American Mineralogist, Volume 58,pages 405-411, 1973
Cavansiteand Pentagonite, NewDimorphous Galcium Vanadium Silicate Minerals From Oregon
Lrovp W. Srlprps Department ol Geology, Uniuersity of Oregon, Eugene, Oregon 97403
Hownno T. EvlNs, Jn., ANo Jevrns R. LlNnsav U. S. GeologicalSuruey,Washington, D. C.20244
Abstract
Cavansite, Ca(VO) (SLO'").4ltrrO, a new mineral found in cavities and veinlets in basalt and vugs in tuff at Goble and at Owyhee Dam in Oregon, occurs as radiating greenish-blue prismatic crystals associated with calcite, analcime, thomsonite, heulandite, stilbite, and apophyllite. The crystals are orthorhombic, space group Pcmn (D^'"), and have a unit cellwitha-9.778(3),b-13.678(4),c-9.601(2)A,containing4formulaunits.Thev are optically biaxial positive and strongly pleochroic. The composition was determined by X-ray fluorescence analysis and crystal structure analysis. Pentagonite, a dimorph of cavan- site, was found at Owyhee Dam in prismatic crystals twinned to form fivelings with star-shaped cross sections. It is also orthorhombic, space group Ccm2' (C^"), and has a unit cell with a = 10.298(4), b,- 13.999(7) and c - 8.891(2) ,A, containing 4 formula units' These cell dimensions, and those for c.avansite,are typical and tend to vary over a small range' pre- sumably because of varying zeolitic water content. The crystals are optically very similar to cavansite, but are biaxial negative. Both cavansite and pentagonite have silicate layer structures in which the layers are held together by VG' groups and Ca'* ions, but they differ in the way the SiO. tetrahedra link to form the layers. The crystal chemical study fully accounts for the morphological and physical properties of these unusual minerals.
fntroduction a crystal structure analysis,which showsit to be a In the fall of 1960, Mr. and Mrs. Leslie Perrigo layer silicate of a completelynovel type' The name' of Fruitland, Idaho, observeda blue mineral partly cavansite(ca'-van-site) has beenselected as a mne- coating a Tatgerock face in a new roadcut near monic word from the chemical composition. The Owyhee Dam, Malheur County, Oregon. Together mineral and the name have been approved by the with Mr. and Mrs. Frank Zimmerman of Payette, Commissionon New Mineral Names, International Idaho, they collected some of the material the fol- Mineralogical Association. lowing spring and sent it to Dr. Paul Desautelsof the During the crystallographicstudy, one small speci- United StatesNational Museum, who reported that men from Owyhee Dam was examinedwhich con- it was probably a new mineral. tained crystals that were clearly twinned. These Mr. John Cowlesof Rainier, Oregon,in February proved to be different from cavansite,though they 1963, discovereda similar-appearingblue mineral have very similar appearanceand physical proper- in a quarry near Goble, Columbia County, Oregon, ties, and the same chemical composition.Crystal approximately 350 miles northwest of the Owyhee structure analysisshowed the twinned crystals also locality. He presentedsome of his material to one have a silicate layer structure, but with a different of the authors (L. W. Staples) for identification in type of layer than that of cavansite.The twinned the fall of that year, and it was determinedto be a material, therefore, is a dimorph of cavansiteand new mineral. Subsequently,some of the Owyhee constitutesyet anothernew mineral species.We have material was obtained and comparisonof specimens called it pentagonite(pen-ta'-gon-ite) in allusion to from both localities proved them to be the same its striking twin aspect;this namehas alsobeen ap mineral. Our study of the new mineral has included provedby the Commissionon New Mineral Names. 405 406 STAPLES, EVANS, AND LINDSAY
