Introduction to Crystallography

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Questions and Answers

What is the main focus of crystallography?

Study of minerals and their uses

Study of rocks and geological formations

Study of crystals and their properties (correct)

Study of gases and liquids

What defines a crystal according to its internal structure?

Ordered arrangement of atoms (correct)

Liquid state of mineral

No specific arrangement of molecules

Random arrangement of atoms

What term describes the visible external shape of a mineral's crystal?

Crystal habit (correct)

Crystal framework

Crystal form

Crystal structure

Which of the following crystal habits describes crystals that grow in fine needles?

Acicular Signup and view all the answers

What is a characteristic of an amorphous solid?

Lacks a definite crystal structure Signup and view all the answers

Which of the following correctly describes a prismatic habit?

Crystals develop prism faces Signup and view all the answers

What is the smallest building block of a crystal's internal structure called?

Unit cell Signup and view all the answers

Which type of crystal habit involves crystals forming radiating sprays?

Stellate Signup and view all the answers

What is the definition of a unit cell in crystallography?

The smallest possible structural unit that is repeated, three dimensionally Signup and view all the answers

According to the law of rational indices, what can crystal faces make on the crystallographic axes?

Either infinity or small rational multiples Signup and view all the answers

What distinguishes a mineral from other natural substances?

It has a uniform, definite chemical composition and atomic arrangement Signup and view all the answers

Which of the following is NOT a type of mineral?

Synthetic mineral Signup and view all the answers

Which aspect is NOT considered a strong clue to identifying a mineral?

Location of discovery Signup and view all the answers

What does the term 'law of constancy of symmetry' imply about crystals of a given mineral?

They have the same symmetry across all specimens Signup and view all the answers

Which of the following descriptions applies to subhedral crystals?

Crystals that are partially formed and exhibit some faces Signup and view all the answers

Which of the following is an example of an organic mineral?

Uricite Signup and view all the answers

What characterizes fibrous crystals?

They form parallel fibers. Signup and view all the answers

In contrast to inert monoatomic gases, which materials exhibit short range order?

Water vapor and nitrogen gas Signup and view all the answers

What defines a simple form of a crystal?

All its faces have the same properties. Signup and view all the answers

What is the defined characteristic of euhedral crystals?

They have obvious, well-formed flat faces. Signup and view all the answers

How is the interfacial angle of a crystal defined?

It is the angle between any two intersecting faces. Signup and view all the answers

Which of the following forms a clear distinction between simple and combination crystal forms?

Combination forms consist of two or more simple forms. Signup and view all the answers

What is NOT a characteristic feature of all crystals?

They always appear in euhedral forms. Signup and view all the answers

What distinguishes dendritic crystals from other types of crystals?

They exhibit branching, tree-like growths. Signup and view all the answers

What is the primary focus of physical mineralogy?

Physical attributes of minerals Signup and view all the answers

Which branch of mineralogy involves the identification and classification of minerals based on their chemical composition?

Chemical mineralogy Signup and view all the answers

What does biomineralogy study?

Stabilization of minerals by biological entities Signup and view all the answers

What is the purpose of determinative mineralogy?

To scientifically identify minerals Signup and view all the answers

Which type of mineralogy summarizes results of studies performed on mineral substances?

Descriptive mineralogy Signup and view all the answers

In optical mineralogy, which of these is likely to be studied?

Light interaction with mineral materials Signup and view all the answers

Which of the following is NOT included as a physical attribute in physical mineralogy?

Chemical composition Signup and view all the answers

What does the study of crystal habit in mineralogy refer to?

The way a mineral grows or appears in nature Signup and view all the answers

What defines a mineral in geological terms?

A substance with a crystalline structure and ordered atomic arrangement Signup and view all the answers

What are mineraloids characterized by?

Absence of crystalline structure Signup and view all the answers

Which of the following examples illustrates polymorphism?

Calcite and Aragonite, both with the formula CaCO3 Signup and view all the answers

What characterizes isomorphic minerals?

Different chemical composition with identical structures Signup and view all the answers

Which term is used for minerals that result from a substitution process while retaining constant appearance?

Pseudomorphs Signup and view all the answers

Which of the following is a correct pair of minerals exhibiting isomorphism?

Galena and Halite Signup and view all the answers

What identifies minerals classified as amorphous?

Lack of a defined crystal structure Signup and view all the answers

In which scenarios do mineraloids typically form?

