THE AMERICAN MINERAIOGIST, VOL' 49, JULY_AUGUST' 1964
MINERALOGICAL NOTES THERMAL STABILITY OF AZURITE AND MALACHITE Der,B R. StlrrsoN, Rrcn.q'nl Frsnon aNo Kenr' Ltnscu, Departmentof Geology,Lehigh Unittersity,Bethlehem, Penna'
Although azttrite and malachite are the two most common secondary copperminerals, there is no agreementamong the reporteddecomposition temperatures for the minerals. This paper presents the results of experi- ments conducted to establish the temperature of decomposition of azur- ite and malachite at one atmosphereof air pressure. In 1851 Rose reported that malachite began to decomposeat 220" C. and after long heating had lost only 0'76 per cent weight' However, at 250. C. the weight losswas 2,16 per cent and at 300" c. it was 27.7 | pet cent. Rose further states that the weight Iossof azutite is insignificant at 250oC. but at 300oC. it changedto copperoxide' Kelley and De Noyer (1960) report that malachite shows tan spots when heated in a micro- coil from 220 to 262" C. and that fragments of the mineral turn brown when heatedto 3250C.+. They alsoreport that azuritebecomes deep blue to black in the temperature range of 240" c. to 295o C. and that grains
minute (Beck, 1950a). The apparent temperature of decomposition of azurite and malachite from the Dixie mine, Utah, and reagent CuCOs (very finely divided malachite) was established by thermo-gravimetric analysis using a heating rate of t.25" C. per minute Fig. 1). Azurite beginsto decompose at about 300' c. and reachesa maximum rate of decompositionat 400oc' All of volatiles are lost below 500' C. Malachite showssome weight loss at 275"C.; however,the maximum rate of decompositionis not reached until 440o C. Decomposition of malachite continued to a temperature greater than 600o C. In contrast, finely divided malachite begins to lose weight at lessthan 100oC. and at 300oC. its weight lossis more than 5 times that of azurite or malachite. At 400o C. the decompositionis complete. At a heating rate of 0.15' C. per minute, azttite and malachite are com- pletely decomposedat a temperatureunder 450oC., and they show the maximum rate of weight lossat a temperatureunder 380" C' (Fig' 1)' At 1111 III2 MINERALOGICALNOTES
this slow heating rate the decomposition curve ior azttite is similar to that for malachite; however,at the fast heating rate, marachiteappears to be stable to a higher temperature than aztrite. with a heatingrate of 0.01' c. per minute, malachiteis completelyde- composedat 340oc., and very finely divided malachiteis completelyde- composedat 240" c. (Fig. 1). Decomposition of malachite reachesits maximum rate of 280' C. when heatedat 0.01oC. per minute. *'l
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5tot520253035
WEIGHTLOSS IN PERCENT Frc'1' rherm o-;:ffi teand azurite' ",lrl'":l'"tJiJ'J:ffi,i:: ;lrachi
A solid will decomposewhen the partial pressureof componentsof the solid becomesgreater than the vapor pressureof the samecomponents in the surroundingmedia. Becausethe vapor pressureof a solid is strongly temperaturedependent, and becausethe rate of decompositionof a solid is dependent in part on the difference in vapor pressureof the solid and the surroundingmedia, an infinitely slow heatingrate must be usedif the true decompositiontemperature of a solidis to be accuratelydetermined. When malachiteis heatedat 1.25,0.15and 0.01. C. per minute the maximum rate of weight loss begins at temperatures of 440, 375 and, 280' C. respectively, and the log of these heating rates plotted against the temperaturesgives a straight line (Fig. 2). This semi-logplot there- MINERALOGICAL NOTES 1113
=2
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E
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OF MALACHITE "C
Frc. 2. Semi-log plot of heating rate ?s. apparent decomposition temperature of malachite.
20
t5
Weiqht Loss in Perc€nl
FrG;3. Weight lossof azuiite and malachiteat about 200oC' 1114 MINERALOGICAL NOTES
fore indicates the apparent temperature of decomposition of malachite with different heating rate, but it does not establish the minimum de- compositiontemperature of malachite.It doesshow, however, that if the decompositionof malachite is to be tested at, for example,200o C., the heatingrate can be no more than approximately0.001' C. per minute. To test whether decompositionof malachiteproceeds at 200oC., sam- pleswere held at a constant temperaturefor 20 days. During this period of time there was more than 3 per cent weight loss with the products being CuO and malachite(Fig. 3). It would requireabout 200days f or the reaction to go to completion. Azurite was tested in a similar manner and found to decomposeat 190oC. losing 2.76 per cent weight in 45 days. The temperatureof decompositionof azurite and malachite is therefore less than 200' C. In conclusion,the reporteddecomposition temperatures of azurite and malachite are largely explainable by the different heating rates used by the investigators.And further, the minimum possibleheating rate should always be usedfor determinationsof decompositiontemperatures of any material. This study was supportedby the National ScienceFoundation Under- graduate ResearchParticipation Program and Lehigh University. The authors are grateful for this support.
RBlnnnNcns
Brcr, c. w. (1950a) An amplifier for difierential thermal analysis. Am. Mineral.3s, 509- 524. --- (1950b) Differential thermal analysis curves of carbonate minerals. Am. Mi,neral. 35, 985-1013. Krr,r,rv, W C. exo Do Novon, J. M. (1960) Heating microcoil for study of mineral frag- ments and heat-etching of polished sections.Atn. Mineral.45, 1185-1197. Rosr, H. (1851) ueber den Einfluss des wassers bei chemischenZersetzunsen Ann. ph. Chem.84,461-485.
THE AMERICAN MINERALOGIST, VOL. 49, JULY_AUGUST, 1964
QUARTZGLASS PRESSURE VESSELS FOR HYDROTHERMALSTUDIES R. SpnBo AND A. Frr.rco, California Institute of Technology, Jet Propulsion Laboratory, Pasadena,Cal,if ornia.
fNrnooucrroN Sealedqrartz glasstubes have proved usefulas pressurevessels in low PT hydrothermal experimentsin silica-saturatedsystems. These tubes provide an inexpensive means for making simultaneous runs along an