Mineral Identification Lab Answer Key

Mineral Identification Lab: A Comprehensive Guide and Answer Key

Identifying minerals can seem like a daunting task, but with the right tools and knowledge, it becomes an exciting journey of discovery. This comprehensive guide serves as both a practical walkthrough of a typical mineral identification lab and a detailed answer key, assisting students and enthusiasts alike in mastering this essential skill in geology. Understanding mineral properties is key to unlocking the secrets of our planet's composition and history. This guide will cover the essential identification techniques, focusing on physical properties and providing example answers to common lab exercises.

Introduction to Mineral Identification

Mineral identification relies heavily on observing and testing various physical properties. These properties, while sometimes subtle, provide crucial clues to a mineral's identity. We'll explore the most common properties used in mineral identification:

• Color: While often a starting point, color can be deceptive as impurities can significantly alter a mineral's inherent color.
• Streak: This refers to the color of a mineral's powder when it's scratched across an unglazed porcelain plate. It's a more reliable indicator than color alone.
• Luster: This describes how light reflects off the mineral's surface, categorized as metallic (shiny like metal), vitreous (glassy), pearly, resinous, silky, etc.
• Hardness: Measured on the Mohs Hardness Scale (1-10, with 1 being talc and 10 being diamond), this indicates a mineral's resistance to scratching.
• Cleavage/Fracture: Cleavage refers to the tendency of a mineral to break along flat planes, while fracture describes its irregular breakage. Descriptions like perfect cleavage, good cleavage, or conchoidal fracture (shell-like) are used.
• Crystal Habit: This describes the typical shape of a mineral crystal, such as cubic, prismatic, bladed, etc.
• Specific Gravity: This measures the density of a mineral relative to water. It's determined by comparing the weight of the mineral in air to its weight in water.
• Other Properties: Some minerals exhibit unique properties like magnetism, fluorescence under UV light, reactivity with acid (e.g., calcite fizzing with dilute HCl), or taste (halite).

Lab Procedures and Techniques

A typical mineral identification lab involves a systematic approach. Here's a breakdown of the common steps:

1. Visual Observation: Begin by carefully observing the mineral specimen. Note its color, luster, crystal habit, and any other visible features. Sketch the specimen and record your observations.

2. Streak Test: Gently scratch the mineral across an unglazed porcelain streak plate. Note the color of the resulting streak.

3. Hardness Test: Use the Mohs Hardness Scale to determine the mineral's hardness. Attempt to scratch the mineral with common objects of known hardness (fingernail, copper penny, glass plate, steel file). Record which materials scratch the mineral and which do not.

4. Cleavage/Fracture Test: Attempt to break a small piece of the mineral (if allowed and safe). Observe the resulting fracture surfaces. Are they flat and planar (cleavage), or irregular (fracture)? Describe the cleavage/fracture quality.

5. Specific Gravity Test: If equipment is available, measure the specific gravity using a balance and a graduated cylinder. Record your findings.

6. Acid Test: If appropriate, test the mineral's reactivity with dilute hydrochloric acid (HCl). Observe any fizzing or other reactions.

7. Other Tests: Perform any other relevant tests based on the mineral's suspected identity. This might include checking for magnetism, fluorescence under UV light, or taste (only if safe).

8. Identification: Using a mineral identification chart or key, compare your observations to the properties of known minerals. Consider all the properties you've observed to narrow down the possibilities.

Example Mineral Identification Lab Questions and Answers

Let's consider a few examples of minerals you might encounter in a lab setting and how to identify them. Remember that these are examples and the exact properties observed may vary slightly due to mineral variations and sample quality.

Example 1: Mineral A

• Color: Yellow-brown to golden-yellow
• Streak: Yellow-brown
• Luster: Vitreous to resinous
• Hardness: 7
• Cleavage: Poor to none; conchoidal fracture
• Specific Gravity: ~2.65
• Other properties: None observed

Answer: Based on the properties, Mineral A is likely Quartz. Its hardness of 7, conchoidal fracture, and varied colors are characteristic.

