Stishovite is an extremely hard and dense tetragonal form, or polymorph, of silicon dioxide (SiO₂). It is a rare mineral on Earth's surface but may be prevalent in the lower mantle.

What is the chemical formula for stishovite?

The chemical formula for stishovite is SiO₂, which is the same as other common forms of silicon dioxide like quartz.

What crystal system does stishovite belong to?

Stishovite crystallizes in the tetragonal crystal system.

What is the IMA symbol for stishovite?

The IMA symbol assigned to stishovite is Sti.

According to the Strunz classification, where does stishovite fit?

Stishovite is classified under the Strunz system as 4.DA.40, placing it within the Oxides category of minerals.

What are the specific crystal class and space group of stishovite?

Stishovite belongs to the Ditetragonal dipyramidal crystal class (4/mmm) and its space group is P4₂/mnm (No. 136).

What are the unit cell dimensions for stishovite?

The unit cell of stishovite has dimensions of a = 4.1772(7) Å and c = 2.6651(4) Å, containing two formula units (Z=2) per cell.

What color is pure stishovite?

When pure, stishovite is colorless.

How hard is stishovite on the Mohs scale?

Stishovite is exceptionally hard, measuring 9.5 on the Mohs scale of mineral hardness, making it one of the hardest known naturally occurring substances.

What is the luster of stishovite?

Stishovite exhibits a vitreous luster, which is a glassy shine characteristic of many minerals.

What is the diaphaneity (transparency) of stishovite?

Stishovite can range in transparency from completely transparent to translucent.

What are the specific gravity values for stishovite?

The specific gravity of synthetic stishovite is 4.35, while the calculated specific gravity is 4.29.

What are the optical properties of stishovite?

Stishovite is optically uniaxial with a positive (+) sign, meaning it affects polarized light in a specific way along one axis.

What are the refractive indices and birefringence of stishovite?

Stishovite has refractive indices of nω = 1.799–1.800 and nε = 1.826–1.845, with a birefringence (the difference between the two refractive indices) of δ = 0.027.

How does the crystal structure of stishovite differ from other common silica polymorphs like quartz?

Unlike most low-pressure forms of SiO₂, such as quartz, where silicon atoms are tetrahedrally coordinated (bonded to four oxygen atoms), the silicon in stishovite adopts an octahedral coordination geometry, being bound to six oxygen atoms. This structural difference is key to its high density and hardness.

Who is stishovite named after, and what was their contribution?

Stishovite is named after Sergey Stishov, a Soviet high-pressure physicist who first synthesized the mineral in 1961.

When and by whom was stishovite first discovered in nature?

Stishovite was first discovered in nature in Meteor Crater in 1962 by Edward C. T. Chao.

How common are large natural crystals of stishovite?

Large natural crystals of stishovite are extremely rare; they are usually found as small fragments, typically 1 to 2 millimeters in length.

Under what conditions does stishovite typically form naturally?

Naturally, stishovite forms under extremely high shock pressures, exceeding 10 GPa (100 kbar), and temperatures above 1200 °C. These conditions are usually created by hypervelocity meteorite impacts into quartz-bearing rocks.

Where might stishovite be a predominant form of silicon dioxide?

Although very rare on the Earth's surface, stishovite may be the predominant form of silicon dioxide within the Earth, particularly in the lower mantle.

How can stishovite be chemically distinguished from quartz?

Stishovite can be distinguished from quartz using hydrogen fluoride (HF). Unlike quartz, which reacts with HF, stishovite is resistant to this acid.

Is stishovite stable under normal temperature and pressure conditions?

No, at normal temperature and pressure, stishovite is metastable. This means it can exist under these conditions but is not the most stable form of SiO₂.

How does the density of stishovite compare to other silica polymorphs?

With a density of 4.287 g/cm³, stishovite is the second densest polymorph of silica, surpassed only by seifertite.

What other mineral shares a similar crystal structure with stishovite?

The crystal structure of stishovite resembles that of rutile (TiO₂), a mineral form of titanium dioxide.

What does the IMA symbol 'Sti' represent?

The IMA symbol 'Sti' is the official designation for the mineral stishovite, as approved by the International Mineralogical Association Commission on New Minerals, Nomenclature and Classification.

What are some other crystalline forms of silicon dioxide mentioned in the article?

Other crystalline forms of silicon dioxide mentioned include quartz, coesite, cristobalite, tridymite, moganite, seifertite, and keatite.

What is the significance of stishovite being found within diamonds?

The presence of minute amounts of stishovite within diamonds suggests that these diamonds formed under extremely high-pressure conditions, potentially deep within the Earth's mantle.

What are 'post-stishovite phases' and where have they been identified?

Post-stishovite phases are mineral structures that form from stishovite under even higher pressures. They have been identified within ultra-high-pressure mantle rocks.

How can stishovite be synthesized in a laboratory?

