2092
MINERALOGICAL
NOTES
quantitative numbers on the relative proportions of the aforementioned
minerals.The spectrometersof the microprobewere set to the Ka wavelengths of Ca, Mg and P. Olivine is then recognizedby its high NIg but
negligibie Ca content. Rhombic pyroxene has high Mg and small but
measurableCa content. Ca-rich Cr clinopvroxeneis characterizedby its
high Czr, and chlorapatite by its high P content. Plagioclaseis easily
recognizedb)'its bright blue luminescence.In Table 1 the results of the
repeatedmeasurementsof section 1 are presentedto establishzrmeasure
of the reproducibility of the method. The variations in the total number
of counts in different runs are due to the different steppingdistancesused
each time (20-100 microns). In Table 2 the results of the integration of
seven sectionsof different nodules are summarized.The averageof the
sevensectionswas calculatedtaking into erccountthe sizeof eachsection.
The weight percentageswere calculatedfrom the volume pelcentagesby
meansof the following densit-vvaiues: olivine 3.3, rhombic pl.roxene3.1,
Cir-rich Cr clinopvroxene 3.3, plagioclase 2.6, chlorapatite 3.2. It is
apparent that the relative proportions of the siiicatesand of chlorapatite
are quite different from nodule to nodule.
I am indebted to Dr. R. R. Marshall for providing the Odessasamples,
to Mr. J. \rongrey and Mr. J. Erlichman for assistancein the microprobe
rvork and in preparing the samples,and to Miss I{. De }-rancescofor
computing the numbers.
RrlrnnNcns
Brnxs, L. S. (1963) Electron Probe Microanalyszs. Interscience Publ., Nel. York.
Cnavrs, F. (1956) Petrogrophic Mod.al Analysis. John Wiley & Sons, Inc , Nerv York.
Nlansnelr., R R. ,q.NoK Knrl (1965) Poll'mineralic inclusions in the Odessairon meteorite. Icarus (in press)
THE AMERICAN MINERALOGIST,VOL 50, \OVEMBER_DECEMBER,1965
AN IMPROVED
If . C. CocsRAN
HOLDER
ITOR GRINDING
AND J. R. JrixsrN,
THIN
SECTIONS1
LI. S. Geological Surtey,
Denrer, Colo.
A new holder for grinding thin sectionsusing diamondsfor wear points
has beenfound to be highly satisfactoryand a distinct improvement over
a previously describedholder using boron carbide strips.2This holder is
made of two blocks of brass,joined by a pair of f inch stainlesssteelpins
1 Publication authorized by the Director, U. S. Geological Surve-v.
2 Cochran, M. C., and A G. King (1957) Two new types of holders used for grinding
thin sections Atn. M,ineral 42, 422-424.
MINERALOGICAL
NOTES
\_'ae-_\
^\
m
E
ba
o
.q
'd
bo
d
rc
o
t.i
o
E
tn
I
l-
L
2094
],IINERALOGICAL NOTES
on which one of the blocksslides;a rectangulardepressionaccommodar.tes
a petrographic glassslide with its mounted rock chip (seeillustration).
These pins are fixed to the secondblock by a press-fit.The small groove
shown around the alignment platform facilitates milling the platform.
The use of brass,instead of aluminum (used in the previously described
model), gives less binding on the pins, becausether:eis less wear and
fouling by abrasiveand rock powder.l
Four commercial bort diamonds, each weighing about I ceLrat,are
mounted in the ends of screw-likepins of stainlesssteel. The exposed
surfaceof each diamond should be about 2 to 3 rnm in diameter, thus
with about 3 to 8 sq. mm of bearing surface.The diamonds are mounted
similarly to diamonds in a drill bit except that their flatter surfacesare
mounted upward and no points or sharp edgesare exposed.If an experienceddiamond bit setter is not available to do peen mounting, the diamonds can be brazed,b.vuse of minimum heat, into shallowdrilled holes.
The pins are adjusted with a screwdriver until the diamondsprotrude
far enough to give the desiredfinal thicknessof the slide and chip. The
pins are then locked with set screws.The fiber plugs, shorvnin the illustration, prevent damageto the threadsof the pins. 'Ihis type of mounting
is obviously superior to the use of the boron strips previousll-described;
the latter requiresmilling the platform for any adjustment for thickness.
The chip cannot be ground below the plane determinedb1-the diamond
surfaces.A good gaugefor setting the diamond points holder is to use one
of the thicker slidesfrom the available stock on which a chip has been
mounted and ground down to about 0.004inch. The final reduction,using
a conventionalholder, to a thicknessof about 0.001 inch (or to "standard" thickness)requiresverl- little extra grinding. The thinner slidesof
those available, which of course result in correspondingll,'thicker chips
when used with this diamond-controlledholder, require so little additional fi.nishing time that no preliminary gauging of slides is necessarl.
and one diamond-controlledholder can be used for all routine work.
Chips as thick as 0.04 inch can be started with this holder and ground to
gaugethicknesswith a minimum of stoppagefor inspection.
1 An objectionable amount of wear, after
several years of steadl' use, has been satisfactorily corrected bl'adding two small brass bushings to the sliding brass block.