A Study of the Mineral Gedrite Smith College Fall 20XX 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 0.020 ± 0.004, refractive indices α=1.644 ± 0.002, β=1.656 ± 0.002, γ=1.664 ± 0.002 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 ± 0.01. Its α=90.0° ± 0.0, β=90.0° ± 0.0, γ=90.0° ± 0.0 and unit cell volume equals 1,768.72 Å3 ± 0.09. 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 5.5-6 and density of 3.15 – 3.57g/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 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 2 2 2 by the equation δ= nγ-nα. The 2V was determined by the equation: cos Vz=nα (nγ - 2 2 2 2 nβ )/nβ (nγ -nα ), then multiply the answer by two. 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. g in air/(g in air-g in weight in air: 0.86g water 0.86/(0.86- weight in water: 0.59g 0.59)=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. 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 SiO2 60.085 48.53 0.807689 1.615378 14.08478 7.042388 TiO2 79.899 0.48 0.006008 0.012015 0.104762 0.052381 Al2O3 101.961 10.05 0.098567 0.295701 2.578273 1.718849 Fe2o3 159.692 0 0 0 0 FeO 71.846 16.14 0.224647 0.224647 1.958739 1.958739 MnO 70.937 0 0 0 0 MgO 40.311 18.68 0.463397 0.463397 4.040443 4.040443 CaO 56.079 0.44 0.007846 0.007846 0.068411 0.068411 Na2O 61.979 1.17 0.018877 0.018877 0.164595 0.32919 K2O 94.203 0 0 0 0 H20 18.015 0 0 0 0 TOTALS 95.49 1.627031 2.637862 23 15.2104 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 . 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) 10.7 8.26 75 (2100 (440) 27.63 3.23 50 (440) (610) 29.16 3.06 100 (610) (521) 30.9 2.89 50 (521) (161) 34.74 2.58 40 (161) (702) 48.52 1.88 60 (702) Cell Parameters: a=18.63 Å ESD α=90.0° ESD 0.03 =0.00 b=17.95 Å ESD β=90.0° ESD 0.03 =0.00 c=5.29 Å ESD γ=90.0° ESD =0.01 =0.00 Space group: Pmnb Unit Cell Volume: 1768.72 Å3 Z= 4 Crystal system: orthorhombic 3.5 Refractive Indices (n) Optic sign negative Refractive indices α: 1.644 ±0.002 β: 1.656 ±0.002 γ: 1.664 ±0.002 Max birefringence δ: 0.020 ± 0.004 Optic and (2V) 101 ± 8 The range for gedrite refractive indices according to William Nesse are: α=1.588 – 1.694, β=1.602 – 1.710 and γ=1.613 – 1.722. He also lists the maximum birefringence (δ) as 0.013 – 0.028 and the optic angle (2V) as 69 – 90. 3.6 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.