Crystal Habit of Synthetic Ghalcanthite (Copper Sulfate Pentahydrate) As
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Amefican Mineralogist, Volume 59, pages lI05-III2, 1974 CrystalHabit of SyntheticGhalcanthite (Copper Sulfate Pentahydrate)as Relatedto Positionand Orientation in GrowthSolution Eurn CnesnN, Departmentol Geologyand Geography,Hunter College ol The City Uniaersityof New York, New York, New York 10021 nNn C. W. WornE D epartment ol Geology, Boston U niuersity, Boston, M sssachusetts02 2 I 5 Abstract The habit of a crystal of CuSOn.5HrO can be controlled during growth under refrigeration by varying the depth of the crystal in the host solution, its crystallographic orientation during growth, and the maximum size to which it is allowed to grow. Crystals were grown in solution 30 mm deep. Those which were grown at the bottom of these solutions rarely produced the form' {100}; where that form did occur, it was little more than a line face. Seedswhich were set 18 mm up from the bottom yielded crystals with both {100} and {010} forms which were roughly equivalent in size. Those crystals which grew just below the surface irroduggd {l0O} forms but no {010} forms, the opposite of crystals which developed on the bottom. {111} was usually predominant among the terminal forms, but when crystals grew to sizes greater than 0.82 cm" in size, atthe 2 mm and 18 mm level of the solutions, {021} and {121} were almost as large as {ilt}. Crystals which were grown to intermediate size at all levels in the solutions developeda new form, {211}, when either the edge between (010) and (tZt), or that between (010) and (021) was set uppermost and parallel to the solution surface during growth. This new form generally was equal in size to the {121} form. The experimentation demonstrates that as crystals grow, the nuinber of forms diminish in favor of those forms with minimal summation of indices. Introduction Experimental Method Wolfe (1960) has suggestedthat crystal habit is The refrigeration method of crystallization is based influencednot only by such factors as crystal struc- on the fact that most salt solutions which are satu- ture, impurities included during growth, and varia- rated at room temperatureswill be supersaturatedin tions in the nature of the growth medium,but alsoby a refrigerator. A crystal, e.9., seed,will neither grow crystallographic orientation during growth and by nor dissolveif placed in a solution which is saturated crystal ldbation in the growth medium. To test this for refrigerator conditions and capped to eliminate premise,crystals of CuSOr.5H2Owere grown rlnder evaporation. If drops from a room-temperature satu- refrigeration conditions, where positive controls on rated solution are introduced into the refrigerator orientationand locationin solutionscould be main- solution, precipitation will occur, preferably on any tained.Copper sulfatepentahydrate was chosehfor seed crystal already in the refrigerator solution. The the experimentationpartly becausecrystals of that seed crystal is usually placed on a small raised plat- compositioncan be grown so readily by the refrigera- form or pedestal in order to keep other seeds,which tion techniqueand partly becausethe experienceof might form nearby, from interfering with the perfec- Tutton (1922) as Well as our own had indicated tion of growth of the control seed crystal. The seed that variations in its crystal habit were frequent, crystal is thus being fed at such a rate that it can notable,and apparentlyalmost random. take on additional ions sufficiently slowly to develop a homogeneous crystal. In order to keep the crystal 'Form indices are referred to the conventional settine from growing in only one direction and frorn growing (Palache,Berman, and Frondel,1951). around the pedestal, the crystal was turned over each I 105 I 106 E, CHASEN, AND C. W, WOLFE day on the pedestal to expose all surfaces to the 5. The b axis was oriented perpendicularto the precipitatingions in the solution. solution surface. Preparationof Seeds Depth Placementof Seds In the researchdescribed herein, a stock solution To achieve the location at various depths small was prepared on the basis of the solubility curve of pedestalswere contrived. After disappointing trials CuSO+'5HzOin water. The most satisfactorysolu- with red universalwax, we finally settledupon plexi- tion proved to be 100 g of water and 16 g of the glass pedestals.This material is easily shaped, and salt; sucha solutionis unsaturatedat room tempera- slots of various sizes can be cut in it to keep the ture (26-27"C) but is supersaturatedin the re- crystalsin any chosenorientation as they are grow- frigerator(6-7"C). To obtain seeds,30 ml of stock ing. Pedestalheights were cut at2 mm, 18 mm, and solution in 150 ml beakers.covered with watch 28mm. glasses,were placed in the refrigerator.Before there was wholesaleprecipitation, the solutions were de- Growth Procedures cantedinto other beakerswhich had been sitting in the refrigerator. Crystallization proceededin these The saturated(at about 6.6'C) host solutionsin beakers, and another decanting