Crystallinity (crystallinity)
Crystallinity refers to the ratio of crystalline part (mineral crystal) to amorphous part (volcanic glass) in rocks.
1. fully crystalline structure
The rock is completely composed of crystalline minerals (Figure 3-2a) and contains no glass. The fully crystalline structure shows that the rock was formed in a slowly cooled magma system, which gave the crystal enough time to grow, which is usually possessed by intrusive rocks (such as granite).
2. Semi-crystal structure (sub-crystal/semi-crystal structure)
There are both crystalline minerals and glassy rocks (Figure 3-2b), and the semi-crystalline structure is found in volcanic lava and some shallow rocks.
Figure 3-2 Holomorphic and Semicrystalline Structures (According to Williams et al., 1982)
3. Glass texture (transparent texture)
Almost all rocks are composed of amorphous volcanic glass, also known as all-glass structure. Glassy structures are found at the edges of volcanic lava and some hypabyssal and ultrahypabyssal intrusive rocks (condensation edges, etc.). ), such as obsidian.
On the hand specimen, the vitreous body is glassy and conchoidal fracture, usually showing different colors, such as black, brick red, brown, grayish green and so on. The thin slice is isotropic, without cleavage or pearl cleavage, with low negative convex to positive convex, and the refractive index depends on the composition of volcanic glass or rock. With the increase of SiO2 _ 2 content, the refractive index of volcanic glass decreases. Therefore, according to the refractive index of volcanic glass, the composition range of rocks can be roughly judged. Glassy is formed by the rapid cooling of magma. Because the arrangement of atoms is completely disordered and has high free energy, vitreous is a very unstable solid substance. With the increase of geological age, vitreous will gradually transform into stable crystals, a process called devitrification. Generally speaking, it is difficult to see vitreous in ancient volcanic rocks before Mesozoic. Most of the glassiness in Mesozoic volcanic rocks has crystallized, and only the glassiness in Cenozoic volcanic rocks is relatively well preserved.
Depletion is a very slow process. In the initial stage of devitrification, some very fine crystallites will grow. Immature crystals are crystal buds that begin to crystallize, often in the form of hairs, rods or spheres, which have not yet shown the characteristics of crystal substances and have no light reaction under orthogonal polarization. According to the shape of microcrystal, it can be divided into:
◎ Spheroidal crystals: very fine spherical microcrystals.
◎ Sphere crystal: tightly assembled spherical microcrystals.
◎ nacre: spherical microcrystals arranged in chains or beads.
◎ Terry: A terry that bends like hair.
Microcrystalline can be the product of initial devitrification or the product of rapid cooling of magma. If the rock is mainly composed of microcrystals, its structure is called crystal structure (Figure 3-3a). Microcrystalline structure mainly exists in acidic volcanic rocks, such as perlite, turpentine and obsidian.
Embryonic crystals can further grow into skeleton shape, but have not yet formed complete crystals, which are called bone crystals. Rapid cooling of magma can also directly form skeleton crystals. After the bone is crystallized, it grows into a microcrystal. Microcrystals have the properties of crystalline substances, and have light reaction under orthogonal polarization, but they are still unable to be identified. The microcrystal structure composed only of quartz and feldspar is called felsic structure or felsic structure. Microcrystals with the same fiber shape that grow radially from a center are called spheres (Figure 3-3b). Cross extinction often occurs when spherical particles rotate on the animal table under orthogonal polarization. The composition of the pellet can be composed of alkali feldspar, and the gaps between the fibers are filled with glass, or it can be composed of alkali feldspar and Yingshi according to the knot ratio. The rock structure mainly composed of spherulites is called spherulite structure. Spherulite structure is very common in acid lava. When the composition of spherulite is composed of plagioclase and augite, it is called spherulite, and its internal structure is usually thicker than spherulite. Rock structures with spherulite characteristics are called spherical structures. This structure mostly occurs in basic volcanic rocks (basalt), cast stones and meteorites.
