concept 5.3- what is crystallography

2.6 What is a Crystal?

  • Definition:

    • A crystal is a solid substance that exhibits a homogeneous composition and has an internal geometric structure that is symmetrical at the atomic level.

    • Crystals may exhibit external plane faces that are symmetrically arranged on crystal surfaces.

What is Crystallography?

  • Definition:

    • Crystallography is the scientific branch dedicated to the study of the physical and chemical properties of crystals.

  • Focus:

    • Crystallographic studies concentrate on understanding the internal arrangement of atoms within the crystalline structures of gems, minerals, or any chemical substance that possesses a crystalline character.

  • Crystalline Structure:

    • Most pure physical-chemical substances can be found in at least one crystalline form.

    • Some substances may exhibit multiple crystalline forms depending on physical and environmental conditions during formation.

The Nature of Gems and Minerals

  • General Characteristics:

    • Most gems are classified as minerals, which have unique atomic arrangements in their crystal structures.

    • Properties:

    • The physical and chemical properties of the elements within the crystal structure are what give gems their unique characteristics.

  • Perspective from Gemologists:

    • A gemologist—a person who studies, prepares, or sells gems—views minerals as particularly fascinating due to their beauty and potential value.

    • Example: Natural minerals can exhibit great beauty and artistry, even outside of jewelry cases.

    • Some examples of commonly recognized minerals include gypsum crystals.

  • Identifying Characteristics of Gems:

    • Gems can be identified through unique physical characteristics such as:

    • Color,

    • Hardness,

    • Crystal structure.

    • Clarification:

    • Not everything regarded as a gem is a mineral (e.g., amber, pearls, obsidian, natural glass).

2.7 Crystalline vs. Non-Crystalline Substances

  • Definition of Crystalline Substances:

    • A crystalline substance has a structured, form-oriented arrangement at the atomic level.

    • This atomic arrangement is repeated trillions of times, forming a single grain of crystal.

  • Examples of Crystalline Substances:

    • Sodium Chloride (NaCl):

    • Commonly known as salt and referred to in geology as halite.

    • Precipitated from evaporating water without organic processes, hence it qualifies as a mineral.

    • Characteristic: Halite is soft due to its ionic bonds between sodium and chlorine.

    • Sugar (C6H12O6):

    • Also forms crystallized structures when precipitated from water but is classified as organic, therefore it is not a mineral.

    • Rock candy appears crystalline but doesn't qualify as a mineral.

  • Amorphous Solids:

    • Very few solid materials exist that do not fit the crystalline definition.

    • Most observed solids are non-crystalline, largely due to biological processes (e.g., organic solids seen in nature).

    • In the context of geology, while rocks are composed of minerals, only about a dozen minerals (termed rock-forming minerals) are abundant. The variety of organic solids is more prevalent than minerals in everyday observations.

    • Figures:

    • Figure 2-13 displays examples of organized mineral structures (crystalline) and disordered substances (non-crystalline).

  • Non-Crystalline Example:

    • Glass:

    • An amorphous substance lacking an orderly atomic arrangement, categorized as non-crystalline.

    • Often a mixture of chemical compounds.

    • Forms through rapid cooling of melted substances. Includes both natural and man-made glasses, with the latter frequently utilized as gem substitutes in costume jewelry.

2.8 Atomic Structure of Crystals

  • Definition:

    • Minerals are defined as chemical substances made up of atoms arranged in precise and unique crystal structures.

  • Crystal Structure Characteristics:

    • A crystal structure signifies a highly ordered, repeatable atomic arrangement.

    • Distinction:

    • A chemical formula describes the elements in a pure chemical compound, while a mineral must have its molecules arranged in a symmetric crystal pattern.

    • Example: Water (H2O) is not classified as a mineral, but ice is considered one.

Unit Cells in Crystals

  • Definition of Unit Cell:

    • The crystal structure consists of infinitely repeating units called unit cells.

    • The unit cell represents the smallest, simplest configuration of molecules arranged to form a crystal.

  • Structure Visualization:

    • Crystals contain repeating unit cells ranging from atomic levels up to vast quantities combining to form larger, observable crystals (e.g., halite).

Illustrating Atomic Arrangement

  • Visual Representation:

    • An effective way to depict atomic arrangement within a crystal structure is through the stacking of marbles.

    • Figures:

    • Figure 2-14 illustrates vertically stacked marbles, representing atomic arrangements in cubic and rectangular cuboidal forms.

    • Figure 2-15 portrays offset arrangements of marbles producing hexagonal prism and pyramidal configurations.

  • Importance of Stacking Arrangement:

    • Different stacking patterns highlight various geometric crystal structures.

    • A minimum of eight marbles represents the smallest unit cell for creating larger stacked and offset variations of cubic structures.

  • Growth of Crystals:

    • As additional layers of marbles (atoms) are added, the crystal grows larger, showcasing the process of crystallization in a tangible form.

    • Note that for the vertical hexagonal unit cell, six marbles suffice, while four are necessary for the offset-stacking hexagonal unit cell.