In case of triclinic system all the crystallographic are unequal in length and inclined to each other i.e. a,b,c the axial ratio (a: b: c) in case of axinite 0.972:1:0.778; a= 102°41\ (3 = 98° 09′ and y = 88° 08′. Three rules followed in determining the position of the crystallographic axes a orienting a triclinic crystal.

These are (i) c-axis is the zone axis and to most pronounced zone is vertical, (ii) Face intersecting the a-axis in positive side should slope forward to the right and (iii) a- and b-axis directions are determined by the intersections of the faces 010 and 100 with 001 respectively. In old convention and b axes were designated as branchy- and macro-axes respectively. The proper orientation of the crystallographic axes and the method of their notation are shown in.

Two classes’ viz. pinacoidal and pedial belong to this system. The pinacoidal class is known as the normal class of triclinic system as it shows maximum symmetry among all the classes belonging to this system and axinite type after the characteristic mineral axinite that crystallizes in this class.

Symmetry elements:

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Centre of symmetry is the only symmetry element of this class. Axis and plane of symmetry are absent.

Forms: Pinacoids are the characteristic forms of this class. These are given in Table 3.6.

(i) Front or a-pinacoid (100)2:

It is an open form having two facts each of which intersects a-crystallographic 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.

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(ii) Side or b-pinacoid (010)2:

It is an open form having two faces each of which intersects b-crystallographic 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 c-pinacoid (001)2:

It is an open form having two faces each of which intersects c-crystallographic axis remaining parallax with other two crystallographic axes. The form symbol is (001)2 and two faces are 001 and 001.

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(iv) First-order pinacoid:

It is an open form consisting of two faces each of which intersects b- and c-crystallographic axes at unequal lengths remaining parallel with a-crystallographic axis. There are two first-order pinacoid, positive with form symbol (0kl)o and negative with form symbol (0kl)2– The unit first-order pinacoid are shown in. The old name is hemibrachydome as the two faces are parallel with branchy (a) axis.

(v) Second-order Pinacoids:

It is an open form consisting of two faces each of which intersects a- and c-crystallographic axes at unequal lengths remaining parallel with b-crystallographic axis. There are two second-order pinacoid, positive with form symbol (hOlX) and negative with form symbol (h0l) 2. The unit second-order pinacoid are shown in. The old name is hemi macrodome as the two faces are parallel with macro (a) axis.

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(vi) Third-order pinacoid:

It is an open form consisting of two faces each of which intersects a- and b-crystallographic axes at unequal lengths remaining parallel with c-crystallographic axis. There are two first-order pinacoid, positive with form symbol (hkO), and negative with form symbol (hkO) 2. The unit third-order pinacoid are shown

(vii) Fourth-order 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 fourth-order pinacoid, positive right (hkl):, positive left (hkl)2, negative right (hkl)2 and negative left (hkl)r These two-faced forms can exist independently of the others. The combination of unit fourth-order pinacoid is shown in.

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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 fourth-order 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.