Figure 3-3 Structure of Crystals and Pellets (According to Sun Nai and Peng, 1985)
It is worth noting that there are two kinds of fully crystalline structures, namely, obvious crystalline structure and hidden crystalline structure. The mineral crystals that can be distinguished by naked eyes or magnifying glasses (mineral particle size is generally above 1mm) are called crystal structures, which belong to intrusive rock structures; Only under the microscope can mineral particles (generally less than 0.2mm) be distinguished, which is called cryptocrystalline texture and is generally owned by hypabyssal rocks and extrusive rocks (such as felsic rocks). On hand specimens, it is sometimes difficult to distinguish between aphanitic texture and glassy structure, but aphanitic texture does not have the glass luster and conchoidal fracture of glassy structure, and often shows porcelain fracture.
(2) Particle size (particle size or particle size)
Particle size refers to the size of main rock-forming mineral particles in rocks. There are many definitions of particle size, including three-dimensional particle size and projected particle size on the plane. Projected particle size refers to the average particle size on the plane. Due to the anisotropy of particles and the difference of particle size in different directions, there may be obvious differences in particle size and distribution characteristics observed in different sections. Therefore, when observing and grinding thin slices in hand specimens, we should pay attention to the cross-sectional differences in many directions to make the observed surface or ground thin slices representative. In grain size statistics, emphasis should be placed on the main rock-forming minerals. At present, great progress has been made in the quantitative study of igneous rock structure, but the crystal size distribution is the most convenient for quantitative study (Higgins, 2006). The quantitative study of crystal size distribution can provide information about the thermal history of magma system and the mechanism of nucleation and crystal growth.
According to the relative size of mineral particles, it can be divided into two types: equal particle structure and unequal particle structure. Then it is further divided according to the absolute size of particles.
1. Isocrystalline texture
The size of main mineral particles in pure rocks is roughly the same.
According to the particle size of mineral particles, the crystal structure can be further divided into:
◎ Fine grain structure: d = 0.2 ~ 2mm
◎ Medium grain structure: d = 2 ~ 5mm；;
◎ coarse grain structure: d = 5 ~ 25mm.
◎ pegmatite structure): d >25mm.
Cryptocrystalline texture can be further divided into:
◎ microcrystal texture: d < 0.2mm；; This kind of crystal can only be seen under a microscope.
◎ Cryptomeric texture: The crystal is too small to distinguish the grain boundaries under the microscope. This structure is developed in volcanic rocks and rocks affected by strong impermeability.
Figure 3-4 summarizes the corresponding relationship between rock crystallinity, grain size and rock structure.
Figure 3-4 Crystallinity, Particle Size and Igneous Rock Structure (modified according to Raymond, 1995)
2. Unequal grain structure
The sizes of several main mineral particles in rocks are different. If there are particles with different particle sizes, a continuous particle size series will be formed, which is called continuous uneven texture.
3. Speckle structure and Speckle structure.
Mineral particles in rocks are divided into two groups with obviously different sizes, the large one is called porphyrite and the small one is called matrix. Therefore, this structure is actually a bimodal granular structure. According to the customary usage in our country, if the matrix is composed of aphanitic and vitreous, it is called porphyritic structure; If the matrix is crystalline, it is called a porphyritic structure (Figure 3-5). The word porphyritic structure is rarely used in European and American countries. In their textbooks, no matter how crystalline the matrix is, all mineral particles can be divided into two groups, which are collectively called porphyritic structures.
According to China's customary usage, porphyritic structures are common in shallow rocks and extrusive rocks. The formation of porphyritic structure is related to the change of physical and chemical conditions during rock crystallization. The phenocrysts and matrix were formed in different generations. Porphyry crystals generally crystallize in the deep or magma rising process, while the matrix is formed by the consolidation and rapid crystallization of magma on or near the surface. When the phenocrysts generated in the deep underground reach the surface or near the surface, the minerals become unstable due to the obvious changes in physical and chemical conditions. The melting point of minerals will decrease with the decrease of pressure, and the latent heat released by crystallization under the condition of surface oxidation will corrode the crystallized minerals, thus producing dissolved structures at the edge or inside of phenocrysts.
