Ven Richthofen first proposed the term rhyolite in 1860. At that time, it was referred to as acidic lava with flowing belt structure.
Almost at the same time, Roth (1860) named the acidic lava produced on the Mediterranean Lipari Islands as liparite and later adopted it by some European continent petrologists.
In fact, the basic characteristics of these two rocks are almost identical, so the latter is actually synonymous with Rhyolite.
Rhyolite rocks refer to volcanic lava with composition equivalent to granite. The most striking feature of its appearance is that it generally has a streamlined structure.
The crystallinity of Rhyolite rock is low; most are semi-crystalline or glassy. It rarely appears as full crystal. Porphyry structure is common, and phenocrysts mainly consist of fused quartz and alkaline feldspar, and sometimes they are phenocryst free.
The mineral composition of Rhyolite rocks is dominated by aluminosilicate minerals, which are mainly potassium feldspar and acid plagioclase (usually An<30).
At the same time, quartz appears in large quantities, with the content generally exceeding 20-25%. However, there are few mafic minerals, mainly biotite and amphibole, and occasionally pyroxene occurs.
The color of Rhyolite rocks is generally light, usually light gray or light purple gray. The color index is low, often less than 10-15. Due to the uneven distribution of constituent minerals, rocks often have different colors.
The most obvious feature of the chemical composition of Rhyolite rocks is that the content of SiO2 is very high, generally more than 68%, and the content of Na2O+K2O is also high. But the content of calcium, magnesium and iron is very low.
Rhyolite can often be divided into two types: calc-alkaline and alkaline.
The former is mainly composed of K-feldspar and quartz, with a certain amount of acid plagioclase and a small amount of magnesium iron minerals such as biotite and amphibole.
The latter is dominated by sodium sanidine (or albite), anorthite and quartz, and mafic minerals generally contain high sodium content, so sodium-rich hornblende or pyroxene (i.e., sodium amphibole or augite, etc.) often appears.
Rhyolite is far less widely distributed than basalt and andesite. Compared with granite, Rhyolite has a much smaller distribution area than granite.
The most closely associated rock is dacite. Sometimes it is difficult to distinguish between them. They often show a gradual relationship. In addition, it is not difficult to find Rhyolite associated with trachyte or andesite.
Basic properties of Rhyolite
[Mineral composition] Rhyolite is grayish-white or light pink. The phenocrysts are usually quartz, alkali feldspar, and sometimes a small amount of plagioclase.
[Texture and structure] Rhyolite commonly has porphyritic texture, glassy texture, globular texture, and frost fine texture, and presents rhyolite structure.
[Genesis] Rhyolite is a volcanic eruption rock equivalent to granite, and its occurrence is mostly in rock mounds.
[Minerals and uses] The related minerals include kaolinite, montmorillonite, pyrophyllite, alum, pyrite, etc.
[Color] Light gray to medium gray, light pink
[Particle size] less than 0.1 mm
[Minor minerals] biotite, amphibole group, plagioclase, pyroxene
[Texture] It has stripes, occasionally with glass texture and holes.
Mineral composition of Rhyolite
Some common minerals in Rhyolite and their characteristics are as follows:
Alkaline feldspar (potassium feldspar) is one of the main components of Rhyolite. The content range is generally 40-70%. It can often see the following species:
Sanidine is colorless and transparent, like glass. It is mainly plate crystal. Most of them are single crystals; sometimes, simple (card type) twin crystals are developed.
The phenocrysts often appear very clear, especially in the relatively younger Rhyolite.
Orthoclase, often with a little light red or brown-yellow, can be seen from the microscopic observation that its section is not clear, and its crystal shape is similar to that of sanidine, but it is not difficult to distinguish it from sanidine by its color.
Of course, the most reliable method is to measure the optical axis angle (2V). The optical axis angle of orthoclase is usually more than 60 degrees, while that of sanidine is almost no more than 30 degrees.
Orthoclase generally occurs in Rhyolite that has undergone certain changes (or older ages) and is often transformed from sanidine.
Microperthite, which is far less common than sanidine or orthoclase. It is characterized by many albite micro-embedded crystals distributed on the base of orthoclase crystals. It can only be identified under a microscope, but separating it from orthoclase on hand specimens is difficult.
Anorthoclase generally occurs in alkaline Rhyolite and is often associated with alkaline-colored minerals. The crystal is colorless and transparent, similar to sanidine in appearance, but the optical axis angle (30-55 degrees) of Anorthoclase is larger than that of sanidine so that the two can be well distinguished.
The Anorthoclase is rich in Na2O (Na2O≥K2O), which is related to the sodium supersaturation of alkaline rhyolite magma, so it is unlikely to occur in normal (calc-alkaline) Rhyolite.
Microcline is rarely seen in relatively fresh Rhyolite; It is gradually transformed into sanidine or orthoclase.
It is also turbid, with a light reddish brown hue, similar to orthoclase.
