Rock Analog Experiments and notes by Dr. Win Means: 8 - Cracking, Dilatancy, and Twinning in NaNO3

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Cracking, Dilatancy, and Twinning in NaNO₃.
Click image to enlarge (~1MB)

Sodium nitrate has the calcite structure and calcite's familiar cleavage and twinning systems. The photos were taken in plain light. The deformation was a horizontal, sinistral shearing. It was carried out at room temperature, which is about 50% of the absolute melting temperature of the material. The apparatus was a hand-operated one (illustrated in Figure 5c of the review article) and the total deformation time was of the order of a few minutes. At this rate of deformation and temperature, the grain boundaries in NaNO₃ are totally immobile. So grain boundary features, such as triple junctions, can be used to determine the relative displacement of the top and bottom edges of the field of grains photographed.

42. Coarse-grained material occupies most of the field of view, with a finer region near the top. A very small sinistral deformation has already been applied. Its microstructural effects include dilation of grain boundary cracks at (45,55) and (16,52), and horizontal cleavage cracking at (40,43) and (50,33). There are also left-dipping cleavage cracks at (46,35) and (25,25).

43. The left-dipping cleavage crack now at (54,38) has dilated and become connected, through a dark transform feature at (62,32), to an extensive grain boundary fissure that extends nearly to the upper right corner of the picture. The grain boundary at (27, 52), which was dilated in the last picture, has nearly closed again. This is commonly observed. Cracks dilate and later close.

The large grain at (20,20) has begun to twin. The twin lamella boundaries are the brown lines dipping to the right.

44. Twinning is more intense in the grain at (20,20) and now shows up well in three other grains. Can you find them? See how the grain boundary fissure at (65,32) has dilated some more northeast of this point, but has opened less southwest of (65,32). The transform structure that makes this dilation discontinuity possible is the broad twin lamella at (55,30).

45. The fissure at (75,24) is broader still, but its former continuation at (65,35) is now nearly closed.
A grain boundary fissure at (30,32), just beginning to open in the last picture, has now become quite wide, and longer. This dilation is probably linked geometrically to the ongoing twinning in the grain northwest of the fissure.

46. A transgranular crack has appeared at (30,15).

47. The crack just observed has now dilated, as has a nearby grain boundary at (20,24).

Meanwhile the old cleavage fissure at (80,37) is nearly completely closed. If one had just this single view of the grain, as in a thin section of a naturally deformed rock, one would probably think that this was a transgranular crack with incipient opening. In fact it is in the terminal stages of closing. Be careful when interpreting thin sections! (You also have to be careful when interpreting time-sequence pictures like these. For example, we just said that the grain boundary at (20,24) has dilated. This might easily be taken to mean that the boundary is dilatING. In fact, this boundary may be CLOSING currently. It is more open now than in the previous picture, but in between these two pictures it might have been even more dilated than it is here. Any finite number of photographs like these has this kind of "blind-spot", this inherent inability to reveal unambiguously the direction or sense of current structural change. I didn't realize this myself until E.B. Watson (Rensselaer Polytechnic Institute) pointed it out to me.)

This set of pictures has shown an intimate view of some gouge in-the-making. Would you call the behavior brittle or ductile? (ans)

First Previous Section 8 Next Last

Cracking, Dilatancy, and Twinning in NaNO₃.	Click image to enlarge (~1MB)
Sodium nitrate has the calcite structure and calcite's familiar cleavage and twinning systems. The photos were taken in plain light. The deformation was a horizontal, sinistral shearing. It was carried out at room temperature, which is about 50% of the absolute melting temperature of the material. The apparatus was a hand-operated one (illustrated in Figure 5c of the review article) and the total deformation time was of the order of a few minutes. At this rate of deformation and temperature, the grain boundaries in NaNO₃ are totally immobile. So grain boundary features, such as triple junctions, can be used to determine the relative displacement of the top and bottom edges of the field of grains photographed.
42. Coarse-grained material occupies most of the field of view, with a finer region near the top. A very small sinistral deformation has already been applied. Its microstructural effects include dilation of grain boundary cracks at (45,55) and (16,52), and horizontal cleavage cracking at (40,43) and (50,33). There are also left-dipping cleavage cracks at (46,35) and (25,25).
43. The left-dipping cleavage crack now at (54,38) has dilated and become connected, through a dark transform feature at (62,32), to an extensive grain boundary fissure that extends nearly to the upper right corner of the picture. The grain boundary at (27, 52), which was dilated in the last picture, has nearly closed again. This is commonly observed. Cracks dilate and later close. The large grain at (20,20) has begun to twin. The twin lamella boundaries are the brown lines dipping to the right.
44. Twinning is more intense in the grain at (20,20) and now shows up well in three other grains. Can you find them? See how the grain boundary fissure at (65,32) has dilated some more northeast of this point, but has opened less southwest of (65,32). The transform structure that makes this dilation discontinuity possible is the broad twin lamella at (55,30).
45. The fissure at (75,24) is broader still, but its former continuation at (65,35) is now nearly closed. A grain boundary fissure at (30,32), just beginning to open in the last picture, has now become quite wide, and longer. This dilation is probably linked geometrically to the ongoing twinning in the grain northwest of the fissure.
46. A transgranular crack has appeared at (30,15).
47. The crack just observed has now dilated, as has a nearby grain boundary at (20,24). Meanwhile the old cleavage fissure at (80,37) is nearly completely closed. If one had just this single view of the grain, as in a thin section of a naturally deformed rock, one would probably think that this was a transgranular crack with incipient opening. In fact it is in the terminal stages of closing. Be careful when interpreting thin sections! (You also have to be careful when interpreting time-sequence pictures like these. For example, we just said that the grain boundary at (20,24) has dilated. This might easily be taken to mean that the boundary is dilatING. In fact, this boundary may be CLOSING currently. It is more open now than in the previous picture, but in between these two pictures it might have been even more dilated than it is here. Any finite number of photographs like these has this kind of "blind-spot", this inherent inability to reveal unambiguously the direction or sense of current structural change. I didn't realize this myself until E.B. Watson (Rensselaer Polytechnic Institute) pointed it out to me.) This set of pictures has shown an intimate view of some gouge in-the-making. Would you call the behavior brittle or ductile? (ans)
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