A Study of the Mineral Gedrite. Smith College. Fall 20

Transcription

1 A Study of the Mineral Gedrite Smith College Fall 20

2 Abstract A gedrite sample from the Tobacco Root Mountains in southwest Montana was tested for physical characteristics, density, chemical composition, refractive indices, unit cell parameters and decomposition reactions. The results show that it has a Moh s hardness between 5 and 6, {210} cleavage, density of 3.19g/cc ±0.05, birefringence (δ) of ± 0.004, refractive indices α=1.644 ± 0.002, β=1.656 ± 0.002, γ=1.664 ± and a 2V equal to 101 ± 8. The unit cell parameters are a=18.63å ± 0.03, b=17.95å ± 0.03 and c=5.29å ± Its α=90.0 ± 0.0, β=90.0 ± 0.0, γ=90.0 ± 0.0 and unit cell volume equals 1, Å 3 ± Decomposition occurred after four tries between 1000 and 1030 C. 1. Introduction Gedrite is an orthorhombic chain silicate in the amphibole group. Sites M1, M2 and M3 are filled with Magnesium and Aluminum while site M4 is filled with varying amounts of Magnesium and Iron. The A-space can be filled with Sodium, but does not need to be. Gedrite s accepted formula is Na(Mg,Fe)6Al(AlSi7O22)(OH)2 (John Brady 2008). It forms a solid solution with anthophyllite (figure 1), which has less Aluminum and more Silica(Deer at al 1963). It is biaxial positive and has an excellent cleavage along the c-axis (figure 2). It is light green to brown in color with a prismatic habit and vitreous luster. It has a white streak, hardness of and density of g/cc. It is found in metamorphic schists (Simon et al, 1977). The sample used was found in the Tobacco Root Mountains in southwestern Montana. It is brown and mixed with garnets and quartz (only the gedrite is pictured in figure 3). The Tobacco Root Mountains are Precambrian and the layer with gedrite is considered to have once been oceanic crust (Brady, et al). 2. Experimental Procedures 2.1 Physical Description of Sample Luster, color, streak, habit and crystal size were determined by visual observation. Streak was found by scratching the mineral on a white ceramic plate. Scratch tests also determined Moh s hardness. 2.2 Density

3 Density was determined in two ways. One was to find the specific gravity, which is said to be equal to density. First, a 1.5cm piece was weighed in a hanging tray. Then, it was weighed in a hanging tray immersed in water. The specific gravity was calculated with the equation: D=weight in air/weight in air-weight in water. The process was repeated three times to ensure accuracy. The second method was to calculate the density using data collected by the Scanning Electron Microscope (SEM). The total mass per unit cell, derived from compound percentages, was divided by the unit cell volume. 2.3 Chemical Composition The Scanning Electron Microscope (SEM) was used to find the percentage of compounds in the sample. The components were reported as oxides. Scan results were treated as though they were weight percents, which were then turned into moles of the oxides. The mole ratios were used to determine the empirical formula for the sample. 2.4 Unit Cell Parameters A small amount was ground into a powder fine enough to go through a 100 grade mesh sieve. The powder was x-rayed by the Scintag X-ray Diffractometer. Data on 2-theta values, cell volume, angle of optic axis and peak values was collected and evaluated by the Scintag software. Peaks were compared to those in the database of previously analyzed minerals to find a match. Once a match was found, their 2-theta values were used by the Scintag software to calculate the unit cell volume. 2.5 Refractive Indices (n) A 2mm crystal was glued to the tip of the needle of spindle stage. Its c-axis was parallel enough to the stage to be used for measuring refractive indices. Once the directions were determined, oils with increasing refractive indices were applied to the crystal until Becke lines moved out instead of in. Maximum birefringence (δ) was found by the equation δ= nγ-nα. The 2V was determined by the equation: cos 2 Vz=nα 2 (nγ 2 - nβ 2 )/nβ 2 (nγ 2 -nα 2 ), then multiply the answer by two.

