The Composition of Rocks: Mineral Crystallinity and Bonding Types

Summary

This video delves into the fundamental concepts of rocks and minerals, focusing on how geologists define minerals. It explores the characteristics that distinguish minerals, such as crystallinity, chemical composition, and bonding types, while also discussing the classification systems used in mineralogy. The video highlights the significance of chemical bonds, including covalent, ionic, and metallic, in determining a mineral's physical properties. Additionally, it touches on real-world examples and the ongoing discovery of new minerals. The next tutorial will continue exploring how bonding and crystal structure influence mineral properties.

Takeaways

• 🌏 Rocks are composed of minerals, which are naturally occurring solid chemical substances with specific properties.
• 🔍 Geologists define minerals as having a highly ordered atomic structure and specific physical properties, according to James D. Dana.
• 📚 The International Mineralogical Association (IMA) has a set of criteria for minerals, including that they must be crystalline and formed by geological processes.
• 🆕 New minerals are discovered regularly, with examples from the Tolbachik volcano in Russia discovered in May 2021.
• 💠 Crystallinity is a fundamental property of minerals, characterized by a repeating three-dimensional atomic framework known as a lattice.
• ❌ Some substances like opal or obsidian are not minerals because they lack a crystalline structure and are amorphous.
• 📈 Mineralogists use chemical formulas and crystalline structures to classify minerals into classes, aiding in identification and study.
• 🔬 The Nickel-Strunz classification system, used by the IMA, categorizes minerals based on elemental composition, structure, and bonding type.
• 💎 Covalent bonds are the strongest, with diamond being the hardest mineral due to its covalent bonding.
• 📐 Ionic bonds result in minerals that can exhibit cleavage, such as halite (sodium chloride) which shows cubic cleavage.
• 🌐 Metallic bonds are the weakest, found in minerals that are soft, conductive, and insoluble in water, often with a cubic crystalline structure.
• 🔄 Chemical bonds in minerals occur along a spectrum, with pure metallic, covalent, and ionic bonds as endmembers, but not mixed ionic and metallic.

Q & A

What is the primary focus of the geology series before this video?

The geology series previously focused on large-scale geological structures and phenomena.

How do geologists define a mineral?

Geologists define a mineral as a naturally occurring solid chemical substance formed through biogeochemical processes, having a characteristic chemical composition, highly ordered atomic structure, and specific physical properties.

What criteria must a substance meet to be classified as a mineral according to the International Mineralogical Association (IMA)?

According to the IMA, a substance must be naturally occurring, a crystalline solid, formed by geological processes, composed of elements or compounds, and characterized by a standard chemical formula.

What is the most fundamental property of a mineral?

The most fundamental property of a mineral is its crystallinity, or the arrangement of its atoms in a repeating three-dimensional framework called a lattice.

Why are opal and obsidian not considered minerals?

Opal and obsidian are not considered minerals because they have amorphous atomic structures, meaning they lack a repeating three-dimensional crystal lattice.

How are minerals classified according to their elemental composition and crystalline structure?

Minerals are classified into specific classes based on their elemental composition, crystalline structure, and bonding type, using systems like the Nickel-Strunz classification, which is based on James D. Dana’s original classification scheme.

What is the relationship between a mineral's chemical bonds and its physical properties?

The nature of a mineral’s chemical bonds significantly influences its physical properties. For example, minerals with covalent bonds are the hardest, while those with ionic bonds may have good cleavage and are highly symmetrical.

What type of cleavage does halite exhibit, and how does it occur?

Halite exhibits cubic cleavage, where the mineral breaks along three orthogonal planes, creating symmetrical cubes when fractured.

What are the key characteristics of metallic minerals?

Metallic minerals are characterized by weak metallic bonds, making them soft and weak. They are also shiny, opaque, have high conductivity of electricity and heat, and are very insoluble in water.

Can chemical bonds in minerals be a mix of different types?

Yes, chemical bonds in minerals can occur along a spectrum, with combinations like mixed ionic-covalent bonds. However, it is not possible to have bonds that are both ionic and metallic, as they involve fundamentally different processes.

Outlines

00:00

🪨 Introduction to Minerals and their Definition in Geology

This paragraph introduces the concept of minerals within geology, contrasting how different fields (nutritionists, biologists, and geologists) define 'minerals.' The focus is on the geological perspective, emphasizing the definition by James D. Dana and the International Mineralogical Association (IMA). Minerals are defined as naturally occurring crystalline solids with a specific chemical composition formed through geological processes. The discovery of new minerals, such as those found in Russia's Tolbachik volcano, highlights the ongoing expansion of mineral classification.

05:02

💎 Crystallinity and Chemical Bonds in Mineral Structure

This paragraph delves into the crystallinity of minerals, which refers to the ordered arrangement of atoms in a repeating three-dimensional structure known as a lattice. It explains that certain materials like opal and obsidian are not classified as minerals due to their amorphous atomic structures. The paragraph also discusses the significance of a mineral's chemical composition and bonding, which determines its classification and physical properties. Different types of bonds, such as covalent, ionic, and metallic, are explored, with examples like diamond and halite used to illustrate their effects on hardness, cleavage, and melting points.

