VALENTINITE: SECONDARY ANTIMONY MINERAL

 

In an earlier posting (August 21, 2020) I described my specimen of senarmontite, an antimony oxide [Sb₂O₃] that often appears as nice octahedral crystals (Isometric System). It usually has a gray to white color or occasionally red to colorless.  The colorless variety is transparent while the others are translucent to mildly opaque.  Although my specimen is a nice octahedron, senarmontite also occurs as a massive and or granular mineral. Colorless crystals are subvitreous while others have a resinous luster, but all have a white streak.  The mineral also is quite soft at ~2 or a little more (Mohs).

The octahedron of translucent senarmontite.  Width/length crystal ~7 mm.   Specimen collected from  the Djebel Hammimat Mine, Ain Babouche District, Oum el Bouaghi Province, Algeria.

It is always interesting as a rockhound to locate dimorphous minerals, that is minerals with the same chemical formula but crystalizing in different Crystal Systems. So, I was happy to pick up two specimens of valentinite, also an antimony oxide with the same chemical formula as senarmontite [Sb₂O₃]. However, valentinite is a member of the Orthorhombic Crystal System and usually appears as flattened prismatic crystals with complex terminations in fan-shaped aggregates. Most minerals have an adamantine luster, a white streak, and are colorless to white; collectors like to locate rarer yellow, pink and brown crystals, However, as in senarmontite, valentinite may form in granular masses.

The specimen above was collected from the Graf Jost-Christian Mine, Wolfsberg, Sangerhausen, Mansfeld-Sudharz, Saxony-Anhalt, Germany. The mine started extracting antimony ore around 1700. There are tiny acicular crystals on porous quartz crystals. MinDat noted specimens from here are rare. The original label, from Shannon’s Minerals in Tucson was glued on but appears “older,” I remounted the specimen due to the condition of case. Width FOV top is ~9.5 mm. With of bottom specimen ~3.0 mm.

Flat lying radiating sprays of valentinite on a stibnite matrix. Width FOV ~1.7 cm. Collected from Kolarsky vrch deposit, Pezinok, Pezinok District, Bratislava Region, Slovakia. MinDat noted “One of the largest hydrothermal antimony deposits in Slovakia. Small-scale mining of antimony started in 1790 and after some breaks finally stopped in 1991 for economic reasons. The deposit is famous for excellent kermesite specimens. Vein-type antimony deposit, hosted in Late Silurian to Lower Carboniferous phyllites and black slates.”

Both senarmontite and valentinite are secondary minerals and are  weathering/alteration products of other antimony-bearing minerals (commonly stibnite) in the oxidized zone of hydrothermal antimony deposits. Associated oxidized minerals often include stibiconite [Sb³⁺Sb⁵⁺O₆(OH)], cervantite [Sb³⁺Sb⁵⁺O₄], and kermesite [Sb₂S₂O].

A brief word about dimorphic polymorphism—brief because understanding the exact mechanisms causing such are mostly above my pay grade. I best understand the dimorphs calcite and aragonite (high pressure polymorph). However, aragonite is metastable at the low pressures near the Earth's surface and is thus commonly replaced by calcite. In fact, aragonite is rare or absent in rocks older than ~Mississippian age. Many rockhounds "sort of" understand carbon (hexagonal structure) and diamond (isometric structure) where differing pressure and temperature form different minerals. I "sort of" understand that the dimorphism in senarmontite and valentinite evidently is in response to changes in temperature and pressure during their formation. Senarmontite seems to be metastable; however, I don’t have the slightest idea if  there can be a change to valentinite. I just cannot locate appropriate informative literature so will file that problem in “one of life’s persistent questions.”