A Study of Actinolite Smith College Mineralogy, Fall 20XX
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! ! ! ! ! ! ! ! ! !"#$%&'"()"*+$,-(.,$/" ! "#$%&!'())*+*! ,$-*./)(+01!2/))!34 ! ! XX ! ! ! ! ! ! ! ! ! ! ! Abstract A sample of actinolite from Chester, Windsor Co., Vermont was used in observation and determination of physical properties, density, cleavage, optical properties, unit cell parameters, chemical composition, and dehydration characteristics. The sample consists of glassy dark green, fibrous bladed prisms with approximate 55° and 125° cleavage, Mohs hardness range from 5 – 6, specific gravity of 3.06 g/cm3. The calculated unit cell parameters of the sample were a = 9.835 ± 0.008 Å, b = 18.04 ± 0.02 Å, c = 5.288 ± 0.003 Å, ! = 104.65°, volume = 908.20 Å3 ± 3.0. The sample is biaxial negative with refractive indices: n"=1.618 - 1.622, n!=1.628 - 1.632, n#=1.636 - 1.640, maximum birefringence: $=0.024, and optic angle range: 2V=79°- 83°. The sample decomposed into diopside and oxidized iron- silicate when heated to 1150°C. Introduction Actinolite is a double-chain silicate of the form C2/m (Mineral Data Publishing, 2001). It is a monoclinic calcic amphibole, which results from contact and regional +2 metamorphism (Nesse, 1986). The chemical formula [Ca2(Mg,Fe )5Si8O22(OH)2] implies a non-constant ratio of magnesium to iron. Actinolite is the name for any intermediate in the tremolite-ferroactinolite series: +2 Ca2Mg5Si8O22(OH)2 — Ca2Fe 5Si8O22(OH)2 Actinolite is classified as a member of this series with concentration of iron from 20% to 80% and less than 0.5 atoms of aluminum replacing silicon per formula unit (Deer, 1963). Actinolite is a dark green, fibrous amphibole with cleavage at 56° and 124°, perfect on {110} (Mineral Data). Optically, actinolite is biaxial negative and varies in pleochroism depending on iron concentration (Deer, 1963). This sample of actinolite is from Chester, Vermont (Appendix, Figure 1). It exhibits large, elongate crystals with distinct cleavage. Talc, a mineral associated with actinolite, can also be found in this sample. ! 2 Experimental Hand Sample Properties Qualitative physical properties of the hand sample (Appendix, Figure 1) were obtained. Streak color was determined using a porcelain streak plate. Hardness was determined standardized Mohs hardness test kit. Luster, habit, and fracture were observed visually. Approximate angles of cleavage were obtained using a goniometer and a petrographic scope. Specific gravity was determined with the formula ! = (weight in air / weight in air - weight in water). Mineral Identification using Scintag XRD The mineral species was identified using the Smith College Geology Department Scintag, Inc. X-Ray Diffractometer. Scanning for 2% was performed at rate of 1° per minute from 5° to 75° to yield a fingerprint pattern. Significant peaks were identified with the peak finder and were compared with standards using LookPDF software; a best- fit pattern search confirmed the identity of the mineral. Unit Cell The dimensions of the unit cell were obtained using x-ray diffraction. Values for 2% were collected at the rate of 1° per minute from 5° to 75°. Obtained values of 2% were compared with a mineral standard on LookPDF and were used to label hkl peaks on the x-ray diffraction pattern; the peaks were imported into Crystallography software to calculate the dimensions and volume of the unit cell. Estimated standard deviation (ESD) for volume was calculated using the formula: v= a*b*c*sin! (“Unit Cell Dimensions,” 2008). ! 3 Mineral Dehydration/Decomposition Two attempts were made at dehydration of the sample. Differential thermal analysis (DTA) was used to determine the temperatures of phase change of the sample at temperatures ranging from 0°C to 1000°C. Literature values for mineral decomposition were obtained (Deer, 1963) and sample (1.01 g) was placed in the oven at 1150°C for one hour. Compositional changes in final products from DTA and heating above 1000°C were obtained using Scintag, Inc. x-ray diffraction. Optical Properties Optical properties were obtained using a petrographic scope. Optic sign and axes were determined using the cross-polarized light and Bertrand lens. EXCALIBR software was used to determine 2V and to ensure proper orientation of the crystal on the spindle stage; because the mineral is biaxial, the spindle stage was needed to obtain values for the refractive indices of each mineral axis, which was determined using standardized refractive index oils. Birefringence was determined using values obtained of refractive indices. Pleochroic scheme was determined by observation of crystal color with respect to axis orientation. Chemical Composition Chemical composition of the mineral was determined using scanning electron microscopy (SEM). The SEM analysis delivered weight-percentages of each element as in the form of an oxide. The weight-percentage value obtained was divided by the formula weight of the oxide to determine the mole number. The mole number for each oxide was multiplied by the number of oxygen anions in each oxide unit to determine the oxygen number for each oxide unit in the formula. The total number of oxygen was ! 