Figure 3-5 Speckle Structure (According to Williams et al., 1982)
For porphyritic minerals (amphibole, biotite, etc. ) contains volatile substances. Due to low pressure, high temperature, oxidation, dehydration and other reasons, opaque edges often appear at the edges of porphyritic crystals, which is called opaque edge structure. For example, the formation of dark edges of biotite and amphibole can be caused by the following reactions (Qiu Jiaxiang, 1985):
Petrology (Second Edition)
Figure 3-6 Speckle Structure (/openpedia/ Speckle)
The dark edge is composed of extremely fine magnetite and high-temperature anhydrous aggregates, such as sanidine, leucite, olivine and pyroxene. The existence of dark edges indicates that volatile minerals such as amphibole and biotite are unstable under low surface pressure. Therefore, volatile minerals generally do not appear in the matrix of volcanic rocks. If dark amphibole and biotite microcrystals appear in the matrix, they are generally intrusive hypabyssal rocks.
When the mineral crystal changes from high temperature to low temperature, its volume will shrink. For example, when β-stress becomes α-stress at atmospheric pressure, it will shrink by 2. 1% along the A axis and 1.3% along the C axis. This uneven shrinkage occurs rapidly in lava and subvolcanic rocks, which often leads to cracks in phenocrysts. Under the condition of sub-volcanism, volatiles expand and release from higher pressure to lower pressure, but they can't freely escape from the surface, so eddy currents are generated, which further split the broken phenocrysts during rolling, but they are not discrete, forming a broken phenocryst structure. This is a common structure of acidic subvolcanic rocks.
Because the matrix of porphyritic structure is crystalline (Figure 3-6), it belongs to intrusive rock structure. The difference between the porphyritic structure and the porphyritic structure is that the porphyritic crystals and the matrix in the porphyritic structure are the products of different generations, except for the different crystallinity of the matrix. The porphyritic structure, porphyritic crystal and matrix are basically the products of the same generation, but the order of crystallization is different. The evidence that phenocrysts and matrix in porphyritic structures belong to the same generation is that they are close or consistent in structural state and composition, and there is no melting phenomenon or blackening edge at the edge of phenocrysts. Because the phenocrysts grow with the minerals in the matrix at the same time, the particles of the matrix are often embedded in the phenocrysts from the edge, and the phenocrysts may not have flat crystal faces. If there is no significant difference in size between porphyritic crystals and matrix minerals, they will transition to a continuous unequal grain structure.
(3) particle shape
The morphology of mineral particles is determined by crystallization habit, growth environment, erosion and deformation after crystallization. In thin slices, the outline of crystals is mainly reflected by the integrity of mineral crystal faces, which can be roughly divided into three types: self-shape, semi-self-shape and special-shape. Automorphic crystals have complete crystal faces and regular morphology, which are generally the products of early magmatic crystallization. Semi-authigenic crystals only have some fully developed crystal faces and some incompletely developed crystal faces. The shape of the heteromorphic crystal is irregular, so a complete crystal face cannot be found. The gap between crystalline minerals in the early stage of magmatic crystallization is often used for growth, and it can also be the product of melting or transformation of relatively early crystallized automorphic crystals. For example, the rock-forming minerals in intrusive rocks, such as feldspar and syenite, are often semi-automorphic to heteromorphic, while the accessory minerals, such as zircon, apatite and sphene, are often automorphic. Therefore, the structure of intrusive rocks can be described according to the self-shape degree of most particles that make up minerals (Figure 3-7).