In this paper we present a description of the as very small (ca 0.3 mm) crystals, frequently dou- mineralogy and paragenesisof these two new vana- bly terminated, and coating the heulandite. The cal- dium silicate minerals. The details of the crystal cite is of two generations, an amber-colored variety structures are given in a separatebut accompanying in rhombohedrons which preceded the cavansite and paper (Evans, 1973). a colorless variety which is much later. Both heu- landite and analcime enclosepentagonite and cavan- 0ccurrence site and as a result assumea deep blue color, giving Cavansite and pentagonite are found in a brown the impression that more cavansiteis present than is tuff in a roadcut 2.1 miles by road south of Owyhee actually the case.The pentagonite is probably of two Dam and about a quarter mile north of Gordon generations. Gmelinite is found lining cavities at a Gulch, on the east side of the reservoir. The locality location about 500 feet above the cavansite occur- lies within the boundaries of the Lake Owyhee State rence. Its paragenetic relation to the other minerals Park. The mineral partially fills a fault fissure which is not known. The sequenceof deposition at Owyhee strikes N. 40o E. and is almost vertical. Calcite fill- Dam appears to be: (1) amber calcite, (2) cavan- ing of the fault shows slickensides,but rosettes of site, (3) pentagonite and heulandite, (4) stilbite, cavansiteon the calcite have not been deformed. The (5) pentagonite, (6) calcite, (7) analcime, (8) tuff is in the Sucker Creek Formation (Corcoran apophyllite. et al, 19621'Kittleman et al, 1965) of late Miocene One specimen was found that has both cavansite age and is overlain by the Owyhee Basalt, feeders of and pentagonite together. Here the cavansite is which frequently cut the Sucker Creek Formation perched in bold radiating groups on calcite, over (Corcoran, 1965). On the east side of the highway which large, clear, colorless crystals of heulandite for a distance of about 90 feet, the roadcut exposes have formed. Smaller, long prismatic crystals of a face 17 feet high with a coating 2-3 mm thick of pentagonite fill the open space above the calcite and cavansite,pentagonite, and associatedminerals. Vein- cavansite, and penetrate the heulandite crystals in lets of cavansite occur in the tuff for another 300 all directions. This association of minerals suggests feet to the south, but here the mineral is poorly that this specimen during its formation passed exposeddue to lack of excavation. through the temperature of transition between cavan- At Goble. cavansite is found in the Charles W. site and pentagonite, and that the latter is the low- Chapman quarry, 0.5 miles west of U. S. Highway temperature form. 30 on Neer Road. The quarry is in a brown to gray At Goble, the sequenceof deposition is more diffi- vesicular basalt and red tuff breccia belonging to the cult to determinebut a common sequenceis ( 1) cal- Goble Volcanic Series of late Eocene age (Wilkin- cite, (2) heulandite,(3) thomsonite,(4) cavansite, son e/ al, 1946). The cavansite occurs as cavity (5) calcite. Native copper is also present in some of fillings, in amygdules, and in calcite veinlets. Other the specimensand the late calcite may enclose both quarries in the area have blue stains on some of the cavansite and copper. Because the cavansite was secondary minerals, but only in the Chapman quarry deposited about the same time and presumably was cavansite found. under the same conditions as calcite and the zeolites, Pentagonite, found so far only at Owyhee Dam, it is difficult to separate these minerals in order to seems to have formed at a later stage than cavan- obtain a pure sample for analysis. site, but their mineral associationsand mode of oc- The source of the vanadium for these vanadium currence are nearly identical. minerals is not known. At Owyhee Reservoir, a A few small specimens of cavansite and penta- rhyolite intrudes and overlies Kittleman's Sucker gonite from Owyhee Dam, Oregon, and a smaller Creek Formation and basalt feeder dikes of the amount of cavansite from Goble, have been placed Owyhee Basalt prominently cut through the Sucker in the collections of the Smithsonian Institution at Creek tuff. The introduction of vanadium may be the U. S. National Museum, Washington,D. C. related to these intrusions.
