One or more groups of crystal planes related to the symmetry of crystal structure and having the same development are collectively called crystal forms. Crystal forms can be divided into simplex and polymorph, open and closed.

1. Simplex and Polymorph

Simplex is the sum of a set of crystal planes connected by symmetry element, that is, simplex is a set of crystal planes that can be repeated by all symmetry element of symmetric type, so the crystal planes of the same simplex are isomorphic and equal. Geometrically, crystal * * * (figure 1-2- 10) has 47 kinds of simplex.

Figure 1-2- 10 47 Geometric Simplex of Crystal

Polymorphism is the aggregation of two or more simplex. Only simplex belonging to the same symmetry type can be aggregated. As shown in figure 1-2- 1 1a, the crystal is polymerized by cubes and rhombic dodecahedrons, while figure1-2-1b is polymerized by quadrangular prisms and four bipyramids.

2. Open and closed shapes

Open: All simplex crystal faces can't close a certain space, such as parallel double faces (bottom axis face and side axis face) and various cylinders.

Closed shape: it can close any crystal plane in a certain space, such as all simplex of various biconical and equiaxed crystal systems (figure 1-2- 12).

The diagram 1-2- 1 1 shows the relationship between simplex and polygon.

Figure 1-2- 12 Open shape (bottom) and closed shape (top)

Figure 1-2- 13 Time-dependent Cluster (Irregular Connection)

Second, the continuous growth of crystals.

Not all the crystals grown in nature are single crystals, but they are often interconnected. Crystal growth can be divided into irregular growth and regular growth.

1. Irregular connection (arbitrary connection)

It means that two or more crystals are randomly connected with each other, as shown in figure 1-2- 13, which means that the timely crystals are irregularly connected, uneven and patchy. If there is an identical substrate, the irregular connection is also called a cluster.

Figure 1-2- 14 Parallel connection of synchronous crystals.

2. Rules are continuous

Refers to two or more crystals growing together according to certain rules, including parallel connection and twins.

(1) parallel connection

It means that two or more crystals of the same kind are parallel to each other and connected together, and the corresponding crystal axes, crystal faces and crystal edges are parallel to each other. Figure 1-2- 14

Represents the parallel connection of synchronous crystals.

(2) Twins

It is a regular connection formed by two or more crystals of the same kind according to certain symmetry laws. The corresponding faces, edges and angles of two adjacent individuals are not parallel, but through symmetry operation, these two individuals can overlap or be parallel to each other. According to the individual way of twins, twins can be divided into the following types:

Contact twins (1): Twin individuals contact each other on a simple plane, which is called contact twins. It can be divided into:

● Simple contact twin: it consists of two individuals, as shown in figure 1-2- 15a, which is the twin of spinel, and 1-2- 15c, which is the contact twin of emeralds.

● Multi-flake twin: multiple flake individuals are connected by the same twin rule, and the joint surfaces are parallel to each other. Multi-flake twins can often show multi-flake twins on some crystal planes or cleavage planes, such as albite multi-flake twins (figure 1-2- 15d), and their bonding surfaces are parallel (0 10).

● Ring twins: A plurality of twins are connected with each other according to the same twin law, but the joint surfaces are not parallel to each other, but intersect at equal angles in turn. According to the number of twins, there are triple crystals and quadruple crystals. , as shown in figure 1-2- 15b, is an emerald ring twin.

Figure 1-2- 15 Various types of twins

Figure 1-2- 16 interspersed with twins.

2) Twins (through twins): Twins formed by the interpenetration of individuals. As shown in figure 1-2- 16a, it is the interlayer twin of fluorite, and figure 1-2- 16b is the card twin of feldspar.

Third, crystallization habit.

Crystallization habit refers to the characteristic that gem mineral crystals always tend to form a certain form under certain external conditions, which is mainly determined by the chemical composition and crystal structure of the crystals and closely related to external conditions (temperature, pressure, concentration, viscosity, impurities, etc.). ) When crystals are formed. It includes two meanings: one is the common simplex in the same crystal, and the other is the proportion of the crystal extending in three-dimensional space. For example, fluorite is often octahedron, cubic and rhombic dodecahedron, but fluorite of different origins has different simplex: in the process of magma and pegmatite, fluorite is often octahedron; It is often a rhombic dodecahedron in the process of high temperature hydrothermal; Cubes are the most common in low-temperature hydrothermal processes. There are three crystal habits that extend in three dimensions:

1) Three-dimensional equiaxed: The monomers have basically the same development degree in three-dimensional space and are granular or equiaxed, such as diamond, spinel and garnet.

2) Two-way extension: Monomer develops especially in two directions in three-dimensional space, and the other direction is poorly developed, showing plate, sheet and leaf shape, such as graphite, mica and barite. , its internal structure is often layered.

3) Elongation all the time: Monomer develops only in one direction in three-dimensional space, which is columnar, needle-like or hairy, such as tourmaline, amphibole and pyroxene. These minerals usually have a chain structure.

4. Micro-morphology of crystal surface

In the actual crystal growth process, due to various reasons such as dissolution, micro-patterns of various patterns will be formed on the surface, including crystal plane stripes, growth layers, spiral patterns, growth hills, etched images and so on.

1. Crystal plane stripe

Also known as growth line or polycrystalline stripe, it is a straight stripe composed of a series of adjacent faces on the crystal plane, which is the trace left by the alternate growth of two simplex forms during the crystal growth process. For example, there are often horizontal stripes on the corresponding crystal plane.

2. Growth layer

Refers to a series of parallel stacked layers on a crystal plane. According to the theory of layer growth, crystal planes move outward in parallel during the growth process, forming a pattern similar to topographic contour lines.

3. Spiral lines and growth hills

Spiral refers to the spiral left on the crystal surface due to spiral growth. Growth mound refers to a mound slightly raised on the crystal surface.

Figure 1-2- 17 Crystallization habit and etching image of beryl crystal

Step 4 etch the image

Etching image refers to the pits (dissolution pits) formed by the dissolution of crystal faces after crystal formation. It is controlled by the arrangement of particles on the crystal plane and has a certain shape and direction. Etching images can be used not only to identify gem minerals, but also to identify simplex, symmetry and left and right shapes. Figure 1-2- 17 shows the columnar habit of beryl crystals. As can be seen from the figure, the main simplex of beryl crystal are hexagonal column, hexagonal bipyramid and parallel double surface (also called axial simplex), and the etching images on different simplex are different.