Figure 3-7 Structure related to the degree of automorphism (according to Williams et al., 1982)
1. Automorphism granular structure
If most minerals are authigenic crystals, the structure of rocks is called authigenic granular structure. Automorphic granular structure is rare in natural rocks. Dunite mentioned in some textbooks is "isomorphic mosaic structure", but it should be called mosaic structure. The rocks with this structure are composed of polygonal crystals and belong to metamorphic rock structure.
2. Semi-authigenic granular structure.
The rock structure mainly composed of semi-automorphic crystals is called semi-automorphic granular structure. Another understanding of semi-automorphism granular structure is that some crystals have a high degree of automorphism and some crystals have a low degree of automorphism. In granite, dark minerals are usually authigenic crystals, feldspar is semi-authigenic, and it is irregular in time, which has a typical semi-authigenic granular structure, so it is also called granite structure.
3. Irregular granular structure
Rocks mainly composed of heteromorphic crystals have heteromorphic granular structures. Fine-grained rocks usually have a special-shaped grain structure composed of special-shaped feldspar and quartz, so this structure is also called fine-grained structure.
(4) grain orientation
The orientation of particles refers to the orientation strength of mineral particles that constitute rocks. In magmatic rocks, the orientation of particles mainly reflects the flow direction and flow mechanism of lava flow and rock wall (Nicolas,1992; Simith, 2002), crystal sedimentation and emplacement mechanism of magma. Partial directional structure is the product of structural deformation (solid flow) after magma consolidation, so it is of great significance to determine the orientation of particles for volcanology and magmatic dynamics research. Johannnsen( 1939) described more than ten structural terms related to flow in igneous rocks. Flow texture, layered texture, linear parallel texture, plane parallel texture, linear spot texture, staggered structure, etc. However, some terms have been classified as structural description, and some have obvious genetic significance and are not suitable for descriptive terms. Structures related to structural deformation already belong to the category of metamorphic structures and will be discussed in metamorphic rocks. At present, typical igneous structures related to grain orientation are introduced as follows:
1. Rough texture
The nearly parallel directional arrangement of feldspar microcrystals is called rough surface structure (Figure 3-8a). It should be noted that some textbooks, especially domestic textbooks, define the rough surface structure as the directional arrangement of potassium feldspar microcrystals. This structure reflects the flow and compaction of magma during crystallization. Because only lava with more microcrystals flows, it is easy to show the directionality of microcrystals, so there is a greater chance of coarse texture in trachyte.
2. Tubular structure
Refers to the structure in which the crystals except feldspar in the extrusive rock or any minerals in the intrusive rock are arranged almost in parallel (Johannsen,1939; Phillpotts, 1989).
3. Pilot structure
Internationally, it generally refers to the structure with disordered microcrystal distribution, no obvious orientation or only weak orientation (Figure 3-8b), which often appears in andesite. If there are glassy substances besides feldspar microcrystals, the structure of this matrix is called glassy structure (Figure 3-8c).
Figure 3-8 Rough surface structure, interwoven structure and glass-crystal interwoven structure (according to Williams et al., 1982)
The study of magma dynamics shows that the grain orientation of magma origin appears in the early stage of magma crystallization, that is, when the solid crystal content is less than 70%. At this time, the magma is close to the Newtonian fluid property, and the rigid minerals crystallized in the early stage will rotate and be optimally arranged due to the movement of the magma (Ma Changqian et al., 1994). However, when the crystal content in magma is more than 70%, it has yield strength, shows the rheological behavior of Bingham, and can undergo plastic deformation. Therefore, the directional fabric of magmatic origin has no plastic deformation, and has obvious characteristics such as wave attenuation, fragmentation and knee fracture. The distribution of directional fabric is often related to the position of the sample in the lava flow or rock wall at that time (Figure 3-9). With the flow of magma, the original randomly distributed crystals will change their arrangement. Near the two walls of the rock wall, the crystal orientation is obvious, but the crystal direction in the center of the rock wall is still randomly distributed.
