Lecture_4-Crystallography

Crystallography

The study of the arrangement of atoms in minerals and the relationship between crystal symmetry and building block symmetry.

Unit cell

The basic building block of crystals, which may have any of seven fundamental shapes.

Translational symmetry

A type of symmetry where an object can be moved in a specific direction without changing its shape or position.

Steno’s Law of Interfacial Angles

The principle stating that the angles between two corresponding crystal faces of any mineral are constant regardless of the size or shape.

Crystal lattices

An ordered internal arrangement of atoms in a crystal structure that defines how unit cells fit together.

Rotational symmetry

Occurs when an object can be rotated around a point and still look the same; characterized by the order of symmetry.

Symmetry elements

Basic geometric operations, including mirror planes and rotation axes, that define the symmetry of a crystal.

Bravais Law

The observation that a crystal face is more commonly developed if it intersects a larger number of lattice points.

Mirror plane

An imaginary line that divides a crystal into two identical halves.

Crystal habit

The characteristic external shape of a crystal, which reflects its internal arrangement of unit cells.

Isometric crystal system

A system where all three axes are equal in length and perpendicular to one another.

Monoclinic crystal system

A system where all three axes are unequal in length, and two axes are perpendicular to each other.

Hexagonal crystal system

A system defined by four axes, three of equal length in the same plane, and a fourth axis perpendicular to them.

Triclinic crystal system

A system where all three axes are unequal in length, and none is perpendicular to another.

Dipyramid

A crystal form consisting of two pyramids joined at their bases.

Equant

A term used to describe minerals that have approximately equal dimensions in all directions.

Scalenohedron

A crystal form having 8 to 12 faces, often associated with calcite, resembling a dog's tooth.

Foliated

A texture in minerals characterized by the layering or banding due to different compositions or sizes of mineral grains.

Glide symmetry

An operation that involves a reflection over a line and a subsequent translation along that line.

Screw axis

An operation that combines translation along an axis with rotation around that axis, creating a helix of symmetry.

Rhombohedral symmetry

Characterized by three equal axes that are not perpendicular to each other, used to describe certain crystal structures.

Atomic structure symmetry

The structure of a crystal defined by the shape of its unit cell, the arrangement of its lattice, and the position of its atoms.

Acicular

A term used to describe elongated, slender mineral grains that are needle-like in appearance.