Orthoclase Feldspar Smith College Mineralogy, Fall 20XX

Transcription

1 Orthoclase Feldspar Smith College Mineralogy, Fall 20XX

2 Abstract A sample of orthoclase feldspar with composition of K (0.737) Na ( ) Ca ( )Al (1.0018) Si (3.0095) O (8.00) was collected on the Line Creek Plateau, Red Lodge, Montana. It was tested for physical properties, density, chemical composition, theoretical density, optical properties, unit cell parameters and synthesis behavior. The large pink prismatic crystals in the sample were found to have a Moh s hardness of 6.5, a specific gravity of 2.565, a theoretical density of 2.367g/cm 3, and at least two good cleavages. The refractive indices of this monoclinic mineral were n α = ±.005, n β = ±.005 and n γ= ±.005, with a Maximum birefringence, δ, or ±.005 and a 2V of ± The unit cell parameters were: a=8.525 Å ±.004 Å, b= Å ±.004 Å, c=7.200 Å ±.001 Å, α= ±.000, β= ±.03, γ= ±.000 with a unit cell volume Å 3 ±.5 Å 3. An unsuccessful synthesis was run of orthoclase that produced lucite at 1100 C. 1. Introduction to Orthoclase Feldspar Orthoclase Feldspar, one of the most common minerals in the earths crust, like all feldspars is tectosilicaceous meaning that all oxygen atoms are bonded to near by silicon aluminum tetrahedrons. It is monoclinic, commonly pastel pink, and can be found in both volcanic and metamorphic rocks. It is the potassium rich end member of the alkali feldspars in which albite is the calcium rich end member. Orthoclase is also one of three polymorphs with sanidine and microcline. The difference between the three is in the amount of order in the placement of the aluminum atom in each tetrahedral ring. Microcline cools slowly, and so has time to become completely ordered. Sanidine cools much faster; so it lacks the time to 2

3 establish any order. Orthoclase has partial order in the placement to the Aluminum in the t 1 and t 2 sites (Brady, 2008). 2. Experimental Procedures 2.1 Physical Description of the Sample The first step of observation was to collect all the data possible through careful visual inspection. In the case of some tests like Moh s hardness, and finding streak some simple tools were needed in order to collect the data. 2.2 Density The first, and simplest, way of calculating density was the next test performed. The following formula was used to measure the specific gravity of a small sample of orthoclase. G = ((weight in air) / (weight in air weight in ethanol)) x (ethanol temperature constant) 2.3 Chemical Composition and Theoretical Yield Using a scanning electron microscope, or SEM, the percent composition was measured. Using stoichiometry on a theoretical 100g sample, the ratio of moles in the mineral was calculated for each element. To calculate the final chemical formula, the assumption was made that the amount oxygen was equal to 8, the number in a perfect sample of orthoclase. All the other numbers were ratios of Optical Properties The optical properties, though they may be at first glance overlooked, give useful diagnostic information about a mineral. Because light travels through most minerals at different speeds depending on their orientation, measurements had to be taken to determine the positions of optic axis. With out this information correct refractive indices couldn t be measured. With a biaxial mineral with good cleavage, like orthoclase, when grains of the 3

4 mineral are poured onto a glass slide, it is probable that they will fall only into a few orientations. The side with the cleavage provides a more stable base. To observe all the orientations a grain must be mounted onto the end of a needle which is held in a spindle stage that allows for rotation. From extinction position data, Excalibur, a mathematical algorithm, calculated the stage and spindle stage positions at which to measure the refractive indices. With positions in hand, refractive indices can be measured by doing Becke Line Tests in refractive index oils. Each oil has a known refraction. When the mineral grain disappeared in the oil under plain polarized light, the oil and the mineral had the same refractive index. The only difficulty with this procedure is that the glue on the needle has a tendency to fail. The time lost when a mineral grain falls off the needle can be considerable. For this reason, two of the three refractive indices were collected without the Excalibur calculation. The final two were collected by mounting the needle along the optical normal, and then measuring the refractive indices of the two orientations with the greatest difference in refraction. 2.5 Unit Cell Parameters Scintag software and powder x-ray diffractometry were used to determine the parameters of the unit cell of the mineral. When compared with known values from previous samples in the LookPDF database, the 2 theta peak positions were used to calculate the size of the unit cell by using the Scintag unit cell calculator. 2.6 Synthesis One of the goals of this study was to recreate orthoclase. This could be done by mixing and then cooking salts, or by converting one mineral in to another by applying heat and pressure. In the case of this experiment the choice was made to heat ordered microcline. 4

