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The steam cave and vent sites were selected for their chemical properties. These were initially recognized in flowing hot springs as iron and sulfur. Here, deposition of iron occurred in the anoxic zone of the spring and at the point of disappearance of iron at the anoxic/oxic interface, sulfur deposition took place and finally iron deposits formed on the existing sulfur in the anoxic/oxic zone. The same phenomenon can be seen in very small sulfur caves where the interior has no sulfur, the cave opening is small and sulfur deposition occurs only at the anoxic/oxic interface or on the outer oxic surface, sometimes along with iron deposits over sulfur. The other categories, nonsulfur and salt (the latter so far only seen in Hawai’i), represent a departure from the flowing hot spring analogy. Nonsulfur steam caves have not yet been fully categorized, although unusual thin filamentous organisms have been enriched from the H5 salt cave, and from sulfur caves such as SW 3 in Lassen. Nonsulfur caves farther below are considered for their solid surface chemistry by means of energy dispersive X-ray microanalysis (EDX). Sampling of steam cave or vent ceilings depends on the nature of the surface. With highly acidic solfatara areas, cave ceilings can be delicate and sometimes only careful hand sampling is essential. With deeper nonsulfur caves sometimes encountered in Hawai’i, this is less of a problem because the cave ceilings are formed of hard lava and only shallow samples are obtained. However, sampling through a small opening into a deep cave interior is not without its challenges, principally resulting from the high level of heat encountered, with or without leather gloves.

Table 1.1 Fumarole sampling locations and physical properties.

Location	pH	Temp (°C)	Type	Chemistry
Hawai’i Volcanoes National Park1
Hawai’i (1)	5.5	65.0	Cave	Nonsulfur
Hawai’i (2)	5.2	68.0	Cave	Nonsulfur
Hawai’i (3)	3.0	82.0	Cave	Sulfur
Hawai’i (4)	3.0	82.0	Cave	Iron/sulfur
Hawai’i (5)	5.0	68.0	Cave	Salt
Hawai’i (6)	4.5	66.0	Cave	Salt/sulfur
Lassen Volcanic National Park2
Sulphur Works, lower (1)	4.5	87.0	Cave	Nonsulfur
Sulphur Works, lower (2)	3.2	93.0	Vent	Sulfur
Sulphur Works, lower (3)	4.0	91.5	Cave	Sulfur
Sulphur Works, upper (4)	3.0	85.5	Vent	Iron
Sulphur Works, upper (5)	2.5	81.2	Cave	Iron
Valles Caldera National Preserve3
Sulphur Springs, New Mexico	2.0	89.0	Cave	Nonsulfur
Pozzuoli, Italy4
Solfatara Crater	3.0	94.0	Vent	Sulfur

¹ June 2016 sampling

² Oct 2018 sampling

^{3, 4} included for comparison