force can draw them back to the surface, imparting substantial energy to these dislocations in the process. Simulations of this process in alkali halides suggest that the time required is consistent with the observed delay of the emissions

457

from fracture, and that the process of emergence is suffi- ciently energetic to account for substantial neutral emis- sion. Similar emission spikes accompany the fracture of single crystal Ge, suggesting that a similar process oper- ates in more brittle materials (Dickinson et al. 1991 b). The fracture of LiF and NaC1 generally yields multiple bursts more delayed from fracture than the K bursts observed from feldspar, but a similar process may be responsible for the Na spikes.

Alkali emission has also been observed from sodium silicate glasses (Langford et al. 1991). In addition to bursts similar to the alkali halides, these glasses show a less intense, slowly varying, " con t i nuum" component of emission. The continuum component typically peaks 4~6 ms after fracture, somewhat later than the K peaks observed from feldspar. The decay of the emission is also slower, some tens of milliseconds. This continuum emission is accounted for by Na § diffusion to the sur- face, followed by neutralization and thermal desorption. Na § diffusion is driven by the effective volume of the Na § ion, which is somewhat larger for the free ion than inside the silicate lattice. Thus, a tendency exists for the Na § to move out to the free surface. Charge compensa- tion for Na + diffusion by the counter-diffusion of trapped holes produced during fracture may also be im- portant. The first Na § to arrive at the surface tends to fill defect sites; this Na § is tightly bound and is not available for emission processes. Eventually excess Na § accumulates at less favorable sites, where it can be neu- tralized by trapped electrons and subsequently emitted by thermal processes. The delay between fracture and the Na emission peak is accounted for by the time re- quired for the Na § to fill the surface defect sites. The similarity between the chemistry of the sodium silicate glasses and feldspar suggests that a similar process may be responsible for the atomic K emission observed here. Measurements of K emission from K-containing glasses and minerals would obviously be valuable for compari- son purposes.

The multiple bursts observed in the mass 23 (Na) signal from feldspar is suggestive of the Na bursts ob- served from sodium silicate glasses and the alkali halides. Bursts were observed at least 80 ms after fracture, in contrast with the single mass 39 bursts which typically peaked within 1 ms of fracture. Since most of the Na in this feldspar is concentrated in the albite phase, it is reasonable to suppose that Na emissions involve the albite phase. The difference between the Na and K emis- sions from the two phases is probably due to the con- trasting ionic radii of the cations, which affects the affin- ity of the ions for surface sites as well as the relative diffusivities of the ions through the mineral structures.

Assuming that the signal at mass 39 is entirely due to atomic K, an order of magnitude estimate of amount of emitted K can be made from the area of the peak (the detected charge). Since K atoms adhere well to met- al surfaces (i.e., vacuum chamber walls), it is safe to assume that they pass only once through the quadrupole ionizer. Then the number of emitted K atoms is simply related to the total charge detected by the electrometer output with the mass filter tuned to mass 39, QK (about

2 nC). The output of the quadrupole electrometer may be calibrated by introducing a known partial pressure of a gas whose molecular weight is similar to that of K. Measurements indicate that the sensitivity of the quadrupole/electrometer system to nitrogen gas, SN2, is 0.3 A/torr or 6 x 10-18 A/molecule (i.e. the electrometer output is 6 x 10 -18 Amps for each nitrogen molecule in the quadrupole ionizer). A burst of molecules passing through the ionizer will yield current only for the time

they remain in the ionizer, -c is simply the length of the path through the quadrupole ionizer divided by the molecular velocity. In the case of thermal emission, the average molecular velocity is given by vavg= (~z kT/ 2 rn) l/a, where T is the temperature, m is the molecular mass, and k is Boltzmann's constant. Depending on the distance between the ionizer and the sample, the ionizer will intercept only a fraction of the total emitted mole- cules, t/geo m. The number of emitted molecules, N, is giv- en by:

g Q Sztlgeo,," (1)

With QK~2 nC, S ~ 6 • -18 A/molecule, ~ 3 4 gs (from the average thermal velocity at 300 K), and t/geo,, ~ 7 x 10 -2, the number of emitted K atoms, N, is esti- mated to be 1.4 x 1014. This is a significant amount of K. Assuming a nominal fracture surface area (0.24 cm 2) equal to twice the sample cross section, this amounts to 6 x 1014 atoms/cm a of fracture surface area, equiva- lent to all the K in a 1 nm surface layer with bulk stoichi- ometry. Breaking one cubic meter of this material into 1 mm cubes would thus yield about 0.06 moles of K atoms. Since many of the particles leaving the fracture surface never escape the crack, this is a conservative lower limit.

Water is much more volatile and readily escapes from cracks and voids exposed to vacuum. This creates addi- tional uncertainty in the estimate of the total emitted water, as the evaporation of water from the vacuum system walls allows some to pass through the ionizer more than once. Evaporative cooling of water also re- duces the velocity of subsequently emitted vapor and increases the average time ~ required for water molecules to pass through the ionizer. Both effects lead to overesti- mates of the total amount of emitted water. Although these effects are difficult to treat quantitatively, they are not likely to affect the results by more than a factor of four. A smaller but quantifiable effect is the higher sensitivity of the quadrupole at lower masses, which amounts to a factor of 1.5 higher sensitivity at m a s s 18 relative to mass 39. Applying (1) to estimate the amount of emitted water (Qn2o ~ 500 nC, ~ ~ 19 gs) and discounting by a factor of 1.5 yields 4 x 1016 molecules, or about 1.7 x 1017 mol/cm 2 of nominal fracture surface area. A cubic meter of one millimeter cubes would thus release about 0.3 liters of water into vacuum.