Figure 3-9 Flow of Magma in Rock Wall and Optimum Orientation of Crystals
(According to Higgins, 2006)
(5) mutual relationship.
Including the relationship between minerals and the relationship between mineral particles and glassy or aphanitic components. This kind of structure often records the sequence of mineral growth, or reflects the process of element diffusion, material adjustment, differentiation, reaction and balance between components.
1. symbiotic structure
These two minerals are intertwined and cross together regularly, which is called cross structure. According to the shape of the intersection of minerals, it can also be divided into:
◎ perthitictexture: characterized by the regular intersection of potash feldspar and albite. Feldspar with striped structure is called perthite (Figure 3- 10). The stripe structure has different scales, from X-ray to naked eye. The stripe structure can be divided into positive stripe structure, middle stripe structure and anti-stripe structure. In the regular stripe structure, the guest albite is distributed in the main crystal potash feldspar in stripes; The reverse stripe structure is opposite to the forward stripe structure; The contents of potash feldspar and albite in the middle stripe structure are similar. There are two reasons for stripe structure: solid solution dissolution and metasomatism. Decomposition fringes formed by solid solution dissolution (decomposition) are the most common. At high temperature, potash feldspar and albite are complete solid solutions with uniform composition. With the cooling of magma, the complete solid solution becomes unstable and dissolves, producing potash feldspar and albite, forming a banded structure. After magmatic stage, sodium metasomatic potash feldspar can also form banded structure. The metasomatic stripes are often fresh, mostly irregular dendritic and reticulate textures, with no obvious directionality, and are often distributed along cracks, cleavage and edges.
Figure 3- 10 Normal stripe structure (orthogonal polarization) The main crystal is potash feldspar and the guest crystal is albite.
◎ Wormlike texture: It is characterized by many tiny worm-like time (called vermiculite) interspersed with feldspar, and the extinction grade of time mosaic is consistent (Figure 3- 1 1). This structure is common in granite. Worm structure mainly includes * * * knot worm and metasomatism worm. * * * Arthropods are common in mineral contact. There are timely worms in potash feldspar, and there can also be timely worms in potash crystals, which are the products of timely and potash feldspar in the proportion of * * *. The metasomatic worm is that the early minerals are metasomatized by new minerals, and the remaining components precipitate into worms in the metasomatic process, appearing in the remaining parts of metasomatic minerals. Of course, the worm structure is not limited to two minerals: quartz and feldspar. For example, iron-containing worms can also appear in low-iron pyroxene.
◎ Graphic texture: It has certain shapes (such as sharp edges and hieroglyphs) and is regularly embedded in potash feldspar (Figure 3- 12). The timely inlay goes out simultaneously under orthogonal polarization. This structure is formed by magma whose composition is equivalent to that of feldspar and chronological binary system. When the temperature drops to * * * junction, feldspar will crystallize at the same time. What is visible to the naked eye is called graphic texture, and what is visible under the microscope is called microscopic graphic texture (Figure 3- 14a). In rocks with porphyritic structure, if the matrix has microscopic structure, it is called granite porphyritic structure. Graphic structures are common in pegmatite and some granites.
Figure 3- 1 1 Worm Structure
Figure 3- 12 graphic texture (orthogonal polarization) (according to Chang Lihua et al., 2009)
Step 2 Cover the texture
Refers to the phenomenon that another mineral is wrapped around a larger mineral core by the mantle. The mineral phase of the mantle can be single crystal or polycrystalline aggregate; It can grow on minerals with the same crystal structure (continuous cross, such as alkali feldspar and plagioclase) or on minerals with completely different structures (discontinuous cross, such as amphibole cross in time) (Figure 3- 13). In addition to the dark edge structure mentioned above, the reaction edge structure and the ring spot structure belong to the mantle structure.