4 2.6 Decomposition A 0.411g ±0.005 sample was crushed to powder small enough to go through 100 grade mesh sieve then put into a crucible. Sample was put through XRD for starting point information. It was heated at 900 C, 950 C, 1000 C and 1030 C for one hour at each temperature. Sample was put into the XRD after every heating session to see if anything had changed. 3. Results 3.1 Physical Description of Sample The sample of gedrite studied was found to have a vitreous luster, prismatic habit and hardness between five and six. It is made of 2-5mm crystal medium to dark brown in color. It has a white streak and very good cleavages along the c-axis, {201}. 3.2 Density The density measured by weighing the sample was found to be 3.19 g/cc ± 0.05 using the following data and formula. weight in air: 0.86g weight in water: 0.59g g in air/(g in air-g in water 0.86/( )=3.19g/cc The density calculated from the SEM data was found to be 3.13g/cc. The mass of each oxide was calculated by multiplying the gram formula weight by the oxide units. The mass was then multiplied by the number of formulas per unit cell, Z=4 in the case of gedrite, and then divided by Avogadro s number. This gives the mass per unit cell for each oxide. The oxides were totaled and then divided by the unit cell volume converted from cubic angstroms to cubic centimeters. Figure 9 is a spreadsheet with the calculations.

5 3.3 Chemical Composition The following table is based on data collected by the SEM. See figure 9 for original spreadsheet. Mineral formula calculation and comparison Oxygens per formula= OXY= 23 Mole Oxygen Normalized Atom Oxide GFW Wt% Units Units Ox Units Units SiO TiO Al2O Fe2o FeO MnO MgO CaO Na2O K2O H TOTALS Each of the oxide weight percents were divided by the gram formula weight of itself (mole units). That number was multiplied by the number of oxygens in the oxide formula; the result is called oxygen units in the table. The oxygen units were then multiplied by the number of oxygens in the accepted mineral formula to normalize the oxygen units. Finally, the normalized oxygen units were multiplied by the number of cations in the oxide formulas. Those units are applied to the element in the mineral formula like so: (Na2O)o.33 (MgO)4.04 (Al2O3)1.72 (SiO2)7.04 (CaO)0.07 (TiO2)0.05 (FeO)1.96 becomes Ti0.05 Ca0.07 Na0.66 (Mg4.04, Fe1.96) Al2.40(AlSi7.00O22.00) (OH)2.00.

6 3.4 Unit Cell Unit cell parameters were calculated by the Scintag Software with peak labels found on the pdf card that matched best with diffraction results. Following are lists of the data and figures 4-6 are the diffraction graph, cell parameter calculation output and the pdf card used for calculations. relative intensity 2- d- (1 - Peaks: theta: value: 100): peaks: (210) (2100 (440) (440) (610) (610) (521) (521) (161) (161) (702) (702) Cell Parameters: a=18.63 Å ESD 0.03 α=90.0 ESD =0.00 b=17.95 Å ESD 0.03 β=90.0 ESD =0.00 c=5.29 Å ESD =0.01 γ=90.0 ESD =0.00 Space group: Pmnb Unit Cell Volume: Å 3 Z= 4 Crystal system: orthorhombic 3.5 Refractive Indices (n) Optic sign negative Refractive indices α: ±0.002 β: ±0.002 γ: ±0.002 Max birefringence δ: ± Optic and (2V) 101 ± 8 The range for gedrite refractive indices according to William Nesse are: α= , β= and γ=1.613

7 He also lists the maximum birefringence (δ) as and the optic angle (2V) as Decomposition Oxidation occurred after the first heating (900 C) and then x-ray diffraction results remained the same after subsequent heatings (950 and 1000 C) until the forth heating (1030 C). A noticeable change was that the (201) and (610) peaks disappeared while other changes were subtle. For example, the (521) and (161) peaks shifted. The starting weight was 0.411g ± and the ending weight was 0.371g ± The difference was 0.040g±0.005 and the percent change was 9.8%. Figure 7 in the appendix is a printout comparing XRD readings. Discussion 4.1 Physical Description of Sample Mindat.org describes gedrite as having a vitreous luster, a hardness of 5.5-6, pale green-grey to brown and a white streak. The physical properties observed in the study sample matched well with this. AA Mineral Specimens describes the habit as massivefibrous, distinctly fibrous and fine grained forms. Crystals of this sample have more of a prismatic appearance because they start at a point and splay out on the other end. A picture of the sample and a drawing of the crystal form can be seen in the appendix (figs. 2 and 3). 4.2 Density William Nesse gives the density range for gedrite as Both of the measured (3.19) and the calculated (3.13) densities are in the middle of that range. This is probably due to the iron, titanium and calcium components. Titanium and calcium, being common impurities, will have added a small amount of weight that would not normally be an issue for purer samples. The iron, on the other hand, is a normal part of end member formulas. Iron is heavier than the magnesium it replaces in the formula so samples with higher iron content will be heavier than those with more

