List of Contents
Crystallographic Axes:
Some minerals like axinite, albite, anorthite, microcline, and kyanite crystallize in the triclinic system. The crystal forms of the triclinic system are referred to as three unequal axes that make oblique angles with each other. The angles between the positive ends of ‘b’ and ‘c’, the ‘c’ and ‘a’, and the ‘a’ and ‘b’ are designated as α (alpha), β (beta), and γ (gamma) respectively.
Classes of Triclinic Crystal System:
The triclinic crystal system comprises two classes, namely, the pinacoidal class and the pedial class. The pinacoidal class has a centre of symmetry. It is also called ‘triclinic normal’. On the other hand, the pedial class has no symmetry. It is alternatively known as ‘triclinic hemihedral’. Now, you may ask what the centre of symmetry is. The answer is that a centre of symmetry is that central point inside a crystal that mirrors and inverts one side of the crystal to the other. Even you can create a crystal’s centre of symmetry by rotating the crystal to 180 degrees and mirroring the image. The normal symmetry class of triclinic system is Pinacoidal Class.
There are more than 400 minerals belonging to the triclinic crystal system, among which 300 are associated with Pinacoidal Class, while 50 minerals are included in the Pedial Class. Apart from these minerals, there are dozens of minerals, which have characteristics of both the classes.
(1). Pinacoidal Class (Axinite Type) of Triclinic Crystal System:
Symmetry elements of Pinacoidal Type: Centre of symmetry is the only symmetry element of this class. Axis and plane of symmetry are absent.
Different Forms of Pinacoidal Class: Each form of this class is made up of two faces, which are parallel to one another, and are symmetrical with reference to the center of symmetry. Pinacoids are the characteristic forms of this class. Detail of the pinacoidal forms is as under:
(i) Front or apinacoid (100)_{2}:
It is an open form having two facts each of which intersects acrystallographic axis remaining parallax with other two crystallographic axes. The form symbol is (100), a two faces are 100 and 100. Its old name is macropinacoi as the faces are parallel with macro (b) axis.
(ii) Side or bpinacoid (010)_{2}:
It is an open form having two faces each of which intersects bcrystallographic axis remaining parallel with other two crystallographic axes. The form symbol is (010)_{2} and two faces are 010 and 010. Since the faces are parallel with branchy (a) axis it is also known as branchypinacoid.
(iii) Basal or cpinacoid (001)_{2}:
It is an open form having two faces each of which intersects ccrystallographic axis remaining parallax with other two crystallographic axes. The form symbol is (001)_{2} and two faces are 001 and 001.
(iv) Firstorder pinacoid:
It is an open form consisting of two faces each of which intersects b and ccrystallographic axes at unequal lengths remaining parallel with acrystallographic axis. There are two firstorder pinacoid, positive with form symbol (0kl)_{o} and negative with form symbol (0kl)_{2}– The unit firstorder pinacoid are shown in. The old name is hemibrachydome as the two faces are parallel with branchy (a) axis.
(v) Secondorder Pinacoids:
It is an open form consisting of two faces each of which intersects a and ccrystallographic axes at unequal lengths remaining parallel with bcrystallographic axis. There are two secondorder pinacoid, positive with form symbol (hOlX) and negative with form symbol (h0l) _{2}. The unit secondorder pinacoid are shown in. The old name is hemi macrodome as the two faces are parallel with macro (a) axis.
(vi) Thirdorder pinacoid:
It is an open form consisting of two faces each of which intersects a and bcrystallographic axes at unequal lengths remaining parallel with ccrystallographic axis. There are two firstorder pinacoid, positive with form symbol (hkO), and negative with form symbol (hkO) _{2}. The unit thirdorder pinacoid are shown
(vii) Fourthorder pinacoid:
It is an open form consisting of two faces each of which intersects all the three crystallographic axes at unequal lengths. There are four fourthorder pinacoid, positive right (hkl)_{:}, positive left (hkl)_{2}, negative right (hkl)_{2} and negative left (hkl)_{r} These twofaced forms can exist independently of the others. The combination of unit fourthorder pinacoid is shown in.
(viii) Hemiprism, Hemimacrodomes: Each of these two forms includes two faces. For each form there is a positive and negative form.



 Hemiprism: Positive (hko), Negative (hk o).
 Hemimacrodome: Positive (hol), Negative (hol)


(ix) Quarter Pyramids: Quarter Pyramids are composed of two faces. Each face intersects all three crystallographic axes. The various quarterpyramids are (i) positive right (hkl), (ii) positive left (hk l), (iii) negative right (hkl), and (iv) negative left (hk l).
All the forms of the normal class of triclinic system are open forms. So they cannot occur independently. The combination of a, second, third and fourthorder pinacoid are shown in.
Axinite, microcline, plagioclase feldspars, rhodonite, wollastonite, pectolite, amblygonite, polyhalite, and turquoise, unexcited are some of the important minerals crystallizing in this class.
(2). Pedial Class of Triclinic Crystal System (Analkime Type):
There are no rotational axes of symmetry and no mirror planes in the system. The Pinacoidal class has only a center of symmetry, and the Pedial class has no element of symmetry. Because of this, the minerals Of the pinacoidal class are bounded only by pinacoids (two parallel faces); and those of the pedial class show only pedions (single faces). There are approximately 70 minerals included in this class, which are very rare. Not only are the minerals in this class rare, but good crystals are even more uncommon, and they are most often microscopic. Analcime, a fairly common mineral, is classified in the pedial class but may be cubic, tetragonal, orthorhombic, or monoclinic depending upon slight changes in structural ordering.
Symmetry Elements of Pedial Class: Unlike the Pinacoidal class, the pedial class has no symmetry elements like rotational axes, mirror planes, and a center of symmetry.
Related Lessons:
 Introduction to Crystallography
 Unit Cells of the Crystal Lattice
 Crystallographic Axes
 Interfacial Angle of Crystals
 Parameters of Crystal faces
 Crystallographic Notation
 Forms of Crystal faces
 Symmetry Elements of Crystals
 Six Types of Crystal Systems
 Cubic/ Isometric Crystal System
 Tetragonal Crystal System
 Hexagonal Crystal System
 Orthorhombic Crystal System
 Monoclinic Crystal System
 Triclinic Crystal System