However, microcline commonly has lattice twin crystals. In addition, for the largest microcline, the included angle between the Ng axis and the pole of (010) cleavage plane is 18 degrees, while the pole of (010) cleavage plane of the largest orthoclase coincides with Ng.
In general, orthoclase and microcline can be divided by (010)=9 ° on Ng, orthoclase is less than 9 °, and microcline is more than 9 °. The triclinic degree (>0.5) of microcline is greater than that of orthoclase (<0.5).
Alkaline feldspar (K-feldspar) not only forms phenocrysts in Rhyolite but also widely distributes in matrix.
In many areas, only plagioclase phenocrysts are found in Rhyolite, but no potassium feldspar phenocrysts are found. Because plagioclase is generally crystallized before potassium feldspar when the magma rises rapidly, some plagioclase has crystallized (forming phenocrysts) and potassium feldspar has not crystallized yet, the magma will erupt out of the ground.
2. Quartz is also one of the main components of Rhyolite, but its content is generally 20-40% lower than that of alkaline feldspar. The phenocrysts are mostly high-temperature quartz, and often have a molten shape, sometimes containing inclusions of glass or matrix.
The quartz in the matrix is generally a low-temperature variant, which may be tridymite or even cristobalite.
Sometimes the quartz and K-feldspar microcrystals in the matrix are mixed together, and sometimes they form a mixture and are produced in the form of pellets.
3. The content of plagioclase in Rhyolite is generally lower than that of potassium feldspar. It not only exists in the matrix but also appear as phenocrysts.
The banded structure can also be seen in plagioclase that forms phenocrysts. Its composition is often more basic than plagioclase in granite, often up to feldspar (An30-50).
In the Rhyolite rich in magnesium and iron minerals or rapidly cooled glassy Rhyolite, plagioclase phenocrysts sometimes appear more basic, ranging from feldspar to double feldspar. Plagioclase in the matrix is generally acidic, ranging from albite to trondhjemite.
Plagioclase is mainly plate crystal; twin crystal is common, sometimes containing inclusions such as glass or gas.
4. Biotite is a common magnesium iron mineral in Rhyolite. However, the content is generally low, rarely exceeding 10-15%. Almost all of them are brown, while those showing green are extremely rare.
5. It is mainly composed of phenocrysts and often has dark edges. Magnetite, apatite, zircon and other inclusions are often found in biotite scales.
6. Amphibole is generally less common than biotite. It is mainly brown columnar, mostly in the form of phenocrysts, and dark edges are often seen.
In alkaline Rhyolite, hornblende is rich in sodium, and there are often sodium amphibole and sodium iron amphibole.
7. Pyroxene is rarely seen in Rhyolite. It is generally light green clinopyroxene, such as iron-rich augite and diopside. They are mainly phenocrysts, rarely occurring in the matrix.
In some glassy rhyolites, some pyroxene crystals can also be seen sometimes. In alkaline Rhyolite, in addition to iron-rich augite and diopside, it is also common to see spodumene or aegirine, which sometimes form the shell of diopside.
7. Olivine, an occasional mineral (such as Arran Island), occurs in vitreous rhyolite and forms phenocrysts together with quartz and other minerals. Olivine is rich in iron, mainly iron olivine, which is often associated with small crystals of magnetite.
8. Laminated glass is very common. Colorless to light reddish brown, the refractive index is very low; generally, 1.48-1.50, and the specific gravity is about 2.30. There are often various forms of dimensional crystals.
9. There are many kinds of accessory minerals. The most common are magnetite, apatite, zircon, etc.; The common ones are stone, garnet, anatase, monazite, etc.
Type of Rhyolite By mineral composition
The mineral composition of Rhyolite is equivalent to that of normal granite. It is mainly composed of alkali feldspar, sanidine, orthoclase or a small amount of microcline feldspar, quartz and a certain amount of acid plagioclase, occasionally with a small amount of biotite or ordinary amphibole.
It can be of the following types according to mineral composition:
Dacite Rhyorite: Dacite rhyolite’s mineral composition is equivalent to monzogranite’s. The main mineral components are plagioclase, alkali feldspar (potassium feldspar) and quartz, followed by a small amount of amphibole or biotite.
The plagioclase that forms phenocrysts is generally feldspar or even higher grade, and the plagioclase in the matrix is generally feldspar.
The difference with common Rhyolite is that the content of plagioclase increases, while that of alkaline feldspar (K-feldspar) decreases, which accounts for about 1/3-2/3 of all feldspars.
The structure of Dacite Rhyorite: is basically the same as that of common Rhyolite.
Trachytic Rhyolite: The mineral composition of trachy liparite; is equivalent to that of K-feldspar granite. Therefore, it is also called potassium feldspar rhyolite.
The rock mainly consists of potassium feldspar (sanidine, microcline, orthoclase or anorthite) and quartz.
There are a small amount of magnesium iron minerals such as biotite and amphibole. The content of plagioclase is very low or even completely absent.
This is a transitional rock type from Rhyolite to quartz trachyte. The main difference between the two is that the former has a high quartz content (>20%), while the latter has a low quartz content (<20%).