Paragenesis Chemical ComPositions At Owyhee Dam the principal associatesof cavan- Because of the intimate association of cavansite site and pentagonite are calcite, heulandite, stilbite, with other minerals, especially zeolites, it was diffi- analcime, and apophyllite. The apophyllite occurs cult to obtain a sample large enough for analysis by CAVANSITE AND PENTAGONITE 407 wet methods.The OregonState Department of Geol- tagonite, was estimated by an alternate X-ray fluo- ogy and Mineral Industrieskindly suppliedus with rescence technique, using a General Electric Com- some analytical information obtained on a gross pany Model xno-6 Semifocussing Spectrometer. In samplefro'm Goble. Their spectrographictest showed it the specimen, which may be less than 1 mm in Si, V, and Ca as major constituents.Their wet analy- diameter, is bathed in intense tungsten (or chro- sis did not conform well stoichiometricallyto any mium) radiation from a type rn-75 dual target X-ray reasonableformula. Since the purity of the large tube, and the excited radiation is collimated by a 1 sampleused was open to question,other meansof mm beam tunnel. The collimated X-radiation is then analysiswere sought. analyzedwith a curved LiF crystal and detectedwith X-ray fluorescenceanalysis carried out on a 20 a flow proportional counter. Four different analyzing mg sampleproved to be more reliable.The heavy crystals were available to provide optimum focussing absorber-fusiontechnique (Rose et al, 1963) was over a wide range of wave lengths. The apparatus used to prepare the sample anC standards.The and procedure have been described by Rose e/ c/ standardswere prepared by mechanicallyincorporat- (1969) and its adaptationfor study of small single ing V2O5 with a set of calcium silicate reference crystals used in X-ray crystallography by Evans and materialsto givethe appropriateconcentration ranges Christian (1972). This technique is not capable of of CaO (9.8 to 29.4%), VrOu(7.5 to 22.5Vo), and the accuracy of that described above for cavansite, SiOz (25 to 75% ). The analysiswas carriedout but can yield (with a sample of - 0.1 pg) semi- with a vacuum spectrometer(built by the General quantitative information that can decisively deter- Electric Company) equippedwith a chromiumtar- mine critical ratios. In this case, a single crystal of get X-ray tube (operatedat 40 ma and 60 kv), an pentagonite was compared with a similar crystal of ethylene diamine tartrate analyzing crystal, and a cavansite,and one of pascoite(Ca'rVroO,.:n'17HrO). flow proportionalcounter. The relative peak heights for Ca, V, and Si for the The elementsCa, V, andSi weredetermined using different crystals indicated clearly that the ratio of the Ka line of each and countingfor a period long Ca:V:Si is the same for both cavansiteand pen- enoughto give approximately10,000 counts on the tagonite. loweststandard. In this way the followingcomposi- The amount of water present was again the un- tion was found: known quantity. The question was answered in this case by comparison of the unit-cell volumes of ,Yt Mole ratios % cavansite and pentagonite (Table 1). Since they CaO r1.5 .998 are nearly identical (AZ for one H,O unit would VOz t1.t 1.w2 be -80 A'), we may safely conclude that the SiOz 49.4 4.000 HrO (21.o) chemical formulas of the two speciesare the same. Rem. .8 The final conclusive confirmation of the formu- lation CaO'VOz'4SiO2'4HrO for cavansite and Total 99.8 pentagonite was provided by complete crystal struc- The water content was estimated by the Oregon ture analyses,described in the paper (Evans, 1973) State Department of Geology and Mineral Indus- that follows. In all these considerations the oxida- tries Laboratory (see above). The mole ratios tion stateof vanadiumwas assumedto be IV, mainly clearly establisha formula of the form CaO.VO:. because of the weak red optical absorption shown 4SiOz.nHgO, but the water content was relatively by both materials, which is characteristicof the VO:- uncertain and the value of r could not be determined cation in many of its compounds. Again, this as- by the usual analytical methods. The amount found sumption was confirmed by the crystal structure de- above suggestsn = 6, but this predicts a specific termination. gravity of 2.52 from the X-ray unit cell, which For chemical tests, the following properties may compares very poorly with the highest measured be noted. Cavansite and pentagonite are difficultly value of 2.31 (see below). The true ideal water soluble in acids. In HNO,;, the blue color of the content was finally established by crystal structure minerals bleachesout and turns olive brown. A fine analysis, giving the formula for cavansite: CaO. white crystalline precipitate is slowly formed on a VOr.4SiOr.4HrO, or Ca(VO) (Siroro).4HrO. microscope slide when the minerals are heated in The composition of the twinned material, pen- dilute HzSOr. 408 STAPLES, EVANS, AND LINDSAY