5 Adding heat would reverse the ordering process and turn the microcline into orthoclase. X ray diffractometry would confirm what mineral was created. Two grams of powdered microcline were heated in the oven at 1100 C for two weeks while the transition took place. 3. Results 3.1 Physical Description of the Sample This sample of orthoclase feldspar (KAlSi 3 O 8 ) comes from a hand sample from the Line Creek Plateau, Red Lodge, Montana. The rock is an intrusive porphyry. The crystals of interest are the large pink hexagonal crystal that can be seen in the following image. Descriptions of other properties of the mineral follow the photograph. For scale, the large pink crystals are 1 to 1.5 cm across. Luster: Non-metallic, pearly Hardness: 6.5 to 7 Color: Pastel pink Streak: Creamy white Cleavage: In each broken crystal two cleavages could easily be observed dipping into the mineral. Each cleavage was approximately parallel to a separate face its crystal. 5

6 Habit: Large, short, hexagonal, prismatic crystals. 3.2 Density Collected Data Weight of Feldspar sample in air: g Weight of Feldspar sample in ethanol: g G ethanol : g/cc Calculations G = ((0.4220g) / (0.4220g g)) x (0.7875g/cc) = Chemical Composition Original SEM Data: Formula Calculations: Oxide GWF Wt. % Mole units Oxygen units Normalized units Atom units Na2O Al2O SiO K2O CaO Sums: oxygen in perfect formula/ sum oxy units = Final Formula: K (0.737) Na ( ) Ca ( ) Al (1.0018) Si (3.0095) O (8.00) Theoretical Density Calculations: Theoretical Density Z=4 Atoms Atom units Atomic wt. Wt. oxides per 1 formula amu Wt. oxide/ 1 unit cell amu Na Al Si

7 K Ca O Theoretical density=mass of one unit cell=mass of one molecular formula x Z volume of 1 unit cell volume of 1 unit cell Theoretical density= amu x x10-24 g 1amu = x10-21 g 718.3Å 3 x (1x10-8 cm) x10-22 cm 3 1Å 3 =2.367g/cm Optical Properties Refractive indices for this biaxial negative mineral were found to be: Measured Values n α ±.005 n β ±.005 n γ ±.005 Maximum ±.005 birefringence: δ Optic Angle:2V ±1.596 The range in values is due to the fact that in most cases the refractive indices were between two of the oils. In some cases it was possible to tell that the mineral s refractive index was closer to that of one oil than that of the other, but it was difficult to quantitatively determine to what extent this was true. Also, the refractive properties of the oils have an uncertainty of ± A printout from Excalibur, showing optic axis positions, can be seen in Appendix A, Figure Unit Cell Parameters 7

8 Below are the peak positions and relative intensities used by the Scintag software to calculate the unit cell parameters for this sample of orthoclase feldspar. LookPDF data was used to match the peaks to the hkl values. A print out from the unit cell calculation, as well as a peak display and card for orthoclase, can be seen in Appendix 1, Figure2-4. h k l 2 Theta Observed 2 Theta Calculated d value Relative intensity Unit Cell Parameters: a=8.525 Å ±.004 Å b= Å ±.004 Å c=7.200 Å ±.001 Å α= ±.000 β= ±.03 γ= ±.000 unit cell volume=718.3 Å 3 ±.5 Å 3 8

9 3.6 Synthesis The objective of this synthesis was to create orthoclase by heating microcline. The act of heating would reverse the ordering process that makes the difference between the two minerals. The first step was to make sure that the mineral sample was really microcline. This was done in two ways. The first way was by using optics. In a grain mount, the sample had the plaid-like twinning pattern unique to microcline. The second way was to use powder x-ray diffractometry. The locations of certain peaks would confirm that the sample really was microcline. The adjacent graph shows the relationship between chemical composition and peak position of the (-201) peak (Hovis, 1997). This sample s (-201) peak was at , which indicates that next to no sodium was present in the sample, and conversely that the sample was very potassium rich and possibly the potassium rich end member. Analysis from the SEM corroborated this result. The second component that needed to be checked was the ordering of the sample. This could be done by looking at the positions of the (131) and (-131) peaks. If the difference in the peak positions in the given order was close to than the sample was composed of 9