Comendite: This rock is rich in alkali, with Na2O+K2O ≥8.5-10%, and Na2O is high, often close to or even more than K2O.
The mineral composition is equivalent to that of alkaline granite. The main composition is alkali feldspar (sanidine, microcline feldspar, orthoclase), quartz, and albite, and contains sodium-rich hornblende or pyroxene, such as sodium amphibole, almandine or foraging stone, and sometimes biotite.
Alkaline feldspar and quartz are often interspersed in mafic minerals. The rocks have porphyritic texture or non-porphyritic aphanitic texture.
There are many kinds of matrix structures, often including micro granite structures, micro inlaid structures, micro image structures and glassy structures.
Pantellerite： In terms of mineral composition, in addition to including the composition of Comendite, Pantellerite also contains a lot of plagioclase, and diopside, omphacite, almandine or Cossyrite are common, but these components are rare or even completely missing in Comendite.
In addition, the aegirine content of Pantellerite is much more than that in Comendite, but the content of quartz is relatively small.
In terms of structure, in addition to including the structure of Comendite, It also includes coarse texture-alkali feldspar (plagioclase) crystals arranged nearly in parallel.
Aegirine augite often grows around the edge of diopside, forming a diopside shell.
From the chemical composition, Pantellerite is richer in sodium than Comendite, and Na2O is greater than K2O almost in any case.
As the mafic mineral in Pantellerite has a high level, the matrix color is generally dark, often green or green black, while the matrix color of Comendite is relatively light, often gray or yellowish gray to blue-gray. Therefore, the SiO2 content of the two rocks is different, and the SiO2 content in Pantellerite is often lower than that in Comendite.
Quartz Keratophyre: The term quartz Keratophyre was first proposed by Rosenbusch in 1887. At that time, it meant those rocks with porphyritic structure, the matrix was mostly aphanitic, white, light gray or light green, and the phenocrysts were mainly albite, quartz, microcline feldspar or plagioclase and a small amount of biotite.
Later, people found that the definition of Rosen giving was not completely accurate. Through repeated practice, the characteristics of quartz Keratophyre are as follows:
In appearance, the rocks are relatively dense, generally light in color, usually gray white or light gray mineral composition, mainly albite (or Ordovician albite) and quartz, followed by a small amount of potassium feldspar. There are few mafic minerals, mainly chlorite and braided mica, sometimes biotite;
In structure, it is generally full crystalline, and volcanic glass is rare.
There are two types: one with porphyritic structure, the phenocrysts are mainly albite (anorthite) and quartz, and the matrix has micro granite structure, micro inlaid structure, fine structure, etc.
The other is no phenocryst structure. Quartz phenocrysts cannot be found at this time, so it should not be mistaken for Keratophyre.
The main difference between the two is that there is less quartz in the Keratophyre matrix (<20%), while quartz in the quartz Keratophyre matrix is often more (>20%).
In structure, it is mainly layered structure, and the mineral arrangement is often directional. Sometimes, the rhyolitic structure (or strip structure) is also found;
In terms of chemical composition, the content of Na2O is high, almost always greater than K2O.
In addition, due to the high content of quartz, plagioclase is very acidic, so the content of SiO2 is also high, often more than that of ordinary Rhyolite.
Rhyolite healing properties
It is believed that Rhyolite has magical power, which can stimulate the potential and creativity of the soul, help to explore and enhance the knowledge on the soul level, and obtain wisdom ;
At the same time, it also strengthens the performance of body, mind and spirit, provides great help for developing the limits of self, and helps to experience deep loneliness.
The crystal therapist believes that Rhyolite helps people to enter the past and integrate it with the present.
According to them, no matter how complex the problem is, this crystal can solve it easily and inspire people to move forward – it tells people not to indulge in the past but to focus on real-life, down-to-earth.
In addition, crystal fans also believe that Rhyolite can improve self-esteem and self-worth, enable people to understand their own strengths and respect themselves to accept their true selves and calmly face the challenges in life.
In terms of emotion, Rhyolite has the function of coordination, which can promote emotional release at an appropriate time, and has a mild effect without stimulation.
Rhyolite can enhance the body’s resistance; treat vascular diseases, rashes, skin diseases, and infectious diseases; Promote the absorption of vitamin B; Eliminate kidney stones and tissue sclerosis.
How to use Rhyolite
Wearing Rhyolite can bring strength and improve muscle flexibility.
Wear Rhyolite on your body or place it in an appropriate position. It is said that placing it on the forehead will help to return to the past (it must be carried out under the guidance of a professional physiotherapist), and placing it on the solar plexus will help to release emotions.
The composition of Rhyolite is equivalent to that of granite, with porphyritic texture. The phenocrysts are mainly composed of quartz and alkaline feldspar.
Rhyolite is mainly used to produce sanitary ceramics and matt-glazed exterior wall tiles. Rhyolite with suitable large color can be used as granite facing stone and cut into jewelry decoration.