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Geology · 200 Level
This is the complete lecture note for GEY 202 Crystallography and systematic mineralogy, prepared for Geology students at 200 Level. You can read it online below, open it in the full viewer, or download the PDF for offline study. It pairs with the course's video lectures, so you can read and watch together.
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS GEY 202: Crystallography and Systematic Mineralogy (2 Units C: LH 15; PH 45) Course Code: GEY 202 Course Title: Crystallography and Systematic Mineralogy Course Unit: 2 Units C Series 1: Morphological Properties of Crystals Series 2: Classification of Rock-Forming Minerals Series 3: Modes of Occurrence of Rock-Forming Minerals Series 4: Principles of Crystal Systems Series 5: Identification of Crystal Systems and Mineral Series 1 Morphological Properties Of Crystals • Part 1.1: External features of crystals • 1.1.1 Crystal faces, edges, and corners • 1.1.2 Crystal forms and habits • Part 1.2: Symmetry elements in crystals • 1.2.1 Planes of symmetry • 1.2.2 Axes of symmetry • 1.2.3 Centre of symmetry • Part 1.3: Crystal morphology and identification • 1.3.1 Relationship between morphology and internal structure • 1.3.2 Practical approaches to identifying crystals by morphology Introduction Crystals are among the most striking features of minerals, and their external forms often provide the first clues to identification. The study of crystal morphology helps geologists and mineralogists to recognise minerals in hand specimens and to understand the internal arrangement of atoms that gives rise to external shapes. By examining crystal faces, edges,
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS corners, and symmetry elements, we can appreciate how geometry reflects the underlying structure of minerals. The aim of this Series is to introduce you to the morphological properties of crystals, explain the role of symmetry in crystal forms, and demonstrate how morphology can be used in mineral identification. We shall commence this Series by exploring the external features of crystals, including faces, edges, corners, and common habits. Next, we will move on to symmetry elements, considering planes, axes, and centres of symmetry. Finally, we will conclude by examining how morphology relates to internal structure and how it can be applied in practical identification of minerals. Series Learning Outcomes Upon completing this Series, you should be able to: • SLO 1.1 describe the external features of crystals including faces, edges, corners, and habits, • SLO 1.2 explain the role of symmetry elements such as planes, axes, and centres in crystal morphology, • SLO 1.3 analyse the relationship between crystal morphology and internal structure, • SLO 1.4 demonstrate practical approaches to identifying crystals based on morphological properties. 1.1 External Features Of Crystals 1.1.1 Crystal faces, edges, and corners Crystals are recognised by their external geometry, which reflects the internal arrangement of atoms. The most visible features are crystal faces, edges, and corners. Crystal faces are the flat surfaces that bound a crystal. They form naturally during crystal growth and represent planes of atoms arranged in a regular pattern. These faces often reflect light, giving crystals their characteristic lustre and appearance. Edges are the lines where two crystal faces meet. They define the outline of the crystal and can be sharp or rounded depending on the growth conditions. Corners are the points where three or more crystal faces intersect. Together, faces, edges, and corners give crystals their distinctive external morphology and are the first features observed in hand specimens. Fill in the blank: The points where three or more crystal faces meet are called ______.
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Figure 1.1 Crystal faces, edges, and corners 1.1.2 Crystal forms and habits Another important aspect of external features is the crystal form and habit. A crystal form is the set of crystal faces that are related by symmetry. For example, a cube has six identical square faces, while a hexagonal prism has six rectangular faces arranged around a vertical axis. Crystal forms are determined by the internal arrangement of atoms and the symmetry of the crystal system. Crystal habit refers to the general shape or appearance of a crystal, which may be influenced by growth conditions. Common habits include prismatic (elongated crystals), tabular (flat crystals), acicular (needle-like crystals), and massive (without distinct external shape). Habit is therefore a descriptive term that reflects how crystals appear in nature. Fill in the blank: The general shape or appearance of a crystal, influenced by growth conditions, is called crystal ______. Plate 1.2 Crystal forms and habits
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS Pilot questions 1.1 1. Define crystal faces and explain how they form. 2. What is the difference between edges and corners in crystals? 3. Explain the meaning of crystal form and give one example. 4. What is meant by crystal habit? 5. Why are external features such as faces, edges, corners, forms, and habits important in crystal identification? 1.2 Symmetry Elements In Crystals 1.2.1 Planes of symmetry A plane of symmetry is an imaginary plane that divides a crystal into two equal halves that are mirror images of each other. Crystals may have one or more planes of symmetry depending on their internal arrangement of atoms. For example, a cube has several planes of symmetry that pass through its centre. Fill in the blank: An imaginary plane that divides a crystal into two equal halves is called a plane of ______. Figure 1.3 Planes of symmetry in crystals
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS 1.2.2 Axes of symmetry An axis of symmetry is an imaginary line around which a crystal can be rotated to produce identical appearances more than once in a full rotation. The number of times the crystal looks the same during a 360° rotation determines the order of the axis. For example, a square prism has a four-fold axis of symmetry, while a triangle has a three-fold axis. Fill in the blank: A line around which a crystal can be rotated to produce identical appearances is called an axis of ______. Figure 1.4 Axes of symmetry in crystals
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS 1.2.3 Centre of symmetry The centre of symmetry is a point within a crystal such that any line drawn through it will meet identical features at equal distances on opposite sides. Not all crystals possess a centre of symmetry. For example, a cube has a centre of symmetry, but a tetrahedron does not. Fill in the blank: A point within a crystal from which identical features are found at equal distances on opposite sides is called the centre of ______. Figure 1.5 Centre of symmetry in crystals Pilot questions 1.2 1. Define a plane of symmetry and give one example.
LA-NET | ONLINE LEARNING VIDEO PLATFORM FOR POST-SECONDARY & HIGHER INSTITUTION STUDENTS 2. What determines the order of an axis of symmetry? 3. Explain the difference between a cube and a tetrahedron in terms of centre of symmetry. 4. Why are symmetry elements important in crystal morphology? 5. How do symmetry elements reflect the internal arrangement of atoms in crystals? 1.3 Crystal Morphology And Identification 1.3.1 Relationship between morphology and internal structure The external shape of a crystal, known as its morphology, is a direct reflection of its internal atomic arrangement. The repeating pattern of atoms within the crystal lattice determines the orientation of crystal faces, the angles between them, and the overall symmetry. For example, cubic crystals such as halite display square faces because their internal structure is based on a cube-shaped lattice. Similarly, hexagonal crystals such as quartz exhibit six-sided prisms due to their hexagonal atomic arrangement. This relationship between internal structure and external form is fundamental in crystallography, as it allows scientists to infer atomic arrangements by observing external morphology. Fill in the blank: The external shape of a crystal that reflects its internal atomic arrangement is called crystal ______. Figure 1.6 Relationship between internal structure and external morphology
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