Lab4_Spectroscopy_24

Geo Sci 120: Geology of the Planets Name _________________ Spring 2024 LAB DUE: FRI, FEB 23, 11 PM LAB 4: SPECTROSCOPY OBJECTIVES for the overall lab: I. Review the basics of spectroscopy, including how to identify different materials using spectra. II. Develop an understanding of general spectroscopic features of rocks and minerals III. Learn how to identify specific rocks and minerals in the visible and near-infrared wavelength range BACKGROUND The above figures illustrate the electromagnetic radiation spectrum, including visible light. Spectroscopy is the study of the interaction between radiation and an object, as a function of wavelength. A spectrum is a 2-d plot of the intensity of radiation from an object vs. its 1

wavelength. [Note the word ‘ spectrum’ is singular, and ‘spectra’ is plural]. The radiation that is emitted from an object is modified by its compositional properties. The emitted radiation of a rock is controlled by its chemical makeup (the atoms and bonds within its minerals). Since the spectrum of a rock will change based on variations in rock chemistry, this is one way we can identify different rock compositions using spectra. In the visible wavelength region, the chemistry of an object is reflected in its color. This color strongly controls the shape of its spectra, as we will see later in lab. INTRODUCTION A trained geologist can gather diagnostic information just by looking at the color, texture, density, and other physical properties. A typical human eye will respond to wavelengths from about 380 to 750 nm (the ‘visible’ portion of the wavelength spectrum). To generate the data for the first part of this lab, we used a portable reflectance spectrometer to extend our “vision” to a slightly longer wavelength range. The ALTA II spectrometer wavelength range is 470-940 nm (or 0.47-0.94 m m -- see table below). In the second part, we will look at the reflectance spectra of minerals out to even longer wavelengths into the near-infrared. Useful Wavelength Units Converting nanometers to meters: 1 nm = 1x10 -9 m Converting microns to meters: 1 m m = 1x10 -6 m Converting microns to nanometers: 1 m m = 1000 nm Watch the lab video where your instructor demonstrates the use of the ALTA II spectrometer, and uses it to collect the data you’ll need for the 1 st part of the lab exercise. 1. Below is the data table generated using the ALTA II Reflectance Spectrometer on the orange and the basketball. It is your job to plot this data: either sketch and label their spectral shape on the plot below or graph it (with labels) in Excel if you are comfortable with using that software for graphing. Note that relative reflectance value is only needed to draw the spectral shape. While the overall shape of a spectrum should remain constant, it may shift up and down along the y-axis due to changing lighting conditions. Since these are RELATIVE values only, this is one of the ONLY times in this class you are not required to specify the units). Wavelength (nm) Basketball Orange 470 blue 80 180 525 cyan 113 375 560 green 133 545 585 yellow 236 1030 600 orange 344 1150 645 red 404 1380 700 deep red 302 1462 735 IR1 426 1233 810 IR2 376 1413 880 IR3 383 1130 940 IR4 360 1005 2

b) Now that you have visually examined the suite of minerals, use the ALTA II spectrometer data provided and the spectral library plot on the following page to identify the mineral name of each sample (A, B, C, and D).  Be aware that the wavelength unit of the plot is in microns ( m m) so you’ll need to convert your units to compare directly (see page 2 for the conversion factor).  A spectral library is just a set of spectra taken in a laboratory setting of a set of standard, already-identified minerals. (Note that while the reflectance value on the y-axis may be shifted up or down due to lighting conditions, the overall shape of the spectra must match).  Again, feel free to plot this in Excel if you’d like- the table is also provided in Excel format on Canvas. Wavelength (nm) Mineral A Mineral B Mineral C Mineral D 470 164 206 382 240 525 228 259 826 298 560 185 256 848 330 585 178 223 998 379 600 185 185 958 306 645 184 186 918 297 700 169 192 881 281 735 161 150 837 219 810 152 161 844 197 880 162 153 865 168 940 210 152 848 161 Reflectance 1000 900 800 700 600 500 Wavelength (nm) 4

Malachite Olivine Galena Chlorite Sulfur Fluorite Spectral Library Mineral Plot: Use this to identify the minerals associated with the graph you have drawn. Identify the mineral name of each sample (A, B, C, and D): Mineral A= Mineral B= Mineral C= Mineral D= 5

Part III Finally, we will look at an example spectrum from the planet Mars. CRISM (Compact Reconnaissance Imaging Spectrometer for Mars) is a spectrometer on board the Mars Reconnaissance Orbiter spacecraft, orbiting Mars. Planetary scientists have used the CRISM visible and near-infrared spectra to interpret some of the mineralogical variations on the surface of Mars, including the identification of new outcrops of phyllosilicates (clay minerals). Below is an example image that CRISM has taken in a region called Nili Fossae. CRISM scene from the Nili Fossae region We’ve taken spectra from both an area rich in clay:  “On Clay,” signified with red arrow, and  “Off Clay,” signified with green arrow in area with little or no clay Below we have them plotted together on the same graph. Note that they both have a similar overall slope, but that the “Off Clay” spectrum is more “bland” with no distinct absorption features. Off Clay (bland) On Clay N 5 km 7

To enhance the signature in the “On Clay” spectrum, as planetary spectroscopists often do, we will divide (ratio) the “On Clay” spectrum by the “Off Clay” (or bland) spectrum. This is done by dividing the reflectance value of every point in the “On Clay” spectrum by the reflectance of the “Off Clay” spectrum at the same wavelength. Ratioed Spectra = On clay spectrum Off clay spectrum Why is this important ? Dividing one spectrum by the other removes the features in common to both spectra (for example, the overall sloping of the spectra) and enhances the features that make them different. This gives us a so-called “ratioed spectrum” shown here called the CRISM Scene Ratioed Spectra . Finally, let’s compare the ratioed spectrum of our clay-rich outcrop with several different phyllosilicates (sheet silicates, including clays) from a spectral library (samples from Earth, measured in a laboratory spectrometer). On the figure below, we have the Mars clay-rich outcrop (“Outcrop”) plotted with other laboratory phyllosilicates (“SMECTITE”, “KAOLINITE”, and “ILLITE”). Note: smectite, kaolinite, and illite are common clay minerals on Earth . 8