Stishovite can be synthesized in a laboratory by replicating the high-pressure and high-temperature conditions found during meteorite impacts, using either isostatic pressure or shock wave methods.

What is the typical appearance of stishovite when found naturally?

When found naturally, stishovite usually appears as small, rounded gravel-like particles embedded within a matrix of other minerals.

What is the significance of stishovite's extreme hardness?

Stishovite's extreme hardness (around 30 GPa Vickers) made it historically significant as one of the hardest known oxides, indicating its unique structural properties under high pressure.

What is the relationship between stishovite and the rutile crystal structure?

Stishovite's crystal structure is analogous to that of rutile (TiO₂), meaning they share a similar atomic arrangement, particularly the octahedral coordination of the central cation (silicon in stishovite).

What does the image accompanying the article depict?

The image accompanying the article depicts the crystal structure of stishovite.

What does the pressure-temperature diagram for SiO₂ illustrate regarding stishovite?

The pressure-temperature diagram for silicon dioxide shows that stishovite is stable under specific high-pressure and moderate-temperature conditions, typically located in the upper left region of the diagram.

Besides stishovite, what other rare mineral is mentioned as having hexacoordinated octahedral silica?

Thaumasite is mentioned as another rare mineral that features hexacoordinated octahedral silica.

What does the term 'polymorph' mean in the context of stishovite?

The term 'polymorph' signifies that stishovite is one of several different crystal structure forms that silicon dioxide (SiO₂) can take, each having the same chemical composition but distinct physical properties.

What is the specific gravity of calculated stishovite?

The calculated specific gravity for stishovite is 4.29.

What is the refractive index range for stishovite?

The refractive indices for stishovite range from nω = 1.799–1.800 for the ordinary ray and nε = 1.826–1.845 for the extraordinary ray.

What is the birefringence value for stishovite?

The birefringence of stishovite, which is the difference between its highest and lowest refractive indices, is measured at δ = 0.027.

What happens to stishovite when heated to its melting point?

Instead of melting, stishovite decomposes when subjected to high temperatures approaching its melting point.

What is the connection between stishovite and Meteor Crater?

Stishovite was first discovered in nature within Meteor Crater, Arizona, in 1962, shortly after its initial synthesis.

What are some topics related to impact cratering where stishovite is relevant?

Stishovite is relevant to the study of impact cratering, particularly concerning phenomena like shocked quartz, impactites (rocks formed by impacts), and understanding the extreme conditions created during meteorite impacts.

What is the implication of stishovite's potential prevalence in the Earth's lower mantle?

If stishovite is common in the lower mantle, it indicates that this region experiences the immense pressures required for its formation, significantly influencing the mantle's physical properties and seismic wave propagation.

What is the Pearson symbol for stishovite?

The Pearson symbol for stishovite is tP6, which is a shorthand notation used in crystallography to describe its crystal structure.

What role does high pressure play in the formation and structure of stishovite?

High pressure is the critical factor for stishovite's formation and unique structure. It forces the silicon atoms into a more compact octahedral coordination (six oxygen neighbors), which is energetically favorable only under extreme pressure, unlike the tetrahedral coordination (four oxygen neighbors) found in quartz.

What does the presence of stishovite within diamonds suggest about their origin?

The occurrence of stishovite within diamonds suggests that these diamonds originated under exceptionally high-pressure conditions, likely originating from deep within the Earth's mantle.

How does the coordination number of silicon in stishovite differ from that in quartz?

In stishovite, silicon has a coordination number of 6, meaning each silicon atom is bonded to six oxygen atoms. In contrast, silicon in quartz has a coordination number of 4, bonded to four oxygen atoms.

What is the relationship between stishovite and the mineral group it belongs to?

Stishovite is classified as a tectosilicate mineral and belongs to the quartz group, indicating it shares fundamental structural or chemical similarities with quartz.

What is the significance of stishovite being the second densest silica polymorph?

Being the second densest silica polymorph highlights stishovite's highly compact crystal structure, a result of the octahedral coordination of silicon, which allows atoms to pack more closely together than in less dense forms like quartz.

What does the term 'vitreous luster' mean for stishovite?

Vitreous luster describes a shiny appearance similar to glass, which is common for minerals with a relatively smooth, non-metallic surface fracture.

What is the IMA symbol for stishovite?

The IMA symbol for stishovite is Sti.

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What is Stishovite?

A High-Pressure Polymorph

Stishovite represents an exceptionally dense, tetragonal crystalline form of silicon dioxide (SiO₂). It is a polymorph, meaning it shares the same chemical composition as other forms of silica, such as quartz, but possesses a distinct crystal structure due to differing formation conditions.

Rarity and Significance

While exceedingly rare on the Earth's surface, stishovite is theorized to be a predominant form of silicon dioxide within the planet's lower mantle. Its existence provides critical insights into the mineralogy and physical conditions of the deep Earth.