8 magnesium. The iron content of the study sample is low to moderate, according to the solid solution series in figure Chemical Composition The accepted formula for gedrite is Na(Mg,Fe)6Al(AlSi7O22)(OH)2. The formula calculated from the SEM data is Ti0.05 Ca0.07 Na0.66 (Mg4.04, Fe1.96) Al2.40(AlSi7.00O22.00) (OH)2.00. There are small amounts of titanium and calcium in the samples formula, which do not appear in the accepted formula but are typical impurities for gedrite. There is more aluminum in this sample and it has more iron than the purer gedrite (see figure 1). Also, roughly half of the A-spaces in this sample are filled. The sample s formula is still very similar to the accepted gedrite formula. 4.4 Unit Cell Parameters According to mindat.org, gedrite is categorized as an orthorhombic amphibole chain silicate. Accepted unit cell parameters of gedrite are a=18.59 angstroms, b=17.89 angstroms, c=5.3 angstroms and a unit cell volume of 1, cubic angstroms. The findings for the study sample are: a=18.63 angstroms, b=17.95 angstroms, c=5.29 angstroms and cell volume = 1, cubic angstroms. The experimental values are all within 1 angstrom of the accepted values except for the unit cell volume, which is till rather close. The unit cell may be larger due to: 1) the larger size of the iron compared to the magnesium atoms they replaced, 2) sodium in the A-space and3) titanium and calcium impurities. 4.5 Refractive Indices In Introduction to Optical Mineralogy, William Nesse gives the refractive indices range for gedrite as: α= , β= and γ= He lists the maximum birefringence as and the optic angle as The finding for this sample of gedrite (α=1.644, β=1.656 and γ=1.664, δ=0.020 and 2V=101) are mid-range except the optic angle which is at the highest end of its range. Refractive indices increase as electron content increases. The larger iron atoms, sodium, along with the

9 titanium and calcium impurities could be the reason this sample has slightly higher refractive indices. 4.6 Decomposition The study sample changes between 1000ûC and 1030ûC and an XRD scan shows that it shares five peaks with enstatite. The authors of Rock Forming Minerals volume 2 reported their gedrite sample as changing around 1020ûC and that it also changed into enstatite. Figure 8 shows the matching peaks by overlapping the XRD scan with the pdf card. Conclusion The sample of gedrite studied was found to have properties within the appropriated ranges of properties studied by others in the field. Its iron content places toward the middle of the solid solution gedrite makes with ferrogedrite. When dehydrated it becomes enstatite. References Brady, John B.; Mineralogy class lecture and handouts; Smith College 2008

10 Burger, H. Robert; Brady, John B.; Cheney, John T.; Harms Tekla A; Archean and Proterozoic history of metamorphic rock, Tobacco Root Mountains, southwestern Montana; 2004 keckgeology.org/files/pdf/symvol/11th/montana/brady_et_al.pdf Elkins Lynne; A Study of the Mineral Diopside; Smith college; Fall 1998; use of paper format Mindat; Nesse, William D.; 1986 Introduction to Optical Mineralogy, (1 st edition); Oxford University Press, NY, USA p212 Prinz, Martin; Harlow, Geairg; Peters, Joseph; Simon and Schuster s Guide to Rocks and Minerals

11 Appendix Gedrit Ferrogedrit Study Sampl Anthophyllit Ferro-anthophyllite Figure 1 The solid solution series of gedrite and anthophyllite from Rock Forming Minerals vol. 2: Chain Silicates by Deer, Howie and Zussman. I have put a box in the general area where my sample goes. It has almost two iron atoms in its calculated formula so it is a little toward the

12 Figure 2 A picture of the crystal shape and indices directions from Introduction to Optical Figure 3 A chunk of gedrite crystals with a pencil for scale. The crystals are 2-5mm long.

13 Figure 4 A graph of the x-ray diffractometer results with major diffraction lines, their relative intensities and their Miller indices.

14 Figure 5 program. Cell parameter calculations done by Scintag

15 Figure 6 samples. Pdf card of mineral that had peaks matching my Peak values on this card were used to calculate unit cell

16 Figure 7 Compare results of heated samples.

17 Figure 8 XRD scan of sample at 1030 C for one hour overlapping the pdf peaks for enstatite.

18 Figure 9. SEM results with formula and density calculations.