Olmiite

{{Short description|Calcium manganese silicate mineral}} {{Infobox mineral|boxbgcolor=#e07c58|name=Olmiite|boxtextcolor = #fff|image=Olmiite-122298.jpg|formula=CaMn2+[SiO3(OH)](OH)|IMAsymbol=Olm|strunz=09.AF.90|system=Orthorhombic|dana=78.01.08.02|class=Dipyramidal H-M symbol: ''mmm'' (2/''m'' 2/''m'' 2/''m'')|symmetry=''Pbca''|unit cell=868|molweight=203.44|color=White, brown, pale to intense reddish pink|cleavage=None|fracture=Irregular, uneven|tenacity=Brittle|mohs=5 - 5.5|luster=Vitreous|opticalprop=Biaxial (+)|refractive=nα = 1.663(1) nβ = 1.672(1) nγ = 1.694(1)|birefringence=0.031|pleochroism=Non-pleochroic|2V=Measured: 71° Calculated: 66°|dispersion=r > v Distinct|fluorescence=May be deep red fluorescent in shortwave ultraviolet light|streak=White|gravity=3.05|density=Measured: 3.05 Calculated: 3.102|diaphaneity=Transparent, translucent}}

'''Olmiite''' is a rare calcium-manganese silicate that was named after an Italian mineralogist called Filippo Olmi. It was approved by the IMA in 2006, with its first description published in 2007.<ref name=":0">{{Cite web |title=Olmiite |url=https://www.mindat.org/min-30762.html |access-date=2022-08-29 |website=www.mindat.org}}</ref> Around 2001, a large amount of specimens believed to be poldervaartite was discovered at the N'Chwaning II mine, which is near the Wessels mine, where the latter was discovered. Only later were the researchers able to determine through their investigations that the two minerals are different, as they are visually indistinguishable. Until Renato Pagano acquired and examined the specimens, seemingly no specific investigation was carried out.<ref name=":2">{{Cite journal |last1=Pagano |first1=Renato |last2=Guastoni |first2=Alessandro |last3=Pezzotta |first3=Federico |date=2008-09-01 |title=Olmiite and poldervaartite from the Kalahari manganese field northern cape province republic of South Africa |url=https://go.gale.com/ps/i.do?p=AONE&sw=w&issn=00264628&v=2.1&it=r&id=GALE%7CA186599420&sid=googleScholar&linkaccess=abs |journal=The Mineralogical Record |language=English |volume=39 |issue=5 |pages=373–382}}</ref> Olmiite has been misidentified not only once, but twice. The cream-colored specimens were at first thought to be baryte by the mine geologist.<ref name=":3">{{Cite book |last=Cairncross |first=Bruce |url=https://books.google.com/books?id=Xp9nEAAAQBAJ&dq=olmiite&pg=PA582 |title=Minerals & Gemstones of Southern Africa |date=2022-06-01 |publisher=Penguin Random House South Africa |isbn=978-1-77584-754-0 |language=en}}</ref>

