Imagine that you are given two milky white crystals, each about \(2 \mathrm{~cm}\) across. You are told that one of the crystals is plagioclase and the other is quartz. How can you determine which is which? (E)

Plagioclase is a member of the feldspar mineral group, well-known for its significance in both geological and commercial contexts. Its unique identifying features set it apart from other minerals.

Firstly, understanding plagioclase's composition is essential. It's composed of a series of minerals forming a solid solution between albite and anorthite. This results in variations in its chemical composition which, in turn, may slightly affect physical properties like hardness.

With regard to mineral identification, its Mohs hardness typically falls between 6 and 6.5. One could attempt gently scratching it with a steel nail. If the nail scratches the mineral, it suggests the mineral may indeed be plagioclase.

Additionally, examining the cleavage of plagioclase provides another clue. This mineral typically presents two distinct cleavage planes intersecting at nearly right angles, which can often be observed with the naked eye or under magnification. These planes reflect light differently, sometimes creating a characteristic 'striation' pattern on one of the cleavage surfaces, offering a visual cue.

In summary, by considering the mineral's hardness and distinct cleavage characteristics, one can confidently identify plagioclase among other white, milky crystals.

Quartz is one of the most abundant minerals found on Earth's surface, prized for both its aesthetic and durability. Unlike plagioclase, quartz is composed solely of silicon dioxide (SiO2), making it chemically pure.

One distinguishing feature of quartz is its noteworthy hardness of 7 on the Mohs scale. This means that, in a scratch test, common objects like a steel nail or glass plate should not be able to scratch its surface, distinguishing quartz from minerals with lower hardness such as plagioclase.

Another distinctive trait is quartz's lack of cleavage. When quartz breaks, it fractures in a pattern known as 'conchoidal fracture', creating curves rather than flat surfaces. This mode of breakage is related to the atomic structure of quartz and results in smooth, shell-like surfaces which are quite different from the straight cleavage planes found in plagioclase.

Identifying quartz, then, involves checking for resistance to scratching and observing the fracture pattern - both of these properties are key in differentiating it from other mineral candidates.

The Mohs hardness test is a qualitative method used to measure the scratch resistance of minerals, scaled from 1 (talc) to 10 (diamond).

When applying the Mohs hardness test as a method for mineral identification, you would use reference materials of known hardness. For our purposes, considering that a glass plate ranks about a 5.5 and a steel nail around a 6.5, we can compare these to our unknown crystal's resistance to scratching.

How to Perform the Test:

• Try scratching the mineral with a reference object.
• If the object scratches the mineral, the object is harder than the mineral.
• If the mineral scratches the object or remains unscratched, then the mineral is harder than the object.

This simple test can thus help distinguish between plagioclase and quartz. By using the glass plate or the steel nail as the reference point, one can tentatively place the crystal on the Mohs scale and identify it accordingly.

Mineral cleavage is a critical property in mineralogy, referring to the tendency of minerals to break along specific planar surfaces. These surfaces are related to the mineral's crystal structure and can vary in number and angle between each other.

When we consider cleavage in the context of identifying our two white crystals, we're focused on the quality and orientation of these planes. Plagioclase, for instance, has two-pronounced cleavage planes that meet at nearly 90 degrees, a result of its internal lattice structure.

Contrast that with quartz, which does not exhibit cleavage because of its crystal structure. Instead, quartz breaks with a 'conchoidal fracture', forming smoothly curved surfaces. This distinct difference is one of the most definitive ways to differentiate quartz from plagioclase without elaborate equipment.

Cleavage vs. Fracture:

• Cleavage refers to the splitting of minerals along specific planes.
• Fracture describes a break in a mineral that is not along a cleavage plane.

To perform a cleavage test, look at how a mineral breaks and assess if the breakage surfaces are planar or curved. This can help you distinguish between minerals with and without cleavage, such as plagioclase and quartz.