A rough estimate of the molarity of K in the occluded water may be made from the relative intensities of the water and K O H signals at fracture. Since K O H does not readily evaporate, it is not appropriate to compare

458

the total KOH emission with the total water emission. The water signal shows a peak roughly comparable to the KOH peak which probably accounts for most of the water associated with the observed KOH. We pro- pose that both peaks are due to the physical ejection of droplets from inclusions during effervescence. There- fore, the background subtracted water peak is compared with the KOH peak. In neglecting the long tail in the water emission, most of the uncertainties involving cool- ing from repeated evaporation and vacuum system pure- pout are also avoided. The precise portion of the water peak to be included is a matter of judgement, so we have outlined the portion employed in this estimate in Fig. 4. This yields QH2O~ 14 nC and QKoH~470 pC (z = 34 gs). Converting to moles yields a 1.8 M solution. This is a reasonable molarity for a solution in equilibrium with a potassium-bearing feldspar.

Thus two sources of K may be distinguished: (a) oc- cluded fluids, and (b) emission from the crystalline frac- ture surface. The initial concentration of K in the oc- cluded fluids is on the order of 1.8 M. Under geologic conditions, fluid released by fracture would remain asso- ciated with the rock, where it would absorb much of the K "emitted" from fracture surfaces. The fracture event of Figs. 2 and 3 yielded about 6 x 1014 K atoms/ cm 2 and 1.7 x 10 iv H20 mol/cm 2. In these quantities, the emitted K would increase the K activity of the water by 0.2 M. In the case of finely pulverized rock, the ratio between the total fracture surface area and the total re- leased water could be much higher than in the simple fracture geometry employed here. Thus, greater in- creases in the K-activity of rock-dominated fluids are highly probable.

Similar estimates of the total Na emission can be made from the electrometer output at mass 23, QN, ~ 200 pC. This corresponds to about 2 x l013 Na atoms, or about 15% of the observed K emission. The Na-rich albite phase comprises about 10% of this feldspar, so that the total Na emission relative to the overall compo- sition of the sample is comparable to the total K emis- sion. Although there is no a priori reason to expect a simple relation between the amount of emitted Na and K, the relative size of the two signals is reassuring.

Geological Implications and Conclusion

Where the fluid/rock ratio is low the effect of deforma- tion on mineral solubility may be important (Knipe and Wintsch 1985). Two deformation processes that affect mineral solubility have been proposed: (1) high internal strain energies may increase solubility by production of high dislocation densities and (2) generation of new sur- faces by fracturing may promote ion-exchange reactions which affect cation activities in solution (Knipe and Wintsch 1985). The present investigation suggests an- other process: (3) cation activities in solution may in- crease due to fracto-emission of alkali atoms.

Fluid/rock interactions in shear zones are often thought to involve large volumes of aqueous fluids de-

rived from underlying rocks (e.g. Sinha et al. 1986). Such fluids are capable of participating in metasomatic ex- change (Fletcher and Hoffman 1974) provided that large volumes of fluids are available (Etheridge et al. 1984). However, where rock-dominated fluids are present, au- tometasomatism may occur due to reactions driven by intergranular fluids generated by one or more of the above processes.

An example of low temperature microbrecciation and associated mineral reactions involving rock-dominated fluids has been reported by Christoffersen and Wintsch (1988). Intergranular fluids were initially highly alkaline but became less alkaline as the zone developed. In the early stages of brecciation, when fluid/rock ratios were low, alkaline fluids partially dissolved quartz and feld- spar and precipitated a matrix of fine-grained K-feld- spar. Later matrix minerals suggest a fluid-dominated (less alkaline) system. The initial alkalinity of the inter- granular fluids may be attributed to any or all of the three processes, but ion exchange reactions following brecciation would not account for the subsequent pre- cipitation of K-feldspar. Inasmuch as deformation in the breccia zone was predominantly brittle (Cristoffer- son and Wintsch 1988), fracto-emission of alkali atoms during brecciation could have played an important role.

In summary, during the fracture of minerals, a number of volatile products (e.g. KOH, H 2 0 ) can be observed, most of which originate in fluid inclusions that are exposed by the moving crack. However, a significant number of neutral atoms (e.g. K and Na) are also pro- duced by fracture induced emission. This study shows that alkali is released from fractured feldspar by rapid evaporation of solutions present in fluid inclusions and by a solid-state diffusion mechanism accompanying cleavage fracture of the crystalline material. An under- standing of the origin and chemical composition of solu- tions generated in closed systems where fracture is an important process thus requires the consideration of both mechanisms. K atoms emitted by fracto-emission, at least 6 x 1014 cm -2, are sufficient to affect K activities in solution during microbrecciation in the presence of rock-dominated fluids. Additional release of volatiles may also occur long after fracture due to relaxations which may be stimulated by an increase in temperature (Dickinson et al. 1991 c).

Acknowledgments. This work was supported by the Ceramics and Electronics Materials Division of the National Science Foundation under Grant DMR 8912179 and the Washington Technology Center. The samples were prepared by Miluska Hill, Geology De- partment, Washington State University.

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