10 microcline (Hovis, 1997). The positions of the peaks were and consecutively with of difference of , which confirms that the sample was indeed microcline. When the sample was done cooking at 1100 C for two weeks, the hope was to have synthesized orthoclase, but the result also had to be confirmed. When the sample was removed from the crucible in which it was heated it appeared to have melted and resolidified during the cooling process. Instead of being loose grains, the sample was in a single solid mass, which had wetted the bottom of the crucible. When analyzed using powder x-ray diffractometry, the peaks lined up almost perfectly with the peak display for Lucite, a mineral that forms due to the incongruent melting of alkali feldspars. The peak displays and cards for the microcline and Lucite can be seen in Apendix1, Figures Discussion 4.1 Physical Description Accepted physical descriptions of orthoclase were very similar to those observed in the studied sample. Published and measured data described orthoclase as having a white streak (database), a Non-metallic, pearly or vitreous luster, a short prismatic columnar habit, and a color of pink (database). Other colors for orthoclase are also possible such as greenish, grayish yellow, or white (mindat.org, 2008). According to the Dyar, Gunter, Tasa Mineral Database orthoclase has perfect cleavage at {001} and {010} (Database). These refer to the two cleavages observed in the sample. The fact that data fit the sample so well was the first indication that the sample was really composed of orthoclase. 10

11 4.2 Density The measured specific gravity of orthoclase in this study was 2.565, which is well within the range of the accepted values, (mindat.org, 2008), for orthoclase. The similarity of the values suggests a good deal of accuracy in the measurement techniques. 4.3 Chemical Composition and Theoretical Density Alkali feldspars can occur in a range of compositions, as can be seen in the phase diagram (Nelson, 2003) which follows. The studied sample of orthoclase had a composition K (0.737) Na ( ) Ca ( ) Al (1.0018) Si (3.0095) O (8.000). The composition puts this sample in the region of immiscibility where alkali feldspars have been known to separate during cooling. Not only did the hand sample have a second whiter feldspar surrounding the large pink crystals, but the SEM data indicated that there were inclusions of a second mineral in the larger orthoclase crystals. Though never tested, it would be reasonable to hypothesize that the other mineral was a more sodium rich feldspar. When compared with measured and accepted values for density, the theoretical density of this sample, 2.367g/cm 3, was a little low. This is not surprising because few samples are perfect. Impurities and variation in composition throughout the mineral could lead to a difference in density. The accepted value for orthoclase, 2.55 g/cm g/cm 3, is for pure 11

12 orthoclase, but the studied sample has sodium as well as potassium. Since different elements have different masses, having any other elements would lead to a different density. The difference from the measured specific gravity, 2.565, may be due to the fact that the sample used to measure specific gravity wasn t completely pure. More than one mineral had grown together in the sample that were difficult to isolate from the desired orthoclase. 4.4 Optical Properties Values for the optical properties of orthoclase feldspar compared very closely with published data for the mineral. Published data and measured data can be found in the table below. The values for the refractive indices measured are all well within the possible range. The difference in the birefringence is most likely due to the error caused by the uncertainty related to the refractive index oils. The measurement would be more precise if it were possible to get a narrower range of values for each refractive index. The difference in the 2V for this biaxial negative mineral is probably from error in the values used in the Excalibur software. Extinctions for the mineral were not very complete so it was difficult to determine exactly at what position they occurred. Accepted Values Measured Values n α ±.005 n β ±.005 n γ ±.005 Maximum ±.005 birefringence: δ Optic Angle:2V 40-~ ±1.596 (Nesse, 1991) 4.5 Unit Cell Parameters In general, the observed peak positions were very close to the published data for orthoclase feldspar. All 2 theta values were within a hundredth of a degree of the calculated 12

13 values which, indicates that all the hkl values were assigned correctly. Accepted values for this mineral are as follows: a = Å, b = Å, c = Å, β = ; Unit Cell Volume= ų (mindat.org, 2008). These are quite close to the measured values of the mineral, a=8.525 Å ±.004 Å, b= Å ±.004 Å, c=7.200 Å ±.001 Å, β= ±.03 ; Unit Cell Volume = Å 3 ±.5 Å 3. Discrepancies in the values are most likely due to the fact that orthoclase is a solid state solution. Like any solution the ratios of each component are not fixed. The composition of the measured sample may be quite different from the sample used to calculate the accepted values. Atoms are different sizes so a different composition could lead to a different sized unit cell. From the SEM data it was known that the sample included some sodium unlike in pure orthoclase. This may explain the difference in unit cell measurements. Still, the values are close enough to call the mineral orthoclase, to confirm that it is monoclinic, and to establish it in the crystallographic space group B2/m (B1 1 2/m) [C2/m] {C1 2/m 1} (mindat.org, 2008). 4.6 Synthesis In this synthesis, new minerals were created, but they were not the single mineral desired. To synthesize orthoclase, as was the objective, the microcline sample could not melt during the heating process, as microcline melts incongruently. To convert microcline to sanidine Guy L. Hovis, in his paper Phase Fun with Feldspars, recommended heating microcline for three weeks at 1050 C. The idea in this experiment was to raise the temperature, and so make the reaction run faster. Also, the microcline would not have to make the complete conversion to sanidine since orthoclase is partially ordered. The problem most likely came from the fact that, according to the 1910 United States Geological Survey paper on feldspar deposits of the United States, the melting points of potassium-rich feldspars are not definite. 13

