Single crystal X-ray diffractometer study of carbonate minerals and its relationship to their unit cells

This assignment is designed for an upper level, undergraduate course in Mineralogy, Crystallography, or Earth Materials. It can also be utilized for analytical courses, or research experiences.

Students should have been introduced to the concept of unit cells and basic mineral groups before they undertake this activity. An introduction to X-ray diffraction could occur prior to this activity, or, introduced during this activity. It would be helpful if the students had a basic understanding of solid solutions; however, this activity can be used to introduce the concept of atomic cation substitution and solid solutions if this topic has not been previously discussed in the course.

We utilize this activity as an assignment that takes one to two full lab periods of 2 hours and 30 minutes to complete.

The goals of this activity are: 1) to help students better understand the intangible concepts of unit cells; 2) students will make connections between unit cells and their relationship to solid solutions; and, 3) to investigate space groups, as well as other crystallographic parameters like axial ratio and unit cell volume. Axial ratio and unit cell volume will also change as smaller cations, like Mn, are substituted for larger cations thus furthering the connection to solid solutions.

This activity will help develop students' three-dimensional spatial recognition. It will help develop the connection between concepts of unit cells and solid solutions. Students will be able to use instrumentation to generate scientific data. They will be asked to apply that data graphically to help actualize their results. Students will be asked to make spatial connections to cationic radii, their positions within carbonates, and how that is related to distance, size, and volume.

Students will use an online mineral database to collect "expected" results.

Student will first be asked to look up the formula, expected space group, and unit cell parameters of both calcite and rhodochrosite. This can be done with an online mineral database, like Mindat; or, with mineral text books. This will allow them to have a knowledgebase, and set expectations for the outcomes of the experiment. They will then be given calcite and rhodochrosite in the lab. Both samples can be acquired through Wards Scientific or other mineral outlets. Students will then be asked to crush samples in a pestle and mortar to reduce the mineral size to < 1 mm in diameter. The samples will be analyzed under a stereomicroscope where one single crystal will be selected and mounted on the single crystal X-ray diffractometer for analysis. The samples will then be run on the single crystal X-ray diffractometer. Alternatively, a powder X-ray diffractometer may generate similar results, but depends on the instrument and software you may own. Depending on your instrument, this may take anywhere from a few minutes to several few hours. The results from the X-ray experiments will most likely not match the expected results 100%. This is because of a number of variables, like minor substitution, the temperature of mineral crystallization, or X-ray source, and temperature of data collection. This is an excellent opportunity to discuss how application, analysis, and synthesis of "real" data can vary depending on the formation of the mineral.

List of materials:

Computer or smart device with internet access; or, mineral text books such as Rock-forming Minerals. Either of these are needed to look up initial, or expected values.

Calcite

Rhodochrosite

Pestle and mortar

Stereomicroscope for selecting single crystal samples and supplies for mounting the samples on the SCXRD stage. Alternatively, clean area to prepare PXRD mounts.

Single crystal X-ray diffractometer (SCXRD). Alternatively, a powder X-ray diffractometer (PXRD) will generate similar results.

Single crystal X-ray diffractometer study of carbonates and its relationship to their unit cells (Microsoft Word 2007 (.docx) 3.6MB Aug17 21)

KEY_Single crystal diffractometer carbonate lab -- educator-only file

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Any carbonate mineral, such as siderite or magnesite, can be used with calcite instead of rhodochrosite. This is because of the large ionic radius of Ca²⁺, and the smaller ionic radius of Fe²⁺, Mn²⁺, Mg²⁺, etc. This will result in noticeably different sizes of unit cells. It is not recommended that students use calcite and aragonite because aragonite has an orthorhombic unit cell, while calcite and minerals within the calcite or dolomite mineral groups have a rhombohedral unit cell. If the instructor does not have access to a single crystal X-ray diffractometer, you may get similar or the same results with a powder X-ray diffractometer. But, this may be limited to your instrument and software you own. It may be possible to estimate the unit cell dimensions based on your powder X-ray diffractometer pattern's match to databases within software or published results.

Students have poor comprehension of unit cells, X-rays and diffraction, and solid solutions that result in cationic substitution can be difficult concepts at first. Prior instruction about these concepts will help make this lab more impactful.

We have utilized creative exercise assessment of Lewis et al. (2010) to construct pre- and post-testing questions framed around students' understanding of carbonate mineral research. Students demonstrated stronger comprehension of carbonate mineral unit cells, and commonly identified SCXRD as an instrument to study minerals as part of their post-test answers. We also utilized attitude assessment, modified after Lopatto (2010). This attitude assessment displayed that students had greater confidence in their scientific abilities, identified as a scientist, and had a desire to conduct more research after completing SCXRD work on carbonate minerals.

Used to develop pre- and post-test assessment of the students after utilizing the SCXRD to study carbonate minerals: Lewis, S. E., Shaw, J. L., and Freeman, K. A., 2010, Creative Exercises in General Chemistry: A Student-Centered Assessment: Journal of College Science Teaching. v. 40(1), p. 48-53.

Used to develop attitude assessment of the students after utilizing the SCXRD to study carbonate minerals: Lopatto, D., 2010, Science in Solution: The impact of undergraduate research on student learning: Council on Undergraduate Research, Washington, D.C., 113 p.

Mindat can be utilized to look up expected unit cell dimensions/volume, space group, formula, and other crystallographic parameters of the minerals you are considering to investigate with the students: https://www.mindat.org/

Traditional mineralogy books can be utilized to also look up expected unit cell dimensions/volume, space group, formula, and other crystallographic parameters of the minerals you are considering to investigate with the students: Chang, L.L.Y., Howie, R.A., and,

Zussman, J., 1998, Non-silicates: Sulphates, Carbonate, Phosphates, Halides: Rock-Forming Minerals, volume 5B, second edition, The Geological Society (London), 383 p.

The SERC Teaching Mineralogy with Crystal Structure Databases and Visualization Software site can be utilized to enhance this activity or your introduction to the lab which the students complete: https://serc.carleton.edu/7390

The SERC Single-crystal X-ray Diffraction website can be used to enhance this activity or provide background information for the students: https://serc.carleton.edu/207675