Page 272 - Ultimate Visual Dictionary (DK)

GEOLOGY, GEOGRAPHY, AND METEOROLOGY

Mineral CLEAVAGE Cleavage in

Cleavage three directions,
forming a
features in one block cube
direction
CLEAVAGE ALONG
ONE PLANE CLEAVAGE ALONG
MINERALS CAN BE IDENTIFIED BY STUDYING features such THREE PLANES
as fracture, cleavage, crystal system, habit, hardness, color, Horizontal Cleavage in
cleavage four directions,
and streak. Minerals can break in different ways. If a mineral forming a double-
breaks in an irregular way, leaving rough surfaces, it pyramid crystal
possesses fracture. If a mineral breaks along well-defined Vertical
planes of weakness, it possesses cleavage. Specific minerals cleavage
have distinctive patterns of cleavage; for example, mica CLEAVAGE ALONG CLEAVAGE ALONG
cleaves along one plane. Most minerals form crystals, which TWO PLANES FOUR PLANES
can be categorized into crystal systems according to their
symmetry and number of faces. Within each system, several Representation
of tetragonal
different but related forms of crystal are possible; for CRYSTAL SYSTEMS system
example, a cubic crystal can have six, eight, or 12 sides.
Cubic iron Tetragonal
A mineral’s habit is the typical form taken by an aggregate pyrites idocrase
of its crystals. Examples of habit include botryoidal (like a crystal crystal
bunch of grapes) and massive (no definite form). The relative
hardness of a mineral may be assessed by testing its TETRAGONAL SYSTEM
resistance to scratching. This property is usually measured
using Mohs scale, which increases in hardness from 1 (talc)
to 10 (diamond). The color of a mineral is not a dependable
guide to its identity as some minerals have a range of colors. CUBIC SYSTEM Representation
Streak (the color the powdered mineral makes when rubbed of cubic system
across an unglazed tile) is a more reliable indicator. Representation
Hexagonal of hexagonal/
FRACTURE beryl crystal trigonal system Orthorhombic
barytes
Fire opal with Nickel-iron
conchoidal with hackly crystal
(shell-like) (jagged)
fracture fracture
Representation
HEXAGONAL/TRIGONAL SYSTEM of orthorhombic
system
CONCHOIDAL FRACTURE

ORTHORHOMBIC SYSTEM
Monoclinic
selenite
HACKLY FRACTURE crystal
Orpiment with
uneven fracture Representation
Garnierite with
splintery fracture Representation of triclinic
of monoclinic system
system
Triclinic
axinite
crystal
UNEVEN FRACTURE SPLINTERY FRACTURE MONOCLINIC SYSTEM TRICLINIC SYSTEM

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