#sulphides

Now is my time to shine!

Sulphide minerals getting wet:

Orpiment is a striking yellow or orange arsenic sulphide with the chemical formula As2S3. It occurs in volcanic fumaroles, hot springs, and low temperature hydrothermal veins. It is frequently in association with realgar, the other arsenic sulphide mineral. Orpiment may also occur as an alteration product of realgar, but it is chemically distinct from pararealgar, which is the powdery yellow mineral formed when realgar is exposed to light.

Orpiment takes its name from the Latin auripigmentum, “gold pigment”, alluding to its colour. It is transparent, and has a lemon yellow streak and a Mohs hardness of 1.5-2. Like arsenic, it was used in the past as a pigment, despite the fact that both minerals alter and lose their colour on prolonged exposure to light (realgar to yellow, orpiment to white). Due to its arsenic content, orpiment is toxic.

Sources:

Wikipedia

Minerals.net

Webmineral

Stibnite, which may also be called antimonite or antimony glance, is a metallic lead to steel grey antimony sulphide mineral with the formula Sb2S3. It, like many other sulphide minerals, is quite soft (having a Mohs hardness of only 2). It has a specific gravity of 4.63. Its crystals often form in the characteristic slender prismatic habit also possessed by tourmaline, although it may form in the granular habit as well. While it is bright metallic grey when fresh, stibnite oxidises to black when exposed to air. Its streak is more or less the same colour as the mineral itself. Since stibnite contains antimony, it is potentially toxic. Stibnite is actually flexible, although it is not elastic.

Sources:

Wikipedia

Webminerals

Images:

Pyrite, a fairly ubiquitous member of the mineral kingdom, is a sulphide mineral well-known for its cubic or dodecahedral crystals, metallic lustre, and pale brass-yellow colour, of which the latter two characteristics earned this mineral the name fool’s gold. Despite this name pyrite is not difficult to distinguish from gold, through its superior hardness (pyrite’s is 6-6.5 and gold’s is only 2.5-3) and lesser density (pyrite has a specific gravity of 5 g/cm3, less than a third of gold’s density of 15-19 g/cm3). Interestingly, “fool’s gold” is sometimes found with small quantities of gold.

Its name comes from the Greek word pyr, for fire, because it sparked when struck against metal or another hard surface. For this reason pyrite was important to many ancient societies as a common, easy to use firestarter.

Pyrite, when exposed to air and moisture, will decompose into iron oxide and sulphate, the latter of which can combine with water to produce sulphuric acid. Acidithiobacillus bacteria oxidise pyrite, so the presence of this bacterium accelerates the decomposition of pyrite. Oxidisation is an exothermic reaction, producing energy (usually released as heat), and this poses a problem in some high-sulphur coal mines, where the oxidisation of pyrite can occasionally lead to spontaneous combustion.

Sources:

Mindat’s pages on pyrite and gold

Wikipedia

Images:

1: Golden pyrite crystals from Gilman District, Colorado, USA. Source. 2: Large pyrite dodecahedrons with microcrystalline pyrite from Huanzala Mine, Huallanca District, Dos de Mayo Province, Peru. Source. 3: Modified octahedral pyrite crystals with quartz-covered matrix from Butte, Montana, USA. Source.