How to distinguish the inclusion?

Firstly, the inclusions are classified according to the formation time.

The inclusions in natural gemstones can be divided into three types according to their relationship with the formation time of host gemstones: primary type, syngenetic type and epigenetic type.

1. Primary inclusion

Primary inclusions refer to mineral particles formed before the main crystal and then wrapped by the main crystal. Primary inclusions are always solid. It can be formed in the process of magma condensation and rock metamorphism. In the process of magma condensation, minerals precipitate according to a certain crystallization order, and the minerals precipitated in advance can become inclusions of minerals precipitated in the later stage, such as zircon and apatite inclusions found in some magma gem crystals. These inclusions often have good crystal morphology, but they may also be dissolved or replaced by minerals formed in the later stage, so the crystal morphology is destroyed. In the process of rock metamorphism, primary minerals are replaced by new minerals, and the primary minerals that have not been completely replaced remain as inclusions in the new minerals, such as amphibole minerals and mica inclusions in some metamorphic gems. Most of these inclusions eroded the irregular crystal morphology.

2. Syngenetic inclusion

Syngenetic inclusions are formed and wrapped with the main crystal at the same time, which can be solid or holes with various combinations of solid, liquid and gas.

(1) solid inclusion: It coexists with the host crystal and belongs to the associated minerals under the same geochemical conditions. Such as apatite, biotite, calcite, chrome diopside, olivine, pyrite, rutile, zircon, etc.

Dissolution (desolvation) is an important reason for syngenetic inclusions. Some main mineral crystals may contain considerable dissolved impurities. In the process of crystal cooling and solidification, the ability of crystal structure to contain impurities decreases with the decrease of temperature. If the cooling rate is slow, impurities will precipitate into inclusions, mostly small flake or needle-like crystals, whose orientation is parallel to the structural direction of the main crystal. For example, rutile dissolved from corundum crystallizes into three-component 120 fine needle-like crystals. Titanium compounds such as rutile, sphene and ilmenite are the most common dissolved minerals in gemstones. This is because titanium is rich in resources, easily accommodated by the main crystal and easily dissolved from the crystal lattice. A large number of dissolved needles can produce cat's eye and starlight effects in corundum, garnet and spinel. If the speed of temperature change is not suitable for forming the correct orientation, these needles will produce mercerizing effect. Syngenetic inclusions of exsolution origin include beryl, cordierite, hematite in moonstone and albite in moonstone.

The growth rate of fibrous minerals can be as fast as that of host crystals, or even faster, so long filament inclusions can be formed, such as asbestos in andradite and rutile needle inclusions in crystals.

The growth of the main crystal may be temporarily interrupted for various reasons. At this time, some minerals can aggregate and grow on the crystal surface. When the crystal grows again, it will cover these minerals growing on the surface and make them become inclusions. These inclusions often show the orientation parallel to the crystal plane, and some of them can also show zonation, which constitutes the so-called phantom. If this process is repeated many times, multiple layers of phantom crystals may appear.

(2) Liquid inclusions and two-phase and three-phase inclusions: they can be generally called fluid inclusions, and there are many combinations, but most of them are gas-liquid inclusions. During the growth process, the main crystal may be broken and filled with crystallization solution, and then the crack will heal and seal the solution in the crystal. During the growth of the main crystal, there may also be a temporary interruption or uneven growth rate. At this time, some pits will appear on the crystal surface. When the crystal resumes its growth, it will cover the solution gathered in the pit and become a liquid inclusion. The fluid inclusions in these two cases are homogeneous liquid phase at first, but with the change of temperature and other conditions, gases, solids or other liquids will separate and become two-phase or three-phase inclusions.

During the healing process, the shapes of holes and cracks may change. Dissolution occurs in some places, while growth in other places narrows the channel, leading to "necking" or "neck sticking". Sometimes, a three-phase inclusion can be divided into two parts, one with crystals in the liquid and the other with bubbles in the liquid. Sometimes, a gas-liquid inclusion will break into two or three inclusions with different gas-liquid ratios.

Sometimes the holes caused by defects such as lattice dislocations continue to grow along the original lattice direction after being filled with high temperature solution, forming a body cavity similar to the host crystal form. This kind of hole filled with gas and liquid is similar to the crystal form of the host mineral, and it is called negative crystal inclusion or empty crystal. Some people think that empty crystal should refer to the negative crystal form without gas-liquid filling (gas-liquid loss).

3. Epigenetic inclusion

The inclusions are formed after the main crystal stops growing.

Fracture crystallization is the cause of epigenetic inclusions. After the crystal stops growing, foreign substances may penetrate into the cracks and precipitate in them. The most common oxides are iron and manganese, which always form black or brown dendritic inclusions.

Secondly, the inclusions are classified according to their phase states.

The inclusions can be solid, liquid and gas. The homogeneous part of the inclusion system is single-phase, so if a hole contains two separated liquids (immiscible liquids), it should be regarded as two-phase inclusions, and if a hole contains one liquid and two different minerals, it should be regarded as three-phase.

1. Single phase inclusions

It can be a solid inclusion, a liquid inclusion or a gas inclusion.

(1) solid inclusions: amber mainly includes mineral crystal inclusions, fused glassy inclusions, plant debris and insect inclusions. Mineral crystal inclusions include various nonmetallic minerals and metallic minerals. The most common mineral inclusions in gemstones are rutile, zircon, apatite, amphibole, feldspar, mica, calcite, tourmaline, garnet, pyrite, hematite, goethite and chromite (see Figure 6-3- 1). Some of them have complete or relatively complete crystal morphology, such as octahedron and cube, and some are flaky, fibrous, acicular, acicular, granular and irregular. It can be distributed individually or in dense groups. A large number of almost colorless tiny crystal inclusions gather together to produce a hazy appearance called clouds. A large number of round crystal inclusions sometimes produce a syrupy appearance.

