#FocusonExam: Geology Form 5 – 25the April 2020

MODULE 3

This module treats

3.1. Environmental Geology

3.2. Crystallography

3.3. Metamorphic Petrology*

 

INTRODUCTION

  • WHAT IS CRYSTALLOGRAPHY?

Crystallography is that branch of Geology that deals with the study of 3-dimensional geometric solids called crystals. This study includes the formation of crystals, external morphology of crystals, and the internal structure of these crystals.

Crystallography is derived from the Greek word “Crystallon” which means “Cold drop” or “frozen drop” and “grapho” which means to “write”.

 

Any solid that is made up of crystals is said to be crystalline.

 

A crystal is defined as a 3-dimensional solid bounded by flat smooth surfaces called Crystal Faces. The shape of Crystals and the angles between adjacent set of crystal faces are related and are important in the identification of minerals. A unit cell is the smallest building block of a crystal consisting of atoms, ions or molecules.

 

  • CRYSTAL MORPHOLOGY

Crystal Morphology refers to the external characteristics of crystals. They include:

  • Crystal face
  • Crystal Edge
  • Crystal habit
  • Nodes
  • Solid angle
  • SIMPLE FORMS

A simple form is defined as a crystal with only like faces i.e. only similar kinds of crystal Faces. Examples of simple forms include:

  • A simple cube: – A simple cube is a six-faced solid where all the faces are squares.
  • A simple octahedron – This is an eight-faced solid where all the faces are equilateral triangles.
  • A rhombohedron is a six-faced solid where all the faces are rhombus in shape

 

 

A Crystal Face is defined as a flat smooth unbroken surface of a crystal.

A Crystal Edge is a line formed on a crystal by the intersection of two adjacent crystal surfaces.

A Crystal Habit is the characteristic shape of a mineral’s crystal common assume.

A Node is a point formed on a crystal by the intersection of two perpendicular lines.

A Solid angle is one formed by the intersection of 3 or more crystal faces.

  • TYPE OF CRYTAL FACES

 

Crystal Faces have been differentiated based on how they affect crystallographic axes in spaces. The types of crystal Faces include:

  • Pinacoid: – A pinacoid is a crystal Face that cuts the one Crystallographic axis and is parallel to two axes. They have miller indices of 100,010,001
  • Prism is a crystal Face that cuts two crystallographic axes and parallel to one. They have miller indices of 110,011,101
  • A pyramid is a crystal Face that cuts all the three crystallographic axes. They have miller indices of 111,11 ̅1 ̅,111 ̅
  • A dome is a crystal Face that cut the c-axis and one other horizontal axis and is parallel to the other horizontal axis i.e. a dome must cut the c-axis and either the b or the a-axis. All domes are Prims but not all Prisms are domes e.g. 101,011

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