Explain how glass is used to help ID minerals.
Glass is a commonly used material in the field of mineralogy to aid in the identification of minerals. This is because glass has specific and well-defined physical properties that can be compared with the properties of an unknown mineral to determine its identity. Here are some ways glass is used to help identify minerals:
1. Hardness: Glass has a relatively low hardness compared to most minerals. Its hardness is around 5.5 on the Mohs scale, which makes it a useful reference point for testing the hardness of unknown minerals. By scratching an unknown mineral against a glass plate or trying to scratch the glass with the mineral, one can determine if the mineral is harder or softer than glass, providing valuable clues for identification.
2. Streak: The color of a mineral's powder when it is scraped on an unglazed porcelain plate is called its streak. Glass plates can be used to determine the streak color of minerals. By rubbing a mineral against a glass plate and observing the color of the streak it leaves behind, mineralogists can compare it with known streak colors to narrow down the possibilities for identification.
3. Transparency: Glass is transparent, allowing light to pass through it easily. This property is useful for comparing the transparency of an unknown mineral with that of glass. Minerals can be categorized as transparent, translucent, or opaque, and comparing their transparency with a glass plate can help in the identification process.
4. Cleavage and fracture: Glass has a characteristic conchoidal fracture, which means it breaks into smooth curved surfaces. This property can be compared with the fracture or cleavage of an unknown mineral. By examining how a mineral breaks or cleaves in comparison to glass, mineralogists can determine if the mineral has a similar or different fracture pattern, providing valuable information for identification.
5. Specific gravity: Glass has a known and relatively constant specific gravity, which is the ratio of the density of a substance to the density of water. By weighing an unknown mineral in air and then determining its weight in water, one can calculate its specific gravity. Comparing this value with the specific gravity of glass (approximately 2.5) can help narrow down the potential minerals.
In summary, glass is used as a reference material in mineral identification due to its well-defined physical properties such as hardness, streak, transparency, fracture or cleavage, and specific gravity. By comparing these properties between an unknown mineral and glass, mineralogists can make informed and accurate identifications.