Some mineral particles in polycrystalline jade, such as magnetite in nephrite, pyrite in kyanite and chlorite in sandstone, are often described as inclusions in gemology, although they cannot be considered as inclusions from the mineralogical point of view.

(2) Liquid inclusions: water mainly containing various dissolved salts and sometimes containing carbonic acid. Cold water carbonate crystals commonly found in caves.

Fig. 6-3- 1 mineral crystal inclusions

(3) Gas inclusions: the main components are water vapor or carbon dioxide, and occasionally methane. Besides carbon dioxide, bubbles in natural glass also contain hydrogen and nitrogen. The shapes of gas inclusions are round, oval and irregular. It can be distributed individually or in dense groups.

2. Two-phase inclusions

Most of them are gas-liquid two-phase inclusions, and there are also a few gas-solid two-phase inclusions.

Gas-liquid two-phase inclusions, that is, liquid inclusions contain bubbles (see Figure 6-3-2). The main reason is that when the liquid inclusion cools, the volume of the aqueous solution becomes smaller than the volume of the hole, and the water vapor occupies the vacated space, showing a circular bubble, and the liquid inclusion becomes a gas-liquid two-phase inclusion. But it is more a collection of tiny water droplets filled in the cracks and holes of gems. They are called feathers because they look like the thin wings of insects. Gems, such as beryl, produced in an environment rich in aqueous solutions are particularly common. If the primary crack is formed along the direction of cleavage or cleavage, the healed feather can be flat, otherwise it tends to be curved, fingerprint-like, veil-like, lace-like and net-like. A careful study of the above plumes shows that there are often small bubbles and droplets, so these plumes are actually gas-liquid two-phase inclusions (see Figure 6-3-3).

Gas-solid two-phase inclusions are mostly gas-melting inclusions. The melt captured in the crystallization process of minerals formed at high temperature, such as olivine and pyroxene, condenses into glassy state when the temperature drops, and the remaining space is occupied by bubbles and becomes two-phase inclusions. There are similar inclusions in gems synthesized by flux method and other methods.

Figure 6-3-2 Gas-liquid Two-phase Inclusion

Figure 6-3-3 Feathers

3. Three-phase and multiphase inclusions

After the homogeneous fluid is captured, it changes with the decrease of temperature, and gas, solid and liquid are separated into gas, solid and liquid inclusions (see Figure 6-3-4). Usually there is only one crystal in a hole, but there can be more. The solubility of salt in aqueous solution is related to temperature. Due to the change of temperature, the salt dissolved in the liquid will crystallize out. The main crystals are fluoride, chloride, carbonate or sulfate of sodium, potassium, calcium and magnesium, among which the most common are halite, potassium salt and gypsum. Inclusions composed of gas, liquid and one or more crystals of the same variety, or inclusions composed of gas and two immiscible liquids are called three-phase inclusions, while inclusions composed of gas, liquid and more than one crystal are multiphase inclusions.

Figure 6-3-4 Gas-solid-liquid three-phase inclusions

Third, other internal characteristics.

1. Growth area and color division

In the process of crystal growth, the growth environment such as pressure, temperature and chemical composition of ore-forming materials includes the changes of impurity and chromonic ion concentration. It can lead to growth bands and growth stripes with different widths. Most of them are reflected by the change of color depth and become ribbons (see Figure 6-3-5) or colored stripes. Because the distribution of these growth bands and growth stripes is related to the crystal structure, they are mostly straight and angular, as seen in corundum, amethyst and emerald. However, the color distribution is also dotted, blocky and flocculent, which has no obvious relationship with the crystal structure. There are also "tiger stripes" or "zebra stripes" in amethyst and topaz due to the difference in color depth and light and shade, which is the result of twins or partial healing along the direction of rhombus.

Figure 6-3-5 Sapphire Ribbon

Polycrystalline and aphanitic jade materials have layered structure, bands, color groups and spots due to regular and irregular changes in mineral composition, color and particle size. Vortex lines can be seen in natural glass.

2. Twin

Polycrystalline twins can be found in corundum, gold emeralds and some rare gems. Twin was previously considered as evidence of natural origin, but now it is also seen in synthetic gems grown by flame melting and flux method. Twins in minerals may be syngenetic or epigenetic. For example, flaky twins in calcite can be formed by deformation after the crystal stops growing. The same effect may also exist in corundum.

The growth lines or surfaces produced by irregularities such as twins or growth defects in the diamond formation process are called textures and nodules, which can only be slightly seen by the naked eye under a magnifying glass of 10 times.

3. cleavage and cracks

In some gems with cleavage development, the cleavage crack along the cleavage plane is called initial cleavage, which appears as a plane inside the gem. Intersecting cleavage seams can form special patterns, such as "centipede-like" inclusions in moonstone (see Figure 6-3-6). There are also irregular or wavy initial cleavage, usually perpendicular to the C axis, as seen in tourmaline. Cracks can appear in any direction inside the gem, including radial and discoid stress cracks around crystal inclusions. Some cracks can be filled and healed by gas-liquid inclusions during or after formation. A typical example is the water lily leaves contained in olivine. Some gems contain zircon inclusions, and some zircon inclusions contain radioactive elements, which can destroy the crystal lattice of zircon. The volume of zircon increases, and the resulting stress causes cracks to grow radially outward into the main crystal, which is called zircon halo.

Figure 6-3-6 Stress Cracks in Moonstone