Question

A geology student comes back from a field trip with several minerals that he wants to identify. Performing mineral tests in the laboratory, he lists the following properties that describe each mineral. Name the minerals. a. A white mineral with a relative hardness of 1 . b. A translucent mineral that scratches the streak plate. c. A black mineral with cleavage in one direction. d. A mineral reacting with a weak acid. e. A cubic mineral with a salty taste.

Short answer

a. Talc, b. Quartz, c. Biotite, d. Calcite, e. Halite.

Step-by-step solution

Identify Soft White Mineral

The mineral described as a white mineral with a relative hardness of 1 is likely to be Talc. Talc is known for being the softest mineral on the Mohs scale, which makes it easy to scratch and generally appears in white or greenish-white colors.

Identify Hard Translucent Mineral

A mineral that is translucent and is able to scratch the streak plate is likely Quartz. Quartz is common, often translucent, and has a hardness of 7 on the Mohs scale, allowing it to scratch a streak plate which typically has a hardness of 6.5.

Identify Black Mineral with Cleavage

The black mineral exhibiting cleavage in one direction is likely Biotite. Biotite is a type of mica that commonly appears black and has a distinctive cleavage pattern that allows it to easily split into thin sheets.

Identify Mineral Reacting with Acid

When a mineral reacts with a weak acid, such as hydrochloric acid, it often indicates the presence of Calcite. Calcite reacts with acid by effervescing or bubbling due to the release of carbon dioxide gas.

Identify Cubic Mineral with Salty Taste

The mineral described as cubic in structure with a salty taste is Halite. Halite is the mineral form of sodium chloride (NaCl), commonly known as rock salt, and is characteristically cubic.

Key concepts

Mohs scale of hardness

The Mohs scale of hardness is an invaluable tool for geologists seeking to identify minerals through their scratch resistance. Developed by Friedrich Mohs in 1812, this scale ranks minerals from 1, the softest, to 10, the hardest. The softness or hardness is determined by whether the mineral can be scratched by common items or other minerals.
For example, talc is rated the softest mineral at 1, easily scratched by a fingernail. On the other hand, minerals like quartz, with a hardness of 7, can scratch a steel streak plate. The hardest mineral, diamond, with a score of 10, remains unscratched by all other substances. Using this scale, students can quickly ascertain and compare the relative hardness of various minerals they encounter.

• 1 - Talc
• 7 - Quartz
• 10 - Diamond

Understanding the Mohs scale not only helps in identifying minerals but also gives insight into how they will resist wear and tear in different environments.

Mineral cleavage

Mineral cleavage is a key identifier used in mineralogy, referring to how a mineral breaks along specific internal planes. These planes are dictated by its crystal structure, determining how easily and neatly a mineral can split. When examining minerals, observing the direction and ease of this breakage can provide vital clues about its identity.
For instance, biotite, a black mica, exhibits perfect cleavage in one direction. This means it can be effortlessly split into thin, flexible sheets. Such cleavage properties are pivotal in distinguishing minerals that might appear similar in color or texture but break differently.

• Cleavage directions often align with weaker bonds in the crystal lattice.
• Not to be confused with fracture patterns, which are irregular.

Recognizing cleavage allows students to differentiate between minerals, ensuring an accurate identification in both educational and practical applications.

Effervescence in acid

Effervescence in acid is a classic test in mineral identification, especially useful for detecting certain carbonates. When a mineral, like calcite, is exposed to a weak acid such as hydrochloric acid, it will often react by fizzing or bubbling. This reaction is due to the release of carbon dioxide gas.
Calcite, with its formula \( ext{CaCO}_3 \), is the most common mineral to display this behavior. Upon contact with acid, the reaction proceeds as: \( ext{CaCO}_3 + 2 ext{HCl} ightarrow ext{CaCl}_2 + ext{CO}_2 + ext{H}_2 ext{O} \). The visible bubbles are carbon dioxide, indicating the presence of carbonate in the mineral.

• This reaction is not only diagnostic but also visually striking, aiding in quick decision-making while identifying minerals.
• However, some non-carbonate minerals may also effervesce, requiring careful analysis.

Effervescence helps students understand the chemical properties of minerals, providing a crucial link between theoretical knowledge and practical testing.

Halite structure

The halite structure is a fascinating aspect of its identity, illustrating the close relationship between mineral chemistry and visible physical traits. Halite is the mineral form of sodium chloride (\( ext{NaCl} \)), better known as common table salt. This occasionally translucent mineral forms in a highly distinct cubic architecture.
Each halite crystal features a repeating pattern of tiny cubes, a direct consequence of its sodium and chloride ions arranging symmetrically. This cubic structure imparts a smooth, salty taste that is unmistakable but should only be observed safely in a lab setting. The taste test isn't always recommended without knowing exactly what mineral you're dealing with!

• Halite's crystalline structure stems from its atomic arrangement, leading to a perfect cubic cleavage.
• This structure ensures each cut is consistent with its fundamental geometry.

Understanding halite’s structural features enriches a geology student's comprehension of mineral formation, extending beyond mere visual observation to appreciating the crystalline architecture underlying its shape.