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Formula: (NH4)Al3(SO4)2(OH)6
Sulphate, alunite group,
alunite supergroup
Crystal System: Trigonal
Specific gravity: 2.4 measured, 2.58 calculated
Hardness: 2 to 3
Streak: White
Colour: Greyish white
Environments
Localities
At the Dębieńsko Mine heap, Gmina Czerwionka-Leszczyny, Rybnik County, Silesian Voivodeship, Poland,
ammonioalunite, ammoniojarosite and their solid-solution series
occur as yellow, fine-grained crusts and botryoidal masses in the outer part of a sulphate crust found about 1 m below
the surface. The crust is composed mainly of godovikovite and
tschermigite that formed by interaction of sour fire gases or solutions
and waste materials beneath the surface of the burning coal dump at temperatures of at least 80°C. The crystals often
reveal oscillatory zoning due to different Al and Fe contents in thin bands, from near end-members to extensive solid
solutions. Analyses suggest the existence in nature of a complete solid solution between ammonioalunite and
ammoniojarosite. They also carry essential amounts of chlorine
(MM 74.4.731–745).
Ammonioalunite from the Dębieńsko Mine heap -
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The type locality is The Geysers, West Mayacmas Mining District, Sonoma County, California, USA. NH4-rich
feldspars and micas have been found in
a number of hydrothermal environments including recent
gold-mercury hot-spring systems,
volcanogenic lead-zinc deposits,
disseminated gold deposits and
pyrophyllite deposits.
The Geysers is a modern hot-spring system exploited for geothermal energy that contains extremely NH4-rich
fluids.
(AM 73.145-152).
The ammonioalunite occurs as rhombohedral crystals, up to 20 µm in size, forming granular masses. It is formed
in hot springs under very acid conditions, rich in ammonium and sulphate, poor in potassium, below 100oC.
Associated minerals include ammoniojarosite
(Mindat).
At the Ivanhoe Mining District, Elko County, Nevada, USA. three NH4-rich
alunite samples were found near the fossil hot spring locality with
mercury-gold mineralisation. These
NH4-rich alunites occur with
opal and quartz, and one occurs with
low-ammonium alunite. NH4-rich
alunite is indicative of a chemical environment with acidic solutions less
than 100oC with abundant (NH4)1+ and (SO4)2-, and little
K1+.
Ivanhoe is a hot-spring deposit that was formerly mined for mercury and
subsequently prospected for gold. The three NH4-bearing
alunite specimens from Ivanhoe occur with
opal and quartz, and one specimen occurs
with alunite. All are in a host rock of hydrothermally altered
basalt. Many other alunite
samples from this locality contain little (NH4)1+, although these
alunites frequently occur with (NH4)1+-bearing
feldspar
(AM, 73.145-152).
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