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The cause of the fading of black granite in China

The cause of the fading of black granite in China

The minerals in Chinese black granite are mainly composed of feldspar, quartz, mica, pyroxene, and hornblende. In the main silicate mineral structure, each silicon ion is surrounded by four oxygen ions to form a silicon tetrahedron. This basic structural unit is rather stable in nature, resistant to acids and alkalis, difficult to dissolve, resistant to weathering, and does not have good discoloration.
The discoloration of black granite in China is also different due to different types of stone. In general, darker, lighter-colored granites tend to fade, as the dark-colored (black, dark green) granites are mainly composed of pyroxene, hornblende, basic plagioclase, and magnetite. Wait. Light-colored granite is mainly acidic plagioclase, potassium feldspar, biotite, quartz and so on. Minerals are generally formed underground, with some minerals forming first and others forming later.
The first to form is often the lack of atomic weight, the location is deeper, the supply of oxygen is less, the pressure is greater; the latter is the opposite. Therefore, different minerals generally have different formation periods. The order is roughly: dark minerals: olivine-pyroxene-hornblende-biotite; light-colored minerals: basic plagioclase-acid plagioclase-potassium long Stone - Quartz. After the first formed minerals are exposed to the earth's surface, due to the large changes in the environment, they are prone to secondary changes, while those formed later are less variable, and the most stable is quartz.
Another situation is that secondary minerals are easier to fade than primary minerals. For example, green and grass green veneers with epidote chlorite as the main component will fade after 5 to 10 years of age, but with marbles. In comparison, this change is still very weak. The change of color (fade) is also related to the environment. In the industrial area, the acid-base content in the air is high, then it is easy to change and fade. In addition, the degree of maintenance is also very important.
Marble is mainly composed of calcite, dolomite and other minerals, often the majority of the former. The Chinese black calcite molecular formula is CaCO3, and dolomite CaMg[CO3]2 has carbonate [CO3] ion. It is relatively active and easily interacts with sulfur dioxide in the air to form gypsum (Ca[SO4].2H2O). Turbid particles. Calcite easily reacts with water and forms calcium bicarbonate (Ca[HCO3]2) dissolved in water that becomes calcium carbonate after losing moisture. After these changes, the original crystal polished surface of the marble polishing plate no longer exists. Therefore, the exterior wall decoration, especially the open-air decorative materials, is not suitable for marble. As the exterior wall of marble, usually after 2 to 3 years the color will be talked about.

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