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Formula: Cu2+3O(Mo6+O4)2
Anhydrous molybdate
Crystal System: Orthorhombic
Specific gravity: 4.512 calculated
Hardness: 3
Streak: Yellow or light brown
Colour: Primatic crystals honey-yellow to chestnut-brown or dark brown; acicular crystals bright yellow
Environments
Cupromolybdite is a relatively new mineral, approved in 2011 and to date (January 2024) reported only from the
type locality
Localities
At the type locality, the Yadovitaya fumarole, Second scoria cone, Northern Breakthrough, Great Fissure eruption,
Tolbachik Volcanic field, Milkovsky District, Kamchatka Krai, Russia, the
scoria cones formed during the Tolbachik Fissure eruption in 1975. The
Yadovitaya fumarole is a wide-open cavity with a width and depth of 1.5 m and 2 m respectively. Rocks inside the cavity
are covered by thick crusts of sublimates, which are partially weathered and leached by meteoric water. The temperature
inside the fumerole was 338oC in 2010. It appears that most of the sublimates were deposited at an earlier
time, possibly several years or months after the eruption; however, the recent deposition of tiny crystals of sublimate
minerals cannot be excluded.
Among the sublimates cupromolybdite is a rare mineral; it appears within 2 to 5 cm thick sublimate crusts,
along with other fumarolic minerals including oxides (hematite,
magnetite, rutile and
tenorite), oxysalts [salts containing oxygen as well as a given anion]
(piypite, fedotovite,
euchlorine, vergasovaite,
klyuchevskite,
alumoklyuchevskite and
averievite),
sulphates (aphthitalite,
langbeinite and palmierite),
arsenates (lammerite),
vanadates (lyonsite and
pseudolyonsite), silicates such as
filatovite and arsenic-bearing
orthoclase, and native gold. In this
paragenesis, piypite, fedotovite
and euchlorine are the most abundant
copper minerals, and cupromolybdite formed at a late stage after
vergasovaite, in some cases overgrowing or replacing the earlier mineral. The exact temperature of mineral formation remains unclear.
Cupromolybdite is commonly present as well shaped prismatic crystals, which are mostly 30 to 50 mm in length,
but may reach as much as 150 mm. These crystals overgrow piypite and
fedotovite or form clusters within complex intergrowths of associated
minerals. Radiating aggregates of acicular crystals or separate needles are also typical. Moreover, some prismatic
crystals of cupromolybdite are overgrown by its acicular variety, which is
zinc- and sulphur-
enriched. Both prismatic and acicular varieties are well shaped, which suggests that they formed by direct
precipitation from the gas phase rather than by recrystallisation.
(EJM 24.4.749-757).
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