4 divided into 23, the total number of oxygens in the amphibole formula, to derive the normalization constant. The oxygen numbers for each oxide were multiplied by the normalization constant to determine the normalized oxygen number for the chemical formula. The normalized oxygen number was multiplied by the number of cations per oxygen anion for each respective oxide; this value was multiplied by the number of formulas per unit cell (Z) to determine the number of moles of each oxide in the chemical formula. The number of oxide moles in the chemical formula was multiplied by the oxide formula weight; the value for grams per “Z” moles was divided by Avagadro’s constant (6.023E+23) to determine the mass per formula unit cell. The mass per unit cell was divided by the unit cell volume in cubic angstroms (obtained from unit cell calculations) and was converted to grams per cubic centimeter to determine the density of the mineral. Density was compared to specific gravity obtained empirically and to theoretical standard values for actinolite (Mineralogical Society of America, 2001). Results Hand Sample Properties The color of the sample is dark green and the sample is pale green when streaked or powdered. Mohs hardness tests against standards placed the sample between 5 and 6 (between apatite and orthoclase). The sample has a vitreous luster and a columnar, fibrous, prismatic habit, and is brittle. Angles of cleavages measured approximately 55° and 125°. Calculated specific gravity for the mineral is 3.06 g/cm3. ! 5 Mineral Identification using Scintag XRD Scanning was performed at the rate of 1° per minute to obtain an x-ray diffraction pattern for the sample. Peaks were identified using the peak finder and were exported to LookPDF to identify the best-fit pattern (Appendix, Figure 2). Unit Cell Data Table 1: Values of 2% obtained from x-ray diffraction using Scintag, Inc. Peak Finder (Appendix, Figure 3); corresponding peaks labeled in comparison with actinolite standard (Appendix, Figure 4). 2% Intensity h k l 9.7881 7 0 2 0 10.5288 78 -1 1 0 17.4256 5 0 0 1 18.1656 4 -1 1 1 18.6719 3 2 0 0 19.6338 4 0 4 0 22.9000 3 -1 3 1 26.3006 6 1 3 1 27.2300 13 -2 4 0 28.6087 100 2 0 1 30.3800 6 2 2 1 33.0563 17 1 5 1 34.5619 5 0 6 1 35.3919 4 -2 0 2 37.7675 5 4 0 0 38.5244 12 -3 5 1 41.7219 4 2 6 1 44.9000 3 -4 0 2 58.2631 7 -1 5 3 60.4550 14 -3 5 3 Table 2: Unit Cell Parameters determined using Crystallography program (Figure 5). a 9.835 ± 0.008 b 18.04 ± 0.02 c 5.288 ± 0.003 ! 104.65° ± 0.06 volume 908.20 Å3 ± 3.0 ! 6 Table 3: Literature values for unit cell dimensions of actinolite (Mineralogical Society of America, 2001, and LookPDF standard). a 9.891 9.83 ± 0.003 b 18.200 18.067 ± 0.012 c 5.305 5.2837 ± 0.0005 ! 104.64° 104.65° ± 0.01 Mineral Decomposition/Dehydration Figure 1: High temperature differential thermal analysis, low sensitivity (run 1); voltage (mV) with T (°C) Figure 2: High temperature differential thermal analysis, high sensitivity (run 2); voltage (mV) with T (°C) ! 7 Table 4: Percent structural water lost in dehydration of actinolite. Mass before heating Mass after heating at 1150°C Percent H2O 1.01 g 0.98 g 3% Differential thermal analysis was performed at low sensitivity and high sensitivity (Table 1, Table 2). Products after DTA exhibited a color change from pale green to red- brown. X-ray diffraction patterns were obtained for DTA products and were analyzed using LookPDF standard patterns (Appendix; Figure 6, Figure 7). Dehydration by heating to 1150°C also resulted in a color change from pale green to brown. Change in mass before heating and after heating was measured and used to calculate the approximate percent structural water liberated through dehydration (Table 4). X-ray diffraction of the 1150°C dehydration product was performed and identified using LookPDF standard patterns (Appendix, Figure 8). Optical Properties Data Table 5: Optical properties of sample obtained with the use of petrographic scope, refractive index oils, and EXCALIBR software; empirical compared with literature values for refractive indices, 2V (Mineral Data Publishing, 2001), and maximum birefringence (Mindat.org, 2008). optical axes, sign biaxial negative n" 1.620 ± 0.002 n! 1.630 ± 0.002 n# 1.638 ± 0.002 birefringence ($) 0.020 ± 0.004 2V 79° to 83° pleochroic scheme n": colorless, n!: pale blue-green, n#: blue-green literature actinolite RI "= 1.613–1.646, ! =1.624–1.656, # = 1.636–1.666 literature actinolite 2V 79° to 86° literature max.