== Properties == Olmiite is the Mn2+ dominant analogue of poldervaartite.<ref name=":0" /> It typically has a pinkish core color, caused by manganese coloration, and it typically has a colorless outer rim.<ref name=":4">{{Cite web |title=The Earth Story |url=https://the-earth-story.com/post/167575027220/olmiite-this-very-rare-manganese-silicate |archive-url=https://web.archive.org/web/20220829123024/https://the-earth-story.com/post/167575027220/olmiite-this-very-rare-manganese-silicate |url-status=usurped |archive-date=August 29, 2022 |access-date=2022-08-29 |website=The Earth Story |language=en}}</ref> Olmiite shows a wide variety of crystal habits. It can crystallize in radiating spheres, meaning it grows into an orb-like shape, forming slender crystals which grow from a central point. It can grow as botryoidal, which are hemispherical masses; acicular, which is a needle-like crystal habit; in stacked crystal plates, and wheat sheaf crystal groups. It can also form individual pyramidal crystals, however, those are uncommon.<ref name=":1">{{Cite web |title=Olmiite: The mineral olmiite information and pictures |url=http://www.minerals.net/MineralDefinition.aspx?name=olmiite |access-date=2022-08-29 |website=www.minerals.net |language=en-US}}</ref> The aggregates are made of minute crystals.<ref name=":5">{{Cite web |url=https://pubs.geoscienceworld.org/minmag/article-abstract/71/2/193/140348/Olmiite-CaMn-SiO3-OH-OH-the-Mn-dominant-analogue?redirectedFrom=PDF|title=Olmiite, CaMn[SiO3(OH)](OH), the Mn-dominant analogue of poldervaartite, a new mineral species from Kalahari manganese fields (Republic of South Africa) |access-date=2022-08-29 |website=pubs.geoscienceworld.org}}</ref> Olmiite aggregates on the market typically reach a few centimeters, and an individual crystal can reach up to 7 mm.<ref name=":6">{{Cite web |date=2017-03-18 |title=Olmiite |url=https://www.nationalgemlab.in/olmiite/ |access-date=2022-08-29 |website=National Gem Lab |language=en-US}}</ref> In nature, specimens can range from thumbnail- to museum-sized slabs covered in olmiite crystals. However, most of the minerals only occur as tiny grains in rocks, which have to be found using a microscope.<ref name=":4"/> Olmiite mainly consists of oxygen (39.32%), manganese (22.95%) and calcium (22.65%), but otherwise contains silicon (13.81%), and has a negligible amount of hydrogen (0.99%) and iron (0.27%) in its formula as well. It shows no signs of radioactivity.<ref>{{Cite web |title=Olmiite Mineral Data |url=http://webmineral.com/data/Olmiite.shtml |access-date=2022-08-29 |website=webmineral.com}}</ref>

== Identification == Olmiite forms a solid solution series with the much rarer mineral poldervaartite, meaning they have the same basic chemical formula, but there's an element substitution in one or more atomic sites.<ref name=":6" /> As mentioned before, the atomic arrangement of the two are similar, the only difference being is in the case of olmiite, all the Mn cations are ordered on the ''M''2 site, while poldervaartite is Ca dominant at the ''M''2 site, hence the poldervaartite name stays reserved.<ref name=":5" /> One would think that its type locality would help in the identification, however, they both can be found in the Wessels mine in South Africa.

A way to differentiate them is that olmiite shows more variety both in color, and in crystal habits.<ref name=":2" /> One way to identify it more easily is that the cream-colored olmiite specimens have a deep red fluorescence under shortwave UV light.<ref name=":3" /> However, without examining its chemical composition, it is near impossible to identify whether the specimen is an olmiite or a poldervaartite.<ref name=":1" /> Hence it must be tested by electron microprobe analysis, as they can only be differentiated by the percentage of calcium versus manganese they contain. Due to the similarities these minerals have, the discovery that most of the poldervaartites on the market are olmiites in fact is quite recent. National Gem Lab further argues that due to the mislabeling of olmiites, it is possible that there's no faceted poldervaartite gemstones in existence.<ref name=":6" /> Later analyses also revealed that the two minerals can be intimately intergrown and thus both can occur within one specimen.<ref name=":3" />

== Occurrences and localities == Olmiite is a product of hydrothermal alteration.<ref name=":0" /> As such, it grows in manganese-rich hydrothermal replacement deposits.<ref name=":1" /> The crystals form by going through metamorphosis due to the hot hydrothermal fluids passing through a manganese deposit, adding silicon and calcium to the pre-existing elements.<ref name=":4" /> To this day, the only known locality of this mineral is in the Kalahari Manganese Field of South Africa. It has been found in both the Wessels Mine in Hotazel and in the N'Chwaning Mines in Kuruman within the Manganese Field's region. It is associated with calcite, manganite, bultfonteinite, oyelite,<ref name=":1" /> hematite, celestine, sturmanite, and poldervaartite,<ref name=":5" /> baryte, datolite, andradite, thaumasite, caryopilite and gageite.<ref name=":3" /> Not only is it a rare mineral, but rare as a gem as well, since most of the crystals are unsuitable for faceting.<ref name=":6" /> Due to its softness, it cannot be used in jewelries, so the faceted specimens are made for the collectors.<ref name=":4" />

==References== {{Reflist}}

Category:Orthorhombic minerals Category:Orthorhombic crystals Category:Orthorhombic crystal system