Keywords

💡Geology

Geology is the scientific study of the solid and liquid matter that constitutes the Earth, including the processes that shape it. In the context of this video, geology is the overarching theme, as it discusses the composition and characteristics of rocks and minerals, which are fundamental aspects of the Earth's structure and history.

💡Minerals

Minerals are naturally occurring, inorganic, solid substances with a specific chemical composition and a crystalline structure. They are the building blocks of rocks. The script defines minerals according to the criteria set by the International Mineralogical Association (IMA) and discusses their importance in understanding geological processes.

💡Crystallinity

Crystallinity refers to the property of a mineral where its atoms are arranged in a repeating, three-dimensional pattern known as a crystal lattice. This is a fundamental characteristic that distinguishes minerals from other substances, such as amorphous materials like opal or obsidian, which are not considered true minerals.

💡Chemical Bonding

Chemical bonding is the force that holds atoms together in a molecule or compound. The script discusses three main types of chemical bonds—covalent, ionic, and metallic—which determine many of a mineral's physical properties, such as hardness, melting point, and conductivity.

💡Covalent Bonds

Covalent bonds are formed by the sharing of electron pairs between atoms. Minerals with predominantly covalent bonds, like diamond, are the hardest and most refractory. The script uses diamond as an example to illustrate the strength and high melting point associated with covalent bonding.

💡Ionic Bonds

Ionic bonds occur when electrons are transferred from one atom to another, creating positively and negatively charged ions that are attracted to each other. Minerals with ionic bonds, such as halite (table salt), tend to have cleavage, lower melting points, and are soluble in water.

💡Metallic Bonds

Metallic bonds are the weakest type of chemical bond and are characterized by the free movement of valence electrons throughout the structure. This gives metallic minerals high electrical and thermal conductivity. The script explains that metallic minerals can be pure elements or alloys and are typically insoluble in water.

💡Cleavage

Cleavage refers to the tendency of minerals to break along smooth, flat planes. The script mentions that minerals with ionic bonds often exhibit good cleavage, such as halite, which breaks along cubic planes, and calcite, which breaks along rhombohedral planes.

💡Nickel-Strunz Classification System

The Nickel-Strunz classification system is a method used by the IMA to categorize minerals based on their elemental composition, crystalline structure, and bonding type. The script notes that this system is heavily based on the original classification scheme by James D. Dana.

💡Evaporite Minerals

Evaporite minerals are those that precipitate from an evaporating body of water, such as a lake or sea. The script identifies most ionic minerals on Earth as evaporites, providing an example of how minerals form and their geological significance.

💡Silicate Minerals

Silicate minerals are a group of minerals that contain silicon and oxygen in their chemical structure, often bonded to metals. The script mentions that most of Earth's minerals are composed of metals bonded to silicate anions, highlighting the prevalence and importance of silicate minerals in the Earth's crust.

Highlights

Introduction to the focus on rocks and minerals, which are the core subjects most people associate with geology.

Explanation that rocks are assemblages of elements and compounds called minerals.

Clarification of the varying definitions of minerals from different disciplines, such as nutritionists and biologists.

James D. Dana's definition of a mineral as a naturally occurring solid with a characteristic chemical composition and a highly ordered atomic structure.

Introduction to the International Mineralogical Association (IMA) and their stringent criteria for mineral classification.

Discussion on the discovery of new minerals, including an example from May 2021 involving arsenic and copper-containing minerals in Russia.

The fundamental property of a mineral is its crystallinity, with a specific focus on the repeating three-dimensional lattice structure.

Distinction between true minerals and other substances like opal and obsidian, which lack a crystalline structure.

Explanation of mineral classes based on elemental composition and crystalline structure, which help geologists in identification and study.

Details on the Nickel-Strunz classification system, which is based on Dana's original scheme and groups minerals by bonding type.

Overview of the different types of chemical bonds in minerals: covalent, ionic, and metallic, and how they affect mineral properties.

Description of the strong covalent bonds in diamonds, making them the hardest known minerals.

Explanation of ionic bonds, which are easier to break and lead to minerals with good cleavage, such as halite and calcite.

Characteristics of metallic bonds, which result in soft, conductive minerals, often with a cubic crystalline structure.

Introduction to the spectrum of bonding types and how most minerals exhibit mixed bonding, particularly ionic-covalent bonds.

Transcripts

So far in this geology series we’ve focused on  large-scale geological structures and phenomena.  

But now it’s time to zoom in and discuss what most  people think of first when geology comes to mind:  

rocks and minerals. In the broadest sense, all  rocks are assemblages of specific elements and  

compounds called minerals, but the precise  definition of a mineral depends on who you  

ask. Nutritionists might use the word mineral  to describe elements like calcium and zinc,  

which are found in certain types of foods or  supplements. Biologists might define a mineral  

as any inorganic material present within a living  organism. But how do geologists define minerals? 