took place after the refrigerator had a depth of 30 mm in 150 ml many seedshad formed. This processwas continued beakers.Seeds were placed on plexiglasspedestals at until the host solution came to equilibrium with re- the three depths in the five previously referred to frigerator conditions, and no more seedingor crystal orientations. Several crystals were grown for each growth wastaking place. of the fifteenconditions listed. Crystal growth was induced by introducing drops Orientationof Crystalson Pedestals of the room-temperaturesaturated solution with a In order to testthe hypothesisthat crystalhabit is medicinedropper three times a day. Care was taken dependentupon the orientation of the crystal during not to overfeedthe host solution becausethe crystals growth, seedswere selectedon the basis of freedom which developedunder too rapid feeding were ir- from flaws and on the degree of geometrical per- regular in form, rough, and included some of the fection. Many more crystals were measured than mother liquor. The faces on such crystals did not were subsequentlyused for growth seeds.Axono- yield good goniometric measurements.Too slow a metric projections (Wolfe, 1953) of the crystals feeding, on the other hand, produced impractically were made in order to have a rigorous picture of the slow growth. Consideration of these factors and starting habit of each crystal. Careful two circle observationof growth rates led to an optimum rate goniometricmeasurements of the phi and rho anglesz of f ml of sourcesolution per feeding. were made and recorded. and the indices for each The crystals were turned over on the pedestals face were determinedin the Dana setting (Palache, once a day to exposeall surfacesuniformly to the Berman,and Frondel, 1944). With thesedata avail- sourceof precipitating ions. Occasionally,additional able, it was possibleto orient the seedin the host seedsformed on the growing crystals, although this solution for growth in any preselectedorientation. effect was minimized by use of the pedestals.The The following orientations were finally adopted as randomly oriented and undesiredseeds were scraped the most usefuland readily obtained: off manually before becoming embeddedwhen the crystalswere being turned. l. The edge betweenfaces (100) and (ll0) was setuppermost and parallel to the surfaceof thesolution. Final Crystal Sizesand Habits 2. The edge betweenfaces (010) and (021) was setas above. It was not possible to maintain a steady rate of 3. The edge betweenfaces (010) and (i2l) was crystal growth. Therefore, the crystals were simply setas above. fed daily until they reachedthe desiredsize, at which 4. The c axis was oriented perpendicularto the time they were measured,drawn, and photographed. solution surface. The ultimate sizesselected were somewhatarbitrary, but they fell into three sizeranges. The starting seeds 'Azimuth and polar distanceof a face normal are called weighed between 0.2 and 0.8 g, with a maximum phi and ry'ro,respectively (Wolfe, 1941). volume of. 1.4 cm3. Crystalswere consideredlarse CRYSTAL HABIT RELATIVE TO POSITION AND ORIENTATION IN GROWTH SOLUTION IIOT for this work when they weighed between 3.4 and A crystal drawing was preparedfor each possible 4.4 g, with a maximumvolume of 2 cm3.The same combinationof variables.These were consolidated crystals at various stagesof growth and in various into a few drawings of representativehabits which orientationswere measuredon a two circle goniome- developedfor many crystals grown to a particular ter and checkedfor habit development. size, at a certain level, and with a particular orienta- A.total of 46 crystalswere grown on which con- tion. These composite drawings are representative trol measurementswere made. Three variables ex- of what could be expectedif the experimentwere to isted in the depth in the solution,five different orien- be duplicated. Small variatiolts from the general tations were used. and three different ultimate sizes habit, of course, are likely. Ten basic habits are were attained. These variables yield 45 possible shown in Figures 1 and 2. The orientation of all of combinations.Since the same crystals were used to the drawings, unless otherwise noted, is with the produce different sizes, fifteen crystals would have positive end of the c axis upward, the normal to face embracedall of the variables.Thus, the 46 crystals (010) to the right and the normal to face (100) which were grown and measuredshould be suffi- quasiperpendicularto the paper. The face symbols ciently great to prove of statisticalimportance. The employedon Figures I and 2 are those listed in the experimentsextended over a period of 12 months, angle table for chalcanthitein Palaehe,Berman, and although singlecrystals reached maximum growth in Frondel (1951), an abridgedform of which is given about three and a half weeks. here in Table 1 for convenienceof reference. Frc. 1 Crystal drawings of Habits A-E. Habit A is oriented with to0il axis up, (0i0) to the right and ( 100) out from paper. All bar scalesrepresent 5 mm' I 108 E, CHASEN, AND C. W. WOLFE \ T 11 I v.a H 1 I t-l Frc. 2.