To determine which of the two milky white crystals is composed of plagioclase and which is quartz, you can use several techniques based on their physical properties. Here are some methods to distinguish between the two:
-
Hardness Test:
- Quartz has a hardness of 7 on the Mohs scale, while plagioclase has a hardness ranging from 6 to 6.5. You can perform a scratch test using a common mineral or object of known hardness. If you can scratch the crystal with the object (e.g., a steel nail), it is likely plagioclase. If it cannot be scratched, then it is likely quartz.
-
Cleavage vs. Fracture:
- Plagioclase exhibits two directions of cleavage, which means it can break along smooth planes. If you see flat surfaces or shiny faces on one of the crystals, that is likely plagioclase. Quartz, on the other hand, typically has a conchoidal fracture and does not exhibit cleavage in any direction.
-
Luster:
- While both crystals are milky white, quartz usually displays a glassy (vitreous) luster, while plagioclase can have a more pearly to dull luster, particularly if it is weathered or altered.
-
Transparency and Color:
- Although both crystals are milky white, if you examine them closely, quartz may be more transparent or translucent, while plagioclase might appear more opaque. You can also look for color zoning or striations, which are common in plagioclase.
-
Striations:
- If you closely examine the surface of the crystals, you may notice fine parallel lines (striations) on the surface of the plagioclase. Quartz will not have these striations.
-
Density:
- If you have access to a balance, you can calculate the density of each crystal. Quartz has a density of about 2.65 g/cm³, whereas plagioclase has a density that generally ranges from about 2.62 to 2.75 g/cm³. Measuring the mass and volume can help you determine which one is which based on their densities.
Using a combination of these methods should allow you to confidently determine which crystal is plagioclase and which is quartz.
1 answer