Project Mineral Final Paper
Orthoclase Feldspar Smith College Mineralogy, Fall 20XX
Abstract
A sample of orthoclase feldspar with composition of K(0.737)Na(0.22218)Ca(-
0.0012)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/cm3, and at least two good cleavages. The refractive indices of this
monoclinic mineral were nα=1.516-1.520±.005, nβ =1.520-1.524±.005 and nγ=1.524-
1.528±.005, with a Maximum birefringence, δ, or 0.004-0.012 ±.005 and a 2V of
37.411±1.596. The unit cell parameters were: a=8.525 Å ± .004 Å, b=13.028 Å ± .004 Å,
c=7.200 Å ± .001 Å, α=90.000° ±.000°, β=116.06° ± .03°, γ=90.000°± .000° with a unit cell
volume 718.3 Å3 ± .5 Å3. 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 establish any order. Orthoclase has partial order in the placement to the Aluminum in the t1 and
t2 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 8.
2.4 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 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 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 (KAlSi3O8) 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
Habit: Large, short, hexagonal, prismatic crystals.
3.2 Density
Collected Data
Weight of Feldspar sample in air: 0.4220g
Weight of Feldspar sample in ethanol: 0.2924g
Gethanol: 0.7875 g/cc
Calculations
G = ((0.4220g) / (0.4220g –0.2924g)) x (0.7875g/cc) =2.565
3.3 Chemical Composition
Original SEM Data: Formula Calculations:
Mole Oxygen Normalized Atom Oxide GWF Wt. % units units units units Na2O 61.979 2.53 0.04085 0.04085 0.111089155 0.22218 Al2O3 101.961 18.78 0.18418 0.55254 1.50264174 1.00176 SiO2 60.085 66.49 1.10663 2.21327 6.018992675 3.0095 K2O 94.203 12.76 0.1355 0.1355 0.368498078 0.737 CaO 56.079 -0.02 -0.0004 -0.0004 -0.00121182 -0.0012
Sums: 100.54 1.46672 2.94171 8.000009827 4.96922
8 oxygen in perfect formula/ sum oxy units =2.7195066
Final Formula: K(0.737)Na(0.22218)Ca(-0.0012)Al(1.0018)Si(3.0095)O(8.00)
Theoretical Density Calculations:
Theoretical Density Z=4 Wt. oxide/ Atom Atomic Wt. oxides per 1 unit cell Atoms units wt. 1 formula amu amu Na 0.444357 22.98977 10.21565653 40.8626261 Al 0.667841 26.98154 18.01937121 72.0774848 Si 1.504748 28.0855 42.26160469 169.046419
6 K 1.473992 39.0983 57.63059363 230.522375 Ca -0.00121 40.078 -0.048567391 -0.1942696 O 8.000009 15.9994 127.995344 511.981376
256.12257 1024.49028
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=1024.49028amu x 1.65979x10-24g 1amu =1.70044x10-21g 718.3Å3 x (1x10-8cm)3 7.183x10-22cm3 1Å3 =2.367g/cm3
3.4 Optical Properties
Refractive indices for this biaxial negative mineral were found to be:
Measured Values nα 1.516-1.520±.005 nβ 1.520-1.524±.005 nγ 1.524-1.528±.005 Maximum 0.004-0.012 ±.005 birefringence: δ Optic Angle:2V 37.411±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 ± 0.005. A printout from Excalibur, showing optic axis positions, can be seen in
Appendix A, Figure 1.
3.5 Unit Cell Parameters
7 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.
2 Theta 2 Theta d Relative h k l Observed Calculated value intensity 1 3 0 23.53 23.53 0.07 100 -1 1 2 25.64 25.66 0.082 9.96 2 2 0 26.99 26.99 0.092 8.25 -2 0 2 27.16 27.15 0.093 5.46 0 4 0 27.36 27.36 0.094 5.45 0 0 2 27.57 27.56 0.096 11.78 1 3 1 29.84 29.84 0.112 0.9 -1 3 2 32.29 32.28 0.13 5.1 -1 1 3 38.68 38.68 0.185 4.23
Unit Cell Parameters: a=8.525 Å ± .004 Å b=13.028 Å ± .004 Å c=7.200 Å ± .001 Å
α=90.000° ±.000° β=116.06° ± .03° γ=90.000°± .000° unit cell volume=718.3 Å3 ± .5 Å3
8
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 21.8797, 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 -0.81 than the sample was composed of
9 microcline (Hovis, 1997). The positions of the peaks were 29.2618 and 30.0390 consecutively
with of difference of -0.7772, 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 5-10.
