Pentagonite
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Pentagonite is named from the Greek word penta meaning five and angle in allusion to the unusual pseudosymmetrical five-fold or pentagonal habit of the fiveling twinned crystals common in this species.

Discovered in 1960;   IMA status:  Valid (IMA approved 1973)

Chemistry

Chemical Formula:

Ca(V⁴⁺O)Si₄O10₁₀ •4H₂O

Hydrated Calcium Vanadate Silicate

Molecular Weight:

451.42 gm

Composition:

Calcium

8.88 %

Ca

12.42 %

CaO

Vanadium

11.28 %

V

20.15 %

V₂O₅

Silicon

24.89 %

Si

53.24 %

SiO₂

Hydrogen

1.79 %

H

15.96 %

H₂O

Oxygen

53.16 %

O

100.00 %

101.77 %

= TOTAL OXIDE

Classification

Mineral Classification:

Silicates (Germanates)

Strunz 8th Ed. ID:

8/H.36-20

Nickel-Strunz 10th Ed. ID:

9.EA.55

9 : SILICATES (Germanates)
E : Phyllosilicates
A : Single nets of tetrahedra with 4-, 5-, (6-), and 8-membered rings

Related to:

Dimorph of Cavansite. Structurally related to Plumbophyllite. Ishida et al. (2009) suggest that Cavansite is a low-temperature form and Pentagonite a high-temperature form.

Varieties:

None

Synonyms:

IMA1971-039

Crystal Data

Crystallography:

Orthorhombic - Pyramidal

Crystal Habit:

Crystals prismatic, to 1 mm, with dominant {110}, {201}, and {001}, elongated || [001]; commonly as spherulitic rosettes, up to 5 mm

Twinning:

Cyclical by reflection across {110}; multiple twins result having pentagonal basal patterns closely simulating five-fold symmetry

Physical Properties

Cleavage:

Distinct/Good; Good on {010} 

Fracture:

n/a

Tenacity:

Brittle

Moh's Hardness:

3.0 - 4.0

Density:

2.33 (g/cm³)

Luminescence:

Not Fluorescent

Radioactivity:

Not Radioactive

Optical Properties

Color:

Greenish Blue, Blue

Transparency:

Transparent

Luster:

Vitreous

Refractive Index:

1.533 - 1.547  Biaxial ( - ) 

Birefringence:

0.014

Dispersion:

Strong; r > v

Pleochroism:

Pronounced; X = Z = colorless; Y = blue

Occurances

Geological Setting:

Fracture filling in tuffs and basalts.

Common Associations:

Cavansite, Heulandite, Stilbite, Analcime, Apophyllite, Calcite

Common Impurities:

None

Type Locality:

Owyhee Dam, Lake Owyhee State Park, Malheur Co., Oregon, USA

Year Discovered:

1960

View mineral photos:

Pentagonite Mineral Photos and Locations

More Information

Mindat.org
Webmineral.com

Pentagonite was discovered in the fall of 1960. Mr. and Mrs. Leslie Perrigo of Fruitland, Idaho found a blue mineral partly coating a large rock in a new roadcut near Owyhee Dam, Malheur County, Oregon. Together with Mr. and Mrs. Frank Zimmerman of Payette, Idaho, they collected some of the material the following spring and sent it to Dr. Paul Desautels of the United States National Museum, who reported that it was probably a new mineral. A crystallographic study proved this material to be a new mineral species and it was named Cavansite. During the crystallographic study, one small specimen from Owyhee Dam was found to contain crystals that were clearly twinned. These proved to be different from Cavansite. Although they have very similar appearance and physical properties and the same chemical composition, crystal structure analysis showed the twinned crystals to be another new mineral species. The twinned material was named Pentagonite in allusion to the unusual pseudosymmetrical five-fold or pentagonal habit of the fiveling twinned crystals common in this species.

Because Pentagonite and Cavansite have very similar appearance and physical properties and the same chemical composition but different crystal structures, they are dimorphs of each other. Both Cavansite and Pentagonite are orthorhombic with unit cells that are somewhat similar, but distinctly different. Ishida et al. (2009) suggest that Cavansite is a low-temperature form and Pentagonite a high-temperature form.

Distribution: Only two locations are known; the type locality at Owyhee Dam, Lake Owyhee State Park, Malheur County, Oregon, USA; Wagholi Quarry and Lonavala Quarry, Pune District (Poonah District), Maharashtra, India.
 

  
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