Trsln 1. Crystal Data for Cavansite and Pentagonite Cavansite and p€ntagonitehave similar, though distinctly different,optical properties.Both are trans- parent, pleochroic blue, biaxial crystals elongated Crystal systen orthorhoEbic parallel to the c axis, which correspondsto the Z Space group pqnn(Dii) ccnzLGri) vibration direction.The indicesof refraction are such Axlal ratlos (fron X-ray powder data) that the acute bisectrix shifts from c in cavansiteto azbzc 0.7149:1:0.7019 0.7356:1:0.5351 b in pentagonite,and the strong dispersionis there- Structure cell (fron X-ray powder data) fore reversed. The critical data (Table 1) were orL 9. 778r0.003 10.298!0.004 measured with the aid of a spindle stage (using b,E 13.57810.004 13. 99910.007 white and sodium light) accordingto the method of arE 9. 601!0.002 8, 89110.002 Wilcox (1959). The cyclic twinning of pentagonite r 13 1284.1! 0,5 1281.8r 0,6 often has the appearanceof polysynthetictwinning Z , fornula unlts 4 in polarizedlight, producing lamellaeparallel to the
Specific gravlty elongation direction of the crystals. Calc (fron X-ray) 2.33 2.33 Obs (Beman balance) 2.2L-2.3L Crystal MorPhologY - Both cavansiteand pentagoniteare orthorhombic Character Biu. positlve Biax, negatlve with unit cellsthat are somewhatsimilar, but distinctly Optlc plane la La different. The habit of both minerals is prismatic, Acute bisectrlx llo Ib elongatedparallel to c, with dome terminations.As I lb, colorLess I lb, coLorless illustrated in Figure l, the dominant forms for J blue tlue I la, I la, cavansitearem I I 101and d { l0l }, andfor pentagonite z colorless I lc, colorless I lc, m u and c {00i}. Minor forms that 1 . 54210.002 1 .533!0.002 lllol, l20l ), havebeen observed for cavansiteare b {010},c [0011, 1. 54410.002 L,544!0,002 I a[100],andpllllf andfor pentagonite,c[001] and ,z 1 ,551!0.002 I.547lO .002 ; a 1001.The axial ratios as determinedfrom the X-ray zv
AB Frc. 2. Photomicrograph of pentagonite fiveling immersed in oil (n - 1.532), showing end view of prismatic crystal. Frc. 3. Multiple twinned crystal habits of pentagonite. The pleochroism is apparent (plane of polarized light is Two types shown are typical bladed habit (A)' and ideal- vertical). The bar represents0.05 mm. ized blunt habit (B). 410 STAPLES, EVANS, AND LINDSAY
Tesrn 2. X-ray Powder Data for Cavansite and Pen- an early stage,the nature of the mineral was not tagonite from Owyhee Dam, Oregon really understooduntil its crystalstructure had been revealed.From the formula we could expectthat a Cavanslte Pentagonite silicatelayer structureis present,unless this wasone hkl d(calc) d(obs) hkl d(calc) d(obs) I of the unusualcases where the SiOagroup incorpo- 110 7.954 7.964 100 110 8.295 8.298 tO rates an OH radical. The structureanalysis proved 101 6.851 6.854 50 020 7.000 7.006 2 that the 111 6.L25 6.L32 25 111 6.065 6.O7L 100 structureis indeed a layer type, and that r2r 4. 840 4 . 84r 6 20L 4.456) 4'446 2s all of the hydrogen is carried by rather loosely o2r 4.530 4.531 13 oot 4.44il' 20L 4.351 4,362 o LLz 3.918 3.920 100 bound, interlayer water molecules.The structure I02 4.309 4.310 6 22r 3.7591 ^ _-- study also establishedconclusively that ideallythere 2rr 4.151 4.r58 6 or, ,'.;;ij 3.755 100 22Q 3.977 3.978 040 3,500 3.500 36 are 4H2O per formula unit. 022 3.930 3,930 25 202 3,365 3.364 9 The structure analysisdescribed in the accom- r22 3.646 3,647 r32 3.072 3,072 2 040 3.4L9 3.420 25 222 3.033 3.034 9 panying paper (Evans, 1973) revealsthe dimor- 2r2 3.323 3.323 2 330 2.765 2.764 13 phism of cavansiteand pentagoniteto be basedon 310 3.170 3.L7L 3 ?11 2^'7-22^]z.tsz r32 3.r32 3.133 ? o42 2.750J'"-' s the differencein linkagesin the silicatelayer. Zig-zag 301 3.086 3.087 331 2.640 2.640 25 (SiO3)" chainsare discernible 222 3,063) 203 2.569 2.569 in both, but in cavan- r'uoz 13 36 r4t 3.059J 401 2.473 2.475 2 site thesechains are joined laterally into sheetsmade 103 3.042 3.040 2 133 2.431 2.43L 25 113 2.969 2.970 I 242 2.426 2,425 7 up of 4-fold and 8-fold rings, while in pentagonite 32r 2.813 2,813 3 332 2.348 2.348 4 they are differently joined so that only 6-fold rings r23 2.779 2.779 25 004 2.223 2.223 9 232 2.739 2.739 3r3 2.215 2.2L3 2 302 2.697 2.696 044 r.876 1.876 9 3I2 2.646 2.648 150 2.634 2.635 223 2.493 2.495 NoEes: 400 2.444 2.444 3 004 2,400 2.402 2 1. d(calc) in A are fron 052 2.377 2.375 2 unit cell paraEeters 014 2.364 2.364 o glven in Table 1. 332 2.32L 2.320 3 233 2,309 2.309 9 2. d(obs) in A are fron 114 2.298 2.298 2 Hagg-Guinier patterns L43 2.273 2.274 nade wiEh CrKorrad., 034 2.L24 2.r23 ^ = 2,28962 4.. 243 2.108 2.I09 350 2.095 2.095 440 r.9886 1.9883 403 L.9426 L.942r L44 r.926L r.9265 170 l. 9161 1 .9150 511 L.8977 L8975 105 1.8843 1.8838 163 L.8242 r.8237