14 To synthesize orthoclase in the future, it would be important to keep the temperature of the oven well below the approximate melting point, so that the desired mineral could be produced. Conclusion In conclusion, the studies sample compared reasonably well with accepted measurements for orthoclase feldspar. Any discrepancies in the measurements are most likely caused by the difference in composition. Failure to properly synthesize orthoclase feldspar was due to a miscalculated attempt to hasten the cooking process. This study showed that even a small difference in cooking temperature can change the composition and structure of the product mineral. 14

15 Works Sited Bastin, Edson S. Economic Geology of the Feldspar Deposits of the United States. Bulletin 420 ed. Department of the Interior United States Geolgial Servey, Dec < rocline+melting+points&source=web&ots=zqafiujx0g&sig=tlesps1yeaoiz74xmw5mxt1 1yoc&hl=en&sa=X&oi=book_result&resnum=9&ct=result#PPA1,M1. Brady, John B. Lecture Notes Feldspars, November 2008 Dyar, Melinda Darby, Mickey E. Gunter, and Dennis Tasa. Interactive Mineralogy. DVD-ROM. Taos, NewMexico: Tasa Graphic Arts, Inc., Hovis, G. L. (1997) Phase fun with feldspars: Simple experiments to change the chemical composition, state of order, and crystal system. In Teaching Mineralogy, J. B. Brady, D. W. Mogk, and D. Perkins, eds., Mineralogical Society of America, Washington, D.C., pp Nesse, W. D. (1991) Manual of Mineralogy (21 st edition). John Wiley & Sons, Inc., NY, U.S.A. Nelson, Stehen A., Prof. "Two Component Phase Diagrams." Geology 212 Petrology course web page. 2 Apr Dec < images/phdifig4.gif&imgrefurl= &h=415&w=414&sz=18&tbnid=v1nhx7kpdxibpm::&tbnh=125&tbnw=125&prev=/images %3Fq%>. "Orthoclase." mindat.org. mindat.org. 24 Dec < Acknowledgements: Thanks so much to all my fellow mineralogy students for keeping in good spirits throughout the course of this project despite the challenges at hand. Also, thanks so much to John Brady for always providing the resources and information necessary to face each problem as it appeared and for his willingness to re-explain even the simplest of concepts. 15

16 Appendix A, Figure 1: Excalibur Printout 16

17 Figure 2: Orthoclase Peak Display 17

18 Figure 3: Orthoclase Card 18

19 Figure 4: Unit Cell Parameter Printout par ******************************************* \par * * \par * SCINTAG/USA LATTICE REFINEMENT PROGRAM * \par * 3.00-WINNT * \par ******************************************* \par CELL PARAMETERS: \par \par A = B = C = \par ESD A = ESD B = ESD C = \par \par ALPHA = BETA = GAMMA = \par ESD ALPHA =.000 ESD BETA =.029 ESD GAMMA =.000 \par VOLUME = \par CRYSTAL SYMMETRY SYSTEM: \par \par MONOCLINIC 2 \par H K L 2-THETA (DEG) Q = (1/D**2) INT(CPS)\cf1 \par OBS------CALC----DELTA OBS------CALC----DELTA \par \par \par \par \par \par \par \par \par \par H K L 2-THETA (DEG) D - SPACINGS INT(CPS)\cf1 \par OBS------CALC----DELTA OBS------CALC----DELTA \par \par \par \par \par \par \par \par \par \par END OF LATTICE REFINEMENT\cf0 19

20 Figure 5: Microcline Peak Display with Microcline Card Superimposed 20

21 Figure 6: Microcline Card 21

22 Figure 7: Microcline Peaks Used to Check the Composition and Ordering of the Mineral

23 Figure 8: Lucite Card

24 Figure 9: Synthesis Product, Lucite, with Lucite Card Superimposed

25 Figure 10: Microcline and Lucite Peak Displays Superimposed 25