Naming and Discovery

The mineral was named in honor of Sergey Stishov, a Soviet physicist who first synthesized it in 1961. Its natural occurrence was subsequently identified in 1962 by Edward C. T. Chao within samples from Meteor Crater.

Crystal Structure and Chemistry

Rutile-like Framework

The crystal structure of stishovite is notably similar to that of rutile (TiO₂). A key distinction from other common silica polymorphs lies in the coordination geometry of silicon. In stishovite, silicon atoms adopt an octahedral coordination, being bonded to six oxygen atoms. Conversely, the oxygen atoms are typically three-connected.

Crystallographic Data

Stishovite crystallizes in the tetragonal system, belonging to the space group P4₂/mnm. Its unit cell parameters are approximately a = 4.1772(7) Å and c = 2.6651(4) Å, with two formula units (Z=2) per unit cell.

Chemical Formula

The fundamental chemical composition of stishovite is silicon dioxide, represented by the formula SiO₂. This simple formula belies the complex structural arrangement dictated by extreme pressure conditions.

Physical Characteristics

Appearance and Optics

Pure stishovite is colorless. It exhibits a vitreous luster and ranges from transparent to translucent. Optically, it is uniaxial with a positive sign. Its refractive indices are measured as n_ω = 1.799–1.800 and n_ε = 1.826–1.845, with a birefringence (δ) of 0.027.

Density and Hardness

Stishovite is remarkably dense, with a calculated specific gravity of 4.29 and a synthetic value of 4.35 g/cm³. It ranks as the second densest polymorph of silica, surpassed only by seifertite. On the Mohs scale of mineral hardness, stishovite registers a formidable 9.5, making it exceptionally resistant to scratching.

Thermal Behavior

Stishovite does not possess a distinct melting point under standard conditions; instead, it decomposes. This characteristic is typical of minerals formed under extreme pressure and temperature regimes.

Formation and Occurrence

Impact Events

The most common terrestrial occurrences of stishovite are found within impact craters, such as Meteor Crater. It forms under the immense shock pressures (exceeding 10 GPa) and high temperatures generated during hypervelocity meteorite impacts into quartz-bearing rocks.

Deep Earth Conditions

Minute quantities of stishovite have also been identified within diamonds and in ultra-high-pressure metamorphic rocks originating from the Earth's mantle. This suggests that stishovite may be a stable and potentially abundant phase in the deep mantle, conditions that are difficult to replicate and study directly.

Natural Crystal Size

Large natural crystals of stishovite are exceptionally scarce, typically found as small fragments measuring only 1 to 2 millimeters in length. Distinguishing these from common quartz often requires detailed laboratory analysis.

Laboratory Synthesis

Recreating Extreme Conditions

Stishovite can be synthesized in laboratory settings by recreating the high-pressure and high-temperature conditions under which it naturally forms. This is achieved through methods such as isostatic pressing or shock wave experiments, effectively mimicking the geological processes of meteorite impacts or deep mantle environments.

Exceptional Hardness

A Benchmark in Material Science

For a considerable period, stishovite was recognized as the hardest known oxide, with a Vickers hardness rating around 30 GPa. While the discovery of boron suboxide has since revealed an even harder oxide, stishovite's extreme hardness remains a significant characteristic, underscoring the material properties achievable under immense pressure.

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References

Stishovite. Mindat.org.

Stishovite. Webmineral.com.

A full list of references for this article are available at the Stishovite Wikipedia page

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Stishovite: The Deep Earth's Dense Silica Marvel

💡 Dive in with Flashcard Learning! 💡

What is Stishovite? ℹ️

💎 A High-Pressure Polymorph

🌍 Rarity and Significance

🔬 Naming and Discovery

Crystal Structure and Chemistry ⚛️

🏗️ Rutile-like Framework

📐 Crystallographic Data

🧪 Chemical Formula

Physical Characteristics 📊

🌈 Appearance and Optics

⚖️ Density and Hardness

🔥 Thermal Behavior

Formation and Occurrence ⛰️

💥 Impact Events

✨ Deep Earth Conditions

🤏 Natural Crystal Size

Laboratory Synthesis ⚙️

🌡️ Recreating Extreme Conditions

Exceptional Hardness 💪

🛡️ A Benchmark in Material Science

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Dive in with Flashcard Learning!

What is Stishovite?

A High-Pressure Polymorph

Rarity and Significance

Naming and Discovery

Crystal Structure and Chemistry

Rutile-like Framework

Crystallographic Data

Chemical Formula

Physical Characteristics

Appearance and Optics

Density and Hardness

Thermal Behavior

Formation and Occurrence

Impact Events

Deep Earth Conditions

Natural Crystal Size

Laboratory Synthesis

Recreating Extreme Conditions

Exceptional Hardness

A Benchmark in Material Science

Teacher's Corner

True or False?

Test Your Knowledge!

Gamer's Corner

Discover other topics to study!

References

Feedback & Support

Disclaimer

Important Notice