Example 2: Mineral B

• Color: Gray to black
• Streak: Gray-black
• Luster: Metallic
• Hardness: 5-6
• Cleavage: Perfect cubic cleavage
• Specific Gravity: ~5.0
• Other properties: None observed

Answer: Mineral B exhibits properties consistent with Galena. Its metallic luster, cubic cleavage, and high specific gravity are key identifiers.

Example 3: Mineral C

• Color: White to colorless
• Streak: White
• Luster: Vitreous
• Hardness: 3
• Cleavage: Perfect rhombohedral cleavage
• Specific Gravity: ~2.7
• Other properties: Reacts strongly with dilute HCl (fizzes vigorously)

Answer: The reactivity with HCl, combined with the other properties, strongly suggests Calcite. The rhombohedral cleavage is a distinctive characteristic.

Example 4: Mineral D

• Color: Various (often green, but can be other colors)
• Streak: White to colorless
• Luster: Vitreous to pearly
• Hardness: 5.5 - 6.5
• Cleavage: Two directions at approximately 90 degrees
• Specific Gravity: ~2.7
• Other properties: None observed

Answer: The combination of hardness, cleavage, and varied color points towards Feldspar (specifically, likely one of the plagioclase feldspars). The wide range of colors means further analysis might be required for more precise identification.

Example 5: Mineral E

• Color: Brassy yellow
• Streak: Yellow-brown
• Luster: Metallic
• Hardness: 3-4
• Cleavage: Poor to none; uneven fracture
• Specific Gravity: ~15
• Other properties: Malleable, can be cut with a knife

Answer: The very high specific gravity, metallic luster, malleability, and brassy yellow color points to Gold. This is a relatively straightforward identification.

Advanced Mineral Identification Techniques

While the above techniques are sufficient for identifying many common minerals, more sophisticated methods exist for more complex scenarios:

• X-ray Diffraction (XRD): This powerful technique provides a definitive identification based on the unique crystal structure of the mineral. It's the gold standard for mineral identification.
• Optical Microscopy: Thin sections of minerals are examined under a petrographic microscope using polarized light, revealing information about the mineral's optical properties.
• Chemical Analysis: Techniques like atomic absorption spectroscopy (AAS) or inductively coupled plasma mass spectrometry (ICP-MS) can determine the precise chemical composition of a mineral.

Frequently Asked Questions (FAQ)

Q: What is the best way to organize my observations during a mineral identification lab?

A: Create a data table with columns for each property (color, streak, luster, hardness, cleavage/fracture, specific gravity, etc.) This makes comparing your findings to a mineral key much easier. Detailed sketches are also invaluable.

Q: What should I do if I can't identify a mineral using only physical properties?

A: If you're stumped, consult a more comprehensive mineral key or guide. Consider using more advanced techniques like XRD or optical microscopy if available. It's also good practice to check your observations to ensure accuracy.

Q: Are there any safety precautions to consider during a mineral identification lab?

A: Always wear safety goggles, especially when using acids. Be cautious when handling sharp or brittle minerals. Never taste minerals unless specifically instructed to do so (and even then, only with approved minerals).

Q: Where can I find more information on mineral identification?

A: Numerous books and online resources are available on mineralogy. Look for resources specifically designed for mineral identification or introductory geology texts.

Conclusion

Mineral identification is a fundamental skill in geology, offering a window into the Earth's composition and processes. By systematically observing and testing the physical properties of minerals, and by using the right tools and resources, you can unlock the secrets held within these fascinating natural materials. This guide provides a strong foundation for your mineral identification journey; practice and continued learning will further hone your skills. Remember to always approach mineral identification with patience, attention to detail, and a healthy dose of curiosity. The world of mineralogy is vast and rewarding – enjoy exploring it!