Plagioclase
Plagioclase is a group of minerals within the feldspar family, which is one of the most common mineral families in the Earth's crust. These minerals are key constituents in igneous rocks such as basalt and gabbro. Plagioclase is distinct for having a series of compositions ranging from calcium to sodium end members. The differences in composition can influence their physical properties such as color and hardness.
When it comes to identification, plagioclase minerals often exhibit striations on the cleavage faces, which can serve as a diagnostic feature. These minerals may show a bluish or grayish tint and have a pearly luster, characteristics that can aid in distinguishing them from other minerals.
Quartz
Quartz is one of the most abundant minerals on the Earth's surface. It is composed of silicon dioxide (SiO2), and is known for its high durability. This durability is due to its chemical structure, which makes it resistant to weathering and chemical alteration. Unlike plagioclase, it lacks any form of cleavage and instead shows a conchoidal fracture, which manifests as curved surfaces on broken pieces.
It's important for students to note that the physical appearance of quartz can greatly vary. While typically milky white, it can also appear in a variety of colors, referred to as varieties, such as purple amethyst or smoky quartz. The key feature for identification is its hardness along with the characteristic fracture pattern.
Mohs Hardness Scale
The Mohs Hardness Scale is a qualitative ordinal scale that characterizes the scratch resistance of various minerals. Developed in 1812 by the German geologist and mineralogist Friedrich Mohs, it's a tool that students can use to easily compare the hardness of minerals.
The scale ranges from 1 (talc) to 10 (diamond). Each mineral on the scale can scratch those below it. Plagioclase falls at a hardness of around 6, which is less than that of quartz, which stands at a hardness of 7. This difference can be tested with common items such as a steel nail, which, if it scratches the sample, indicates a softer mineral.
Mineral Cleavage
Mineral cleavage refers to a mineral's tendency to break along specific planar surfaces, creating flat facets. These cleavage planes are directly related to the crystal structure of the mineral. Minerals may exhibit cleavage in one, two, three, or more directions.
For instance, plagioclase feldspars exhibit two directions of cleavage at nearly right angles. Students should understand that cleavage planes are not random; they reflect the atomic arrangement within the crystal. By contrast, quartz does not have a cleavage since its crystalline structure is highly uniform without planes of weakness.
Streak Test
The streak test is a straightforward method for determining the color of a mineral in its powdered form. It involves rubbing the mineral on an unglazed porcelain tile, referred to as a streak plate. The streak color can be indicative of the mineral's identity and is especially useful when the mineral's surface color is variable and unreliable for identification.
Despite the fact that quartz and plagioclase may both leave a white streak, plagioclase might exhibit a slight gray streak due to the presence of certain other elements mixed within the mineral. Students using this method should note that the streak test is most useful for minerals softer than the porcelain plate, which typically has a Mohs hardness of about 7.
Mineral Luster
Luster describes the way light interacts with the surface of a mineral. This property can be quite useful in mineral identification, as it tells us a lot about the mineral's surface reflectivity. Students will encounter terms such as metallic, glassy (or vitreous), pearly, silky, and more, to describe luster.
Quartz, with its vitreous luster, can appear glass-like and shiny, reflecting light in a manner similar to glass. On the other hand, plagioclase often has a pearly luster, giving it a more subdued shine that can appear similar to a pearl's sheen. This distinction, in combination with other tests and observations, can help students confidently identify specific minerals.