Figure 3- 13 Some typical mantle structures (according to Hibbard, 1995)
◎ Corona structure (reaction edge structure): Early-generated minerals or xenoliths react with molten slurry. When the reaction is incomplete, another new mineral with completely different composition is formed around the minerals generated in the early stage, which completely or partially surrounds the minerals crystallized in the early stage. This structure is called reaction rim structure (Figure 3- 14c). Ordinary olivine has the reaction edge of enstatite, while clinopyroxene has the reaction edge of amphibole. Multilayer reaction edges may also appear, such as pyroxene reaction edge outside olivine and biotite reaction edge outside amphibole. It should be noted that the secondary edge structure is completely similar to the reaction edge structure, but the secondary edge is an "edge" produced by secondary minerals, such as iddingsite with secondary edge in olivine, tremolite and actinolite with secondary edge in spinel (Figure 3- 14b, c). The secondary variable edge is the secondary reaction edge, which is especially common in mafic and ultramafic rocks.
Fig. 3- 14 micropattern texture, reaction edge structure and secondary variable edge structure (according to Williams et al., 1982).
In addition, there are amphibole reaction edges other than clinopyroxene and mica reaction edges other than amphibole. This is because the minerals crystallized in the early stage are often anhydrous. With the progress of the reaction, the water content in the system gradually increases, resulting in amphibole, biotite and other water-bearing minerals, reflecting the increase of water fugacity in the system. The mineral on the reaction side is usually not a single crystal, but an aggregate of multiple crystals.
◎ Ring-porphyry structure: The alkaline phenocrysts in porphyry are mostly oval, with thin feldspar-mesofeldspar shells outside, and alkali feldspar and quartz in rocks are generally the products of two generations (Figure 3- 15).
If the interior is feldspar and the exterior is alkali feldspar, it is called anti-Rapakivi structure.
3. Partition texture
Fig. 3- 15 Photograph of the ring spot structure specimen in Wiborgite, Finland shows that the alkali feldspar egg spot is wrapped by feldspar, and the alkali feldspar egg spot intersects with biotite in the matrix in an image or worm shape.
It develops in some minerals of solid solution series, and it is distributed annularly from the center to the edge of the grain, but it shows different extinction positions. Many minerals can form banded structures, but plagioclase (especially intermediate feldspar) is the most common banded structure. The core of plagioclase is alkaline, and when it becomes acidic towards the edge, it is called a regular band structure. On the contrary, it is called anti-waist seal; Periodic and repeated changes in composition are called oscillation bands.
4. Inclusion structure (polycrystalline structure)
Many smaller mineral particles are embedded in larger mineral particles. This structure usually reflects that the encapsulated mineral crystallizes earlier than the mineral containing it. In olivine pyroxene, it is often seen that large pyroxene crystals contain many eroded small round olivine particles, which is called olivine inclusion structure at this time. In the larger pyroxene, there are many columnar plagioclase crystals with high self-shape, which are called (embedded) feldspar structure. Inclusion structure is very common in ultrabasic rocks and basic intrusive rocks.
5. Gap-filling structure
Dark minerals such as pyroxene, aphanitic and vitreous are filled in the intergranular space of microcrystalline plagioclase in the igneous matrix of shallow or extrusive phase. The fillings are all granular minerals, called intergranular structure, and the fillings are aphanitic-glassy, called interstitial structure. The transition type between them is called intergranular structure. Sometimes the interstitial structure is regarded as a structure in which non-granular minerals such as zeolite and chlorite are filled between plagioclase microcrystals, and the post-magmatic minerals such as zeolite and chlorite are probably the products of glassy devitrification.
It should be noted that the formation of igneous rocks goes through a long and complicated history of magma condensation and crystallization, and even has other functions (deformation, alteration, weathering, etc. ) will be superimposed after the rock is formed. Although all the above structures are collectively referred to as igneous structures, in fact, the structural characteristics and mineral assemblages of igneous rocks we see on thin slices are the final products of complex diagenetic processes and changes after diagenesis.