Some crystals have a special shape. 
Crystals have many shapes. 

Crystals come in many shapes. One group is the monoclinic crystal system. 

Crystals come in many different shapes and sizes. One special group is the monoclinic crystal system. 
To understand this system, we look at three lines called vectors. These vectors act like the edges of a box. In this system, the three vectors have unequal lengths. Two pairs of these vectors meet at right angles. This means they form square corners. The third pair makes an angle that is not 90 degrees.
Scientists study how these shapes repeat in patterns. These patterns are called Bravais lattices. There are two types of monoclinic Bravais lattices. The first type is the primitive monoclinic lattice. The second type is the base-centered monoclinic lattice.
Many different minerals fit into these crystal classes. One common example of a monoclinic mineral is gypsum. 
We can even look at these shapes in flat spaces. This is called two-dimensional study. In two dimensions, there is only one monoclinic lattice. This is called the oblique lattice.
Crystallography is the study of how atoms arrange themselves into repeating patterns. Scientists use seven different crystal systems to categorize these arrangements. One of these is the monoclinic crystal system. 
To define a monoclinic crystal, we look at three vectors. These vectors represent the edges of the crystal's unit cell. In this system, all three vectors have unequal lengths. This is similar to the orthorhombic system. However, the angles between these vectors are unique. Two pairs of vectors meet at right angles, or 90 degrees. The third pair of vectors meets at an angle that is not 90 degrees.
Scientists also study Bravais lattices, which are the different ways these patterns can repeat in space. There are two distinct monoclinic Bravais lattices. The first is the primitive monoclinic lattice, which has the Pearson symbol mP. The second is the base-centered monoclinic lattice, with the symbol mS.
Monoclinic crystals are organized into several different crystal classes. These classes are categorized by their symmetry and space groups. There are three main types of classes: sphenoidal, domatic, and prismatic. Sphenoidal is also known as monoclinic hemimorphic. Domatic is referred to as monoclinic hemihedral. Prismatic is called monoclinic normal. 
The sphenoidal class includes groups like C2. An example of a mineral in this class is halotrichite. The domatic class includes groups such as Cs. One mineral example for this class is hilgardite. The prismatic class is centrosymmetric and includes the group C2h. A very common mineral found in this class is gypsum. 
These classes are defined by how they handle symmetry elements. For example, some groups use screw axes instead of simple axes. A screw axis involves a rotation combined with a translation. In the prismatic class, some groups use glide planes. A glide plane is a reflection combined with a translation. This translation is one-half of a translation vector in the base plane. These complex movements define the exact shape of the crystal's internal pattern.
We can also study these geometric rules in a flat, two-dimensional space. In two dimensions, there is only one monoclinic Bravais lattice. This is called the oblique lattice.
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