Chemical weathering at low temperatures and pressures Signup and view all the answers

Study Notes

Crystallography

Crystallography is a branch of mineralogy focusing on crystal formation and properties.
Crystals are solids with a highly ordered internal arrangement of atoms, molecules, or ions.
This arrangement, known as a crystal structure, defines a mineral; different structures result in different mineral names.
Amorphous solids lack a definite crystal structure.
A crystal's external shape is called its crystal habit.
Common crystal habits include:
- Prismatic/Columnar: Featuring prism faces.
- Acicular: Fine needles.
- Tabular: Flat plates.
- Stellate: Radiating sprays.
- Fibrous: Parallel fibers
- Dendritic: Branching, tree-like growths.

Concepts of Crystallinity

Inert monoatomic gases lack regular atomic ordering.
Materials like water vapor, nitrogen gas, amorphous silicon, and silicate glass exhibit short-range order.
Metals, alloys, ceramics, and polymers demonstrate long-range atomic/ionic order.

Characteristics of Crystals

A simple form crystal has all faces with the same properties (e.g., cube, octahedron).
A combination crystal features two or more simple forms.
A face is a flat surface on a crystal.
An edge is formed where two faces meet.
A solid angle is formed by three or more faces.
An interfacial angle is the angle between two faces.

Crystal Faces and Shapes

Crystals are often recognized by their shape, consisting of flat faces with sharp angles.
Three types of crystals based on their macroscopic shape:
- Euhedral: Well-formed flat faces.
- Anhedral: No clearly defined faces (often part of a polycrystalline solid).
- Subhedral: Incomplete, less distinct face development.

Unit Cell

The smallest repeating structural unit of a crystal, containing a complete description of the entire structure.
Analogous to an atom in a chemical perspective.

Laws of Crystallography

Law of Constancy of Symmetry: All crystals of a specific mineral possess the same symmetry.
Law of Constancy of Interfacial Angle: The angle between similar faces of a mineral is constant, making it useful for identification.
Law of Rational Indices: Crystal faces intersect the crystallographic axes at points that are either infinite or rational multiples of the intercepts made by the unit form. This determines the crystallographic indices of the faces.

Mineralogy

Focuses on the chemical composition, crystal structure, and physical properties of minerals.
In ancient times, "mineral" referred to any naturally occurring inorganic substance.
Today, "mineral" can have different meanings depending on the context:
- Nutrition: Minerals as elements (calcium, iron, sodium).
- Mining: Anything extracted from the ground, including coal, sand, or gravel.

Minerals

A mineral is a naturally occurring homogeneous solid with a:
- Definite (but not necessarily fixed) chemical composition.
- Highly ordered atomic arrangement.
Minerals can be:
- Inorganic (e.g., calcite, quartz, olivine).
- Organic (e.g., uricite, calclacite, hoelite).

Mineral Properties

Properties commonly used for mineral identification:
- Crystal form and habit
- Luster
- Color
- Transparency
- Streak
- Cleavage
- Fracture
- Parting
- Tenacity
- Density
- Hardness

Branches of Mineralogy

Physical mineralogy: Focuses on the physical attributes of minerals for identification, classification, and categorization (color, crystal structure, habit, cleavage, luster, streak, hardness, specific gravity).
Chemical mineralogy: Focuses on the chemical composition of minerals for identification, classification, and potential applications.
Biomineralogy: Studies how plants and animals stabilize minerals under biological control, including mineral replacement after deposition.
Determinative mineralogy: Scientific method for identifying minerals through data gathering and analysis, including procedures for new mineral discoveries.
Descriptive mineralogy: Scholarly recording of mineral identification, classification, properties, and uses based on scientific research.
Optical mineralogy: Uses light sources to identify and classify minerals.

Mineraloids

Mineraloids possess all characteristics of minerals except for a crystalline structure (they are amorphous).
Form at low temperatures and pressures during weathering.
Examples: Limonite and allophane.

Polymorphism

Polymorphism describes two or more minerals with the same chemical composition but different crystal structures.
Examples:
- Pyrite (FeS2) and Marcasite (FeS2)
- Calcite (CaCO3) and Aragonite (CaCO3)

Isomorphism

Isomorphic minerals are crystalline compounds with different chemical compositions but identical structures.
Example: Galena (PbS) and Halite (NaCl)

Pseudomorph

A mineral that appears in a form different from its typical crystal system due to a substitution process.
The original mineral is replaced by another, preserving the shape and dimensions.
Examples:
- Anhydrite (CaSO4) pseudomorphed after gypsum (CaSO4.2H2O)
- Limonite (FeO.OH) after pyrite (FeS2)

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Description

Explore the fascinating world of crystallography, where you will learn about crystal formation, properties, and classifications. This quiz covers the different crystal habits and the concepts of crystallinity, highlighting the distinctions between crystalline and amorphous materials. Test your knowledge on these critical aspects of mineralogy.