What is the hardness of stone?

Stone hardness refers to the ability of the surface of a stone to resist scratches, abrasions, cutting, or pressing of other substances onto the surface. It is related to the mineral composition, structure, and structure of stone. In this article, we will discuss the relevant knowledge points from these aspects.

1.There is a good positive correlation between the hardness of stone and its compressive strength. At the same time, stones with high hardness have good wear resistance, but they are also difficult to process. The hardness of the stone is also related to its structure, structure, and alteration level. For example, the Shaw hardness of slightly silicified volcanic rocks can reach over 115; Many white series of granite in Sichuan, China, such as “Lushan Red”, “Sanhe Red”, “Yingjing Red”, and “Xuehua Red”, has high hardness due to varying levels of alteration.

When the mineral composition is opposite, stones with coarse grain structure have higher hardness than stones with fine grain structure, such as “Maple Leaf Red” in Guangxi, China, and G654 Sesame Black in Fujian, China. The two have very similar chemical and mineral composition, but the hardness of “Maple Leaf Red” with a coarse grain structure is clearly higher than G654 granite with a medium to fine grain structure.

Understanding the hardness of stone materials has a guiding significance in selecting tools and processing techniques for stone processing. For example, when sawing high hardness granite such as G657 and “South African Red”, it is necessary to choose high strength, low concentration diamond and softer joint agent cutting heads, use lower circumferential speed and smaller cutting depth to ensure cutting efficiency; When sawing granite with lower hardness such as sesame black G654 and “Jinan green”, it is necessary to use a blade with medium strength, a higher concentration of diamond and a harder combination agent, and use a higher circumferential rate and larger cutting depth to improve sawing efficiency and the service life of the saw blade.

2. Stone hardness testing: The hardness of stone is usually divided into relative hardness and absolute hardness.

① Relative hardness

Select 10 minerals as the standard and classify them into 10 levels based on their relative hardness. Relative hardness is formulated by mineralogist Mohr, so it is also called Mohs scale hardness.

Mohs scale does not have a strict quantitative standard, so it is seldom used in the analysis of stone properties. However, because the Mohs scale is easy to use, it can be used to roughly judge the type of rock or mineral type in the absence of special equipment. For example, the Mohs scale of iron is 5~5.5, so it is impossible to carve traces on granite with a knife, while the hardness of minerals in marble is mostly less than 4, which can be carved with a knife, so the rock category of stone can be distinguished with a knife.

② Absolute hardness

Absolute hardness is measured using a hardness tester. According to the method of applying load during measurement, it can be further divided into static hardness and dynamic hardness. Static hardness is also divided into Knoop hardness and Vickers hardness, represented by the symbols HK and HV, respectively. Dynamic hardness, also known as Shore hardness, is represented by the symbol HS.

The stone belongs to brittle materials and is more suitable for measuring the hardness of stone using a Shore hardness tester. The Shaw hardness of marble is usually between 35 and 55, while that of granite is usually between 70 and 95, and some can reach as high as 110 or above.

Mohs hardness table The standard for identifying mineral hardness formulated by German mineralogist Mohs.

Take ten common minerals and arrange them according to their hardness: talc, gypsum, Calcite, fluorite, Apatite, feldspar, quartz, topaz, corundum, and diamond. Other minerals can be compared with these minerals to determine their hardness.

Mohs hardness of commonly used hardness testing substances

Lead 1.5/nail: 2-3/aluminum 2. -25/copper 2.5-3/Afghan white jade, marble: 3/knife: 5-5.5/glass: 5.5/hacksaw blade: 6/Hotan Jade: 6-6.5/steel file 6.5/emerald 6.5-7/Yuhua Stone stone, agate, quartz stone: 7

Mohs hardness of common metals

Chromium: 9/Iron: 4-5/Silver: 2.5-4/Copper: 2.5-3/Gold: 2.5-3/Aluminum: 2-2.9/Lead: 1.5

Mohs hardness of common jade

Agate, chalcedony, Beijing white jade, Dongling jade, and other quartz-based jade: 7/Jade: 6.5-7/Dushan jade, Meihua jade: 6-7

All kinds of Tremolite nephrite

The most famous ones are Hotan Jade and Prehnite: 6-6.5/Shuimozi