If you go to the nature museum to see the mineral exhibition, you will find that crystals often grow in hollow caves, just like a carefully carved crystal house. This kind of cave full of crystals is called crystal cave.
The original space of the crystal cave is generally formed by the volume contraction during the cooling of the magma, or it can be the result of the escape of a large amount of gas during the solidification of the magma, or the cavity left after the rock is partially dissolved. Open free space is a necessary condition for the growth of crystals, but space alone is not enough. "A skillful woman cannot cook without rice". Mineral growth must also have a certain material basis. After the formation of the crystal cavity, if the ore bearing hydrothermal solution is poured in, the corresponding mineral crystals will be crystallized and precipitated successively when the solution is supersaturated due to the decrease of temperature and pressure.
When the high-temperature silicon containing hydrothermal solution is poured into the cave, if the cave itself is relatively cold and the temperature is very different from that of the hydrothermal solution, the supersaturation of the mineral solution at the edge of the cave is very large at the beginning, which is easy to form many small "crystal nuclei" in a short time and grow stably and continuously. It is conceivable that so many small crystal nuclei grow along the cave wall at the same time, and will soon touch each other and cannot continue to grow. Therefore, the earliest formed and located at the edge of the cave is often the aggregate of agate cryptocrystalline quartz.
With the crystallization gradually moving towards the center of the crystal cavity, the temperature gradient and supersaturation are decreasing, and the crystallization speed is relatively slow. At this time, it is particularly suitable for the continuous growth of large crystal nuclei. What grows is hexagonal columnar crystal with cone head. Because each crystal has shiny crystal surfaces at different angles, the reflection of these small crystal surfaces in the crystal cave under the light is like the stars twinkling in the night sky, which is very beautiful.
There are obvious differences in the size of crystals grown at different development stages in the same crystal cave. If the ore bearing hydrothermal solution still contains a small amount of trivalent iron ions, when the iron ions enter the crystal lattice, the crystal will show a noble and beautiful violet color. For example, the violet holes produced in Brazil and Uruguay are often seen in the market. With the continuous growth of crystals, the concentration of ore-forming materials in hydrothermal solution gradually decreases until it is exhausted. Without the supply of "nutrients", the crystal naturally stops growing. Therefore, the center of the crystal cave is often empty. The most primitive form of the crystal cave is a hollow sphere or ellipsoid. After mining, one is divided into two. In this way, we can clearly see the beautiful crystals hidden deep in the crystal cave. At the same time, we can also understand the formation process of the crystal cave according to the observed layered structure.