Cavansite and Pentagonite, New Dimorphous Galcium Vanadium
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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 secondaryminerals, 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).