James D. Dana, an American geologist who created  the first, standardized classification system  

for over 5,000 minerals in the 1850s, defined  a mineral as “... a naturally occurring solid  

chemical substance formed through biogeochemical  processes, having characteristic chemical  

composition, highly ordered atomic structure, and  specific physical properties.” The International  

Mineralogical Association, or IMA, has built  off of Dana’s original definition and developed  

a stringent list of criteria for mineral  status. Minerals must be naturally occurring,  

crystalline solids that are formed by geological  processes, composed of elements or compounds,  

and can be characterized by a standard chemical  formula. Amazingly, new minerals are discovered  

and proposed to the IMA every month! For example,  just in May 2021, scientists discovered three new  

arsenic and copper-containing minerals forming  along fissures in the Tolbachik volcano in Russia. 

The most fundamental property of  a mineral is its crystallinity,  

or the arrangement of the mineral’s atoms  in a repeating three-dimensional framework  

called a lattice, a term we should be familiar  with from the general chemistry series.  

Some of the items you might find at a local  rock or gem shop, like opal or obsidian,  

aren’t actually minerals at all. They fail to  qualify because their atomic structures are  

amorphous, meaning that they lack a  repeating, three-dimensional crystal lattice. 

Another defining characteristic of a mineral is  its elemental composition, which is represented  

by a chemical formula that represents one formula  unit of the mineral’s structure. Mineralogists use  

a mineral’s chemical formula along with its  crystalline structure to assign it a mineral  

class. Classes are a convenient way to group  minerals together based on shared properties,  

making it easier for geologists to identify  minerals and study rocks, especially in the field. 

The IMA uses the Nickel-Strunz classification  system, heavily based on Dana’s original  

classification scheme, which divides  all minerals into specific classes  

based on their elemental composition, crystalline  structure, and bonding type. Bonding types are of  

particular interest in mineralogy because the  nature of a mineral’s chemical bonds dictates  

many of its physical properties. Covalent  bonds are the strongest type of chemical bond.  

Minerals with predominately covalent bonds are  the hardest and most refractory minerals. Diamond,  

which is composed of linked carbon tetrahedra, is  the prime example of a covalently bonded mineral.  

It is the hardest substance known and has a  melting point of over 4000 degrees Celsius.  

For a comparison, most rocks begin to melt  somewhere between 700 and 1300 degrees Celsius. 

Ionic bonds are the second strongest type of  chemical bond. Minerals with predominately  

ionic bonds tend to be easier to break, and they  often fracture smoothly along planar surfaces.  

Geologists call this quality “cleavage”, and  a mineral is said to have “good cleavage” if  

it breaks along a single plane of atoms.  Minerals with ionic bonds also tend to be  

highly symmetrical, have lower melting points,  and are highly soluble in water. Table salt,  

or sodium chloride, and calcium sulfate are two  examples of minerals dominated by ionic bonds.  

In the world of geology, sodium chloride is  called halite and calcium sulfate is anhydrite.  

Halite exhibits a type of cleavage called  cubic cleavage, where the mineral breaks  

along three orthogonal planes, creating little  symmetrical cubes when smashed with a hammer.  

Calcite, or calcium carbonate exhibits  rhombohedral cleavage, where the mineral  

breaks along planes at 60- and 120-degree angles,  creating miniature rhombohedrons when fractured.  

Characterization of cleavage is an excellent  way to identify minerals in hand samples,  

especially in the field. Most of Earth’s  ionically bonded minerals are evaporite minerals,  

meaning they precipitate from an evaporating  body of water, such as a lake or sea. 

Metallic bonds are the weakest type of chemical  bond, making their minerals soft and weak. Due to  

the unique chemical properties of metallic bonds,  valence electrons can flow freely throughout the  

entire lattice, which gives rise to the high  conductivity of electricity and heat exhibited  

by metallic minerals. Metallic minerals can be  made of a single element, like iron or platinum,  

or can be composed of an alloy, or mixture of  metals. All metals have a crystalline structure,  

with most being cubic. Metals are shiny,  opaque, and very insoluble in water. 

Now that we have discussed the three main bonding  types, it is important to note that chemical bonds  

occur along a spectrum, with metallic, covalent,  and ionic being endmembers, or distinctive types  

of bonding. For example, there is a complete  spectrum of bonding ranging from pure metallic  

to pure covalent, as well as from pure covalent to  pure ionic. It is not possible to have a bond that  

is mixed ionic and metallic in nature, as they are  not compatible. Ionic bonds involve the creation  

of cations and anions by an exchange of electrons  between atoms of contrasting electronegativity,  

whereas the bonding between metals involves free  roaming valence electrons travelling between  

atoms of near identical electronegativity;  they are essentially mutually exclusive.  

Most of Earth’s minerals are composed of  metals bonded to silicate anions and the  

nature of these bonds is mixed ionic-covalent. So, we now understand what a mineral is, and know  

some details regarding their basic properties,  such as crystallinity, and the types of chemical  

bonding that are present in their lattice  structure. In the next tutorial we will continue  

to discuss how bonding and crystal structure  affects the physical properties of minerals.