4. 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.
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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, 2.55 - 2.63 (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(0.22218)Ca(-0.0012)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/cm3, 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/cm3- 2.63g/cm3, is for pure
11 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 nα 1.514-1.521 1.516-1.520±.005 nβ 1.518-1.530 1.520-1.524±.005 nγ 1.521-1.533 1.524-1.528±.005 Maximum .005-.008 0.004-0.012 ±.005 birefringence: δ Optic Angle:2V 40-~70 37.411±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 values which, indicates that all the hkl values were assigned correctly. Accepted values for this
mineral are as follows: a = 8.5632 Å, b = 12.963 Å, c = 7.299 Å, β = 116.073°; Unit Cell
Volume=724.57 ų (mindat.org, 2008). These are quite close to the measured values of the
mineral, a=8.525 Å ± .004 Å, b=13.028 Å ± .004 Å, c=7.200 Å ± .001 Å, β=116.06° ± .03°;
Unit Cell Volume = 718.3 Å3 ± .5 Å3. 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 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 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, 1910. 23 Dec. 2008
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., 2008.
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. 97-106.
Nesse, W. D. (1991) Manual of Mineralogy (21st 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. 2003. 24 Dec. 2008
"Orthoclase." mindat.org. mindat.org. 24 Dec. 2008
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
Appendix A,
Figure 1: Excalibur Printout
16 Figure 2: Orthoclase Peak Display
17 Figure 3: Orthoclase Card
18 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 = 8.525146 B = 13.027840 C = 7.199508 \par ESD A = .003901 ESD B = .003660 ESD C = .001304 \par \par ALPHA = 90.000 BETA = 116.061 GAMMA = 90.000 \par ESD ALPHA = .000 ESD BETA = .029 ESD GAMMA = .000 \par VOLUME = 718.31 \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 1 3 0 23.5250 23.5314 -.0064 .07004 .07008 -.00004 5554 \par -1 1 2 25.6413 25.6574 -.0162 .08299 .08309 -.00010 553 \par 2 2 0 26.9881 26.9880 .0001 .09177 .09177 .00000 458 \par -2 0 2 27.1569 27.1525 .0044 .09290 .09287 .00003 303 \par 0 4 0 27.3612 27.3605 .0008 .09428 .09427 .00001 304 \par 0 0 2 27.5675 27.5608 .0067 .09567 .09563 .00005 654 \par 1 3 1 29.8380 29.8398 -.0018 .11171 .11172 -.00001 50 \par -1 3 2 32.2894 32.2782 .0112 .13031 .13023 .00009 283 \par -1 1 3 38.6825 38.6844 -.0019 .18487 .18489 -.00002 235
\par H K L 2-THETA (DEG) D - SPACINGS INT(CPS)\cf1 \par OBS------CALC----DELTA OBS------CALC----DELTA \par 1 3 0 23.5250 23.5314 -.0064 3.77857 3.77756 .00101 5554 \par -1 1 2 25.6413 25.6574 -.0162 3.47131 3.46915 .00215 553 \par 2 2 0 26.9881 26.9880 .0001 3.30104 3.30106 -.00001 458 \par -2 0 2 27.1569 27.1525 .0044 3.28091 3.28143 -.00052 303 \par 0 4 0 27.3612 27.3605 .0008 3.25687 3.25696 -.00009 304 \par 0 0 2 27.5675 27.5608 .0067 3.23297 3.23374 -.00077 654 \par 1 3 1 29.8380 29.8398 -.0018 2.99193 2.99175 .00017 50 \par -1 3 2 32.2894 32.2782 .0112 2.77016 2.77109 -.00094 283 \par -1 1 3 38.6825 38.6844 -.0019 2.32577 2.32566 .00011 235 \par END OF LATTICE REFINEMENT\cf0 19 Figure 5: Microcline Peak Display with Microcline Card Superimposed
20 Figure 6: Microcline Card
21 Figure 7: Microcline Peaks Used to Check the Composition and Ordering of the Mineral
Figure 8: Lucite Card
Figure 9: Synthesis Product, Lucite, with Lucite Card Superimposed
Figure 10: Microcline and Lucite Peak Displays Superimposed
25