Quartz Quartz is a hard, crystalline mineral composed of silica ( silicon dioxide). The atoms are linked in a continuous framework of SiO4 silicon-oxygen tetrahedra, with each oxygen being shared between two tetrahedra, giving an overall chemical ...

is the most abundant single mineral in the earth's crust (although behind the

feldspar Feldspars are a group of rock-forming aluminium tectosilicate minerals, also containing other cations such as sodium, calcium, potassium, or barium. The most common members of the feldspar group are the ''plagioclase'' (sodium-calcium) felds ...

group when taken collectively), and as such is present in a very large proportion of rocks both as primary

crystal A crystal or crystalline solid is a solid material whose constituents (such as atoms, molecules, or ions) are arranged in a highly ordered microscopic structure, forming a crystal lattice that extends in all directions. In addition, macro ...

s and as detrital grains in

sedimentary Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

and

metamorphic Metamorphic rocks arise from the transformation of existing rock to new types of rock in a process called metamorphism. The original rock (protolith) is subjected to temperatures greater than and, often, elevated pressure of or more, causi ...

rocks.

Dynamic recrystallization Dynamic recrystallization (DRX) is a type of recrystallization process, found within the fields of metallurgy and geology. In dynamic recrystallization, as opposed to static recrystallization, the nucleation and growth of new grains occurs during ...

is a process of crystal regrowth under conditions of stress and elevated temperature, commonly applied in the fields of metallurgy and materials science. Dynamic quartz recrystallization happens in a relatively predictable way with relation to temperature, and given its abundance quartz recrystallization can be used to easily determine relative temperature profiles, for example in

orogenic belts Orogeny is a mountain building process. An orogeny is an event that takes place at a convergent plate margin when plate motion compresses the margin. An ''orogenic belt'' or ''orogen'' develops as the compressed plate crumples and is uplifted t ...

or near intrusions.

Mechanisms of recrystallization

Previous research has outlined several

dislocation creep Dislocation creep is a deformation mechanism in crystalline materials. Dislocation creep involves the movement of dislocations through the crystal lattice of the material, in contrast to diffusion creep, in which diffusion (of vacancies) is the do ...

regimes present in experimental conditions. Two main mechanisms for altering grain boundaries have been defined. The first is the process by which quartz softens as temperature increases, providing a means for internal stress reduction by migration of

dislocation In materials science, a dislocation or Taylor's dislocation is a linear crystallographic defect or irregularity within a crystal structure that contains an abrupt change in the arrangement of atoms. The movement of dislocations allow atoms to s ...

s in the crystal lattice, known as dislocation creep. These dislocations concentrate into walls, forming new grain boundaries. The other process involves differences in stored strain energy between neighboring grains, resulting in migration of existing grain boundaries. The extent to which these occur is a function of

strain rate In materials science, strain rate is the change in strain ( deformation) of a material with respect to time. The strain rate at some point within the material measures the rate at which the distances of adjacent parcels of the material change ...

and temperature, those being, respectively, the factors controlling introduction of new dislocations and the ability of dislocations to migrate and form subgrain boundaries which themselves migrate.

Recrystallization regimes

Observable

microstructure Microstructure is the very small scale structure of a material, defined as the structure of a prepared surface of material as revealed by an optical microscope above 25× magnification. The microstructure of a material (such as metals, polymers ...

s in quartz can be classified into three semi-distinct groupings that form a continuum of dynamic recrystallization textures. These regimes will be discussed in terms of temperature changes, assuming a constant level of

shear Shear may refer to: Textile production *Animal shearing, the collection of wool from various species **Sheep shearing *The removal of nap during wool cloth production Science and technology Engineering *Shear strength (soil), the shear strength ...

Bulging recrystallization

The lowest temperature texture (~250-400°C), bulging recrystallization (BLG) is characterized by bulges and small recrystallized grains along grain boundaries and, to some extent, microcracks. The at-large proportion and structure of the original quartz crystals is preserved to the greatest extent, compared with the other profiles. Formed by a combination of the two mechanisms mentioned, limited crystal plasticity (due to low temperature) prevents any further separation of subgrains. It follows, then, that an increase in temperature results in an increase in recrystallized grain size and volume proportion (0-25%) as internal stress becomes more resolved.

Subgrain rotation recrystallization

Following an increase in temperature, the dominant texture changes to one marked by the presence of distinct subgrains. Recognizable in thin section by a more polygonized texture, the increased softening of the quartz allows for more thorough reduction of internal stresses. Recrystallized grains show relatively straight grain boundaries and little to no intragranular deformation feature, such as

undulose extinction Undulose extinction or undulatory extinction is a geological term referring to the type of extinction that occurs in certain minerals when examined in thin section under cross polarized light. As the microscope stage is rotated, individual minera ...

or deformation lamellae. Volume proportion of recrystallized grains in this regime roughly ranges from 30-90%, forming subgrains not only in interstitial space, but also within larger crystals or ribbon grains. Subgrains and recrystallized grains are roughly equal in size and shape.

Grain boundary migration recrystallization

The highest temperature of the three textures, grain boundary migration becomes the dominant mechanism at ~500-550°C. Exhibiting much larger recrystallized grain sizes than the other two regimes, in addition to lobate and highly interfingering boundaries, at these temperatures quartz is completely recrystallized. That is, no evidence for original grains can be found. At these high temperatures, grain boundaries are free to sweep across entire grains, resulting in much less localized boundary formation/change. In this case as well, intragranular deformation features have been erased, but may be present from later-stage overprinting.

Trends

Aside from the obvious increase in temperature, there are other trends which arise in this progression of recrystallization.

Recrystallized volume proportion

As mentioned above, with increased temperature there is a marked increase in the proportion of the rock having undergone recrystallization. From 0-30% in bulging recrystallization, up to 90% in subgrain rotation recrystallization and 100% in grain boundary migration, this property may be observed in

quartzite Quartzite is a hard, non- foliated metamorphic rock which was originally pure quartz sandstone.Essentials of Geology, 3rd Edition, Stephen Marshak, p 182 Sandstone is converted into quartzite through heating and pressure usually related to tec ...

, at least well enough to get relative temperature relationships in the field.

Recrystallized grain size

Progressing from around 15 μm (bulging recrystallization) to about 85 μm (subgrain rotation recrystallization) to up to a few millimeters (grain boundary migration), this exponential increase is not only noticeable, but is part of the basis on which the three recrystallization regimes were demarcated.

Utility

Observation of recrystallization in a rock sample can reveal a general temperature, but nothing very precise. This is because the process of recrystallization is strongly affected by the presence of water and the amount of strain present. As such, this information can be applied to determine relative temperatures of different rock much more reliably than it can determine absolute temperatures. Furthermore, this is an analysis that can be done, if only preliminarily, in the field by observing rocks in hand sample.

Relevant Links

Subgrain rotation recrystallization In metallurgy, materials science and structural geology, subgrain rotation recrystallization is recognized as an important mechanism for dynamic recrystallisation. It involves the rotation of initially low-angle sub-grain boundaries until the mi ...

Neomorphism Neomorphism refers to the wet metamorphic process in which diagenetic alterations systematically transform minerals into either polymorphs or crystalline structures that are structurally identical to the rock(s) from which they developed. Coined ...

Pressure solution In structural geology and diagenesis, pressure solution or pressure dissolution is a deformation mechanism that involves the dissolution of minerals at grain-to-grain contacts into an aqueous pore fluid in areas of relatively high stress and e ...

References

{{Reflist Quartz varieties Metamorphic petrology