1973) gavefor cavansitea=9.792, b:13.644, c= 9.6294 and for pentagonitea='10.386, b=14.046, c:8.975A. These variations probably arise from zeolitic variation in the amount of water in the structure, which is accompanied by a change in the dimensions of the unit cell, especially along the c axis. The variation of several specific gravity meas- urements of cavansite over a range below the value predicted by the unit cell dimensions (Table 1) is also probably related to the zeolitic nature of the mineral. Frc. 4. Pictorial view of the crystal structure of cavansite with the silicate layers in horizontal position (b axis verti- Crystal Chemistry cal). The tetrahedra representSiO. groups, the filled circle with attached open circle is a VO'* group, the stippled cir- Although the general chemistry, physical prop- cle is a Ca'* ion, and the line-shadedcircles are H,O mole- erties, and mineralogy of cavansite were known at cules. CAV ANSITE AND PENTAGONITE 411 are formed. In both structures the unlinked tetra- D. E. Apprr.rr.rAN,AND D. S. HeNowrnren (1963) hedral apicesof the chains are coordinatedin pairs The least-squaresrefinement of crystal unit cells with powder diffraction data by an automatic computer index- by VO'- group6,which tie the layerstogether. Thus, ing method [abstr.].Amer. Crystallogr. Ass. Annu. Meet., the vanadium atoms achievetheir characteristicfive- Cambridge, Mass., 1963,Program and Abstr., 42-43. fold, square-pyramidalcoordination. Large cavities eNo R. P. CnnrsrteN (1972) Adaptation of the in the structure are filled zeolitically by Ca'- ions X-ray milliprobe for the examination of small single and HzO molecules.These structural features are crystalsobtained from lunar samples.Appl. Spectroscopy, 26,313-315. in Figure 4. pictorially illustrated for cavansite Ksrr-y, K. L., eNo D. B. Juoo (1955) The Iscc-NBsmethod A1l of the chemicaland physicalproperties of the of designating colors and a dictionary of color names. two minerals are satisfactorily explained by their Nat. Bur. Stand.Circ.553, and Supplement(1965). crystal structures.In particular, the specialgeometry KrrtrtulN, L. R.. A. R. Gnesw, A. R' H.lcooo, A. M. of the pentagonitecrystal structurereadily permits JonwsoN,J. M. McMunuv, R. G. Russprl, .tNo D. A. (1965) Cenozoic stratigraphy of the Owyhee near 72", thus giving rise to WpeosN twinning at an angle region, SoutheasternOregon. Uniuersity of Oregon Mu- the striking fivelingscommonly observed.The struc- seum ol Natural History, Bull. l' 45 pp. ture study (Evans, 1973) has shown that no such Rosp, H. J., Jn., L Aor-En,eNo F. J. Fr.lNe.cex (1963) twinning mechanismcan be imaginedfor the cavan- X-ray fluorescenceanalysis of the light elementsin rocks site structure,and in fact, no twins of this mineral and minerals. Appl. Spectroscopy,17, 8l-85. J. R. LIuosrv, ruo R. R. LARSoN yet R. P. CnnrsrrlN, have as beenobserved. (1969) Microanalysiswith the X-ray milliprobe. U.S. Prof. Pap.650B' B128-B135. References Geol.Suru. Wncox, R. E. (1959) Use of the spindle stage for deter- ConconeN, R. E. (1965) Geology of Lake Owyhee State mination of principal indices of refraction of crystal Park and vicinity, Malheur County, Oregon. Ore Bin, fragments.Amer. Mineral. 44, 1272-1293. 27, 8l-98. WrrxrNsoN, W. D., W. D. LownY, AND E. M. BelowtN R. A. Dolr, P. W. Ponrsn, F. T. PRrrcHEfi, AND (1946) Geology of the St' Helens quadrangle, Oregon' N. C. Pnrvnesxv (1962) Geology of the Mitchell Butte Oregon Dep. of Geology and Mineral Industries, BuIL Quadrangle,Oregon. State Geol. Map Ser. 2, State Dep. 31, 39 pp. Geology and Mineral Industries. EveNs, H. T., Jn. (1973) The crystal structuresof cavansite Manuscript receiued,Iuly 31 1972; acceptedlor publication, and pentagonite.Amer. Mineral. 58, 412424. December14' 1972'