Mogánite

Chemical formula: SiO<sub>2</sub>·<i>n</i>H<sub>2</sub>O

Moganite is a monoclinic polymorph of quartz, often found in mixtures with chalcedony and opal, invisible to the naked eye.

## Characteristics Moganite is a mineral from the silicate group, representing a monoclinic, high-pressure polymorphic variety of quartz. It typically does not form macroscopic crystals and occurs as submicroscopic fibers or grains, most often mixed with chalcedony or opal-CT. For this reason, moganite identification requires advanced analytical techniques, such as X-ray diffraction (XRD) or Raman spectroscopy. Its presence has been confirmed in many varieties of microcrystalline quartz, especially in agates and cherts, where it can constitute from a few to as much as 85% of the material's volume. ## Physical properties Due to its occurrence as submicroscopic admixtures, its physical properties are difficult to measure directly and are usually an averaged value for the material in which it occurs (e.g., chalcedony). Pure, synthetic moganite has a hardness of about 6 on the Mohs scale and a density of 2.55 g/cm³, which is slightly lower than quartz. Luster and transparency are properties of the aggregate – typically a waxy to dull luster and a translucent character, typical of chalcedony. ## Colors and varieties As a constituent mineral, moganite does not have its own color varieties. It adopts the color of the material in which it occurs, and thus can be part of agates and jaspers of any color – from white, through grays, yellows, reds, to black. ## History and name The mineral's name comes from the town of Mogán on the island of Gran Canaria (Canary Islands, Spain), where it was first identified and described in 1984 by Klaus Flörke and Ursula Flörke. It was officially recognized by the International Mineralogical Association (IMA) in 1999, when it was proven to be a distinct mineral phase, not just a mixture.

Properties

Mohs hardness
6
Luster
Waxy
Streak
White
Density
2.55
Cleavage
None
Fracture
Conchoidal
Transparency
Translucent to opaque
Crystal system
Monoclinic

Diagnostic features

## Identification Moganite identification is impossible without specialized equipment. In collecting practice, the material in which moganite occurs is recognized, which is most often chalcedony or chert. Characteristics of such material include: hardness of about 6.5-7, waxy or dull luster, conchoidal fracture, and lack of cleavage. ## Distinguishing from similar minerals The presence of moganite distinguishes chalcedony from "ordinary" microcrystalline quartz only at the level of crystal structure. Macroscopically, these materials are indistinguishable. It is distinguished from opal by its higher hardness and lack of characteristic play-of-color (opalescence), although it often co-occurs with opal-CT. ## Crystal forms Moganite forms submicroscopic, fibrous or bladed crystals that aggregate into spherulitic or radial clusters. These forms are visible only under an electron microscope. On a macroscopic scale, it occurs in massive, reniform, and botryoidal aggregates, typical of chalcedony.

Geological environment

## Genesis Moganite crystallizes under low-temperature and relatively high-pressure conditions, from silica-rich solutions. It forms in evaporitic environments, at alkaline pH. It is a metastable phase that over time (on a scale of millions of years) recrystallizes into the more stable form, quartz. Its presence is often an indicator of a relatively young geological age of the rock. ## Mineral associations It most often co-occurs with quartz (in the form of chalcedony) and opal (mainly opal-CT). It is also accompanied by calcite, dolomite, and various clay minerals. ## Localities As a constituent mineral, it is widely distributed. It has been identified in cherts and agates from many places around the world. In addition to the type locality in Mogán (Gran Canaria, Spain), its presence has been confirmed, among others, in agates from Arizona and California (USA), in Brazil, Uruguay, as well as in flints and cherts from various geological formations in Europe, including Poland.

Rarity

Common

For collectors

## Quality criteria Since moganite is not collected as a separate phase, its value is identical to the value of the specimen in which it occurs. In the case of agates and chalcedonies, quality is determined by: intensity and arrangement of colors (pattern), transparency, size, and absence of fractures. The presence of moganite is a scientific curiosity that does not directly affect the market price of a specimen for a typical collector. ## Popular localities The most prized agate and chalcedony specimens that may contain moganite come from classic localities such as Brazil (Rio Grande do Sul), Uruguay, Mexico (e.g., Laguna agates), USA (e.g., Lake Superior agates), and Poland (Płóczki Górne, Nowy Kościół).

Care and storage

## Cleaning Specimens containing moganite (mainly agates and chalcedonies) should be cleaned like quartz. The safest method is lukewarm water with mild soap and a soft brush. After washing, they should be thoroughly rinsed and dried. ## What to avoid Avoid contact with strong chemicals, especially hydrofluoric acid (HF), which etches silica. Although it is a stable material, extreme and sudden temperature changes should be avoided, as they can cause internal fractures in agates and chalcedonies. ## Storage Chalcedony specimens with moganite are hard and durable. They can be stored in collections without special precautions, protecting them only from heavy soiling and the possibility of scratching by harder minerals (e.g., corundum, diamond).

External references

Sources

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