Serpentine Group Of Minerals

[Luciana]

Serpentinesubgroup (part of the kaolinite-serpentine group in the category of phyllosilicates)[1] are greenish, brownish, or spotted minerals commonly found in serpentinite. They are used as a source of magnesium and asbestos, and as decorative stone.[5] The name comes from the greenish color and smooth or scaly appearance from the Latin serpentinus, meaning "snake-like".

Serpentine subgroup is a set of common rock-forming hydrous magnesium iron phyllosilicate ((Mg,Fe)

3Si

2O

5(OH)

4) minerals, resulting from the metamorphism of the minerals that are contained in mafic to ultramafic rocks.[6] They may contain minor amounts of other elements including chromium, manganese, cobalt or nickel. In mineralogy and gemology, serpentine may refer to any of the 20 varieties belonging to the serpentine subgroup. Owing to admixture, these varieties are not always easy to individualize, and distinctions are not usually made. There are three important mineral polymorphs of serpentine: antigorite, lizardite and chrysotile.

Serpentine minerals are polymorphous, meaning that they have the same chemical formulae, but the atoms are arranged into different structures, or crystal lattices.[7] Chrysotile, which has a fibrous habit, is one polymorph of serpentine and is one of the more important asbestos minerals. Other polymorphs in the serpentine subgroup may have a platy habit. Antigorite and lizardite are the polymorphs with platy habit.

Samples of the oceanic crust and uppermost mantle from ocean basins document that ultramafic rocks there commonly contain abundant serpentine. Antigorite contains water in its structure, about 13 percent by weight. Hence, antigorite may play an important role in the transport of water into the earth in subduction zones and in the subsequent release of water to create magmas in island arcs, and some of the water may be carried to yet greater depths.

Serpentines find use in industry for several purposes, such as railway ballasts, building materials, and the asbestiform types find use as thermal and electrical insulation (chrysotile asbestos). The asbestos content can be released into the air when serpentine is excavated and if it is used as a road surface, forming a long-term health hazard by breathing. Asbestos from serpentine can also appear at low levels in water supplies through normal weathering processes, but there is as yet no fully proven health hazard associated with use or ingestion, although the EPA states an increased risk of developing benign intestinal polyps can occur.[17] In its natural state, some forms of serpentine react with carbon dioxide and re-release oxygen into the atmosphere.

The more attractive and durable varieties (all of the antigorite) are termed "noble" or "precious" serpentine and are used extensively as gems and in ornamental carvings. The town of Bhera in the historic Punjab province of the Indian subcontinent was known for centuries for finishing a relatively pure form of green serpentine obtained from quarries in Afghanistan into lapidary work, cups, ornamental sword hilts, and dagger handles.[9] This high-grade serpentine ore was known as sang-i-yashm in Persian, or 'false jade' in English, and was used for generations by Indian craftsmen for lapidary work.[9][18] It is easily carved, taking a good polish, and is said to have a pleasingly greasy feel.[19] Less valuable serpentine ores of varying hardness and clarity are also sometimes dyed to imitate jade.[19] Misleading synonyms for this material include "Suzhou jade", "Styrian jade", and "New jade".

The lapis atracius of the Romans, now known as verde antique, or verde antic, is a serpentinite breccia popular as a decorative facing stone. In classical times it was mined at Casambala, Thessaly, Greece. Serpentinite marbles are also widely used: Green Connemara marble (or 'Irish green marble') from Connemara, Ireland (and many other sources[citation needed]), and red Rosso di Levanto marble from Italy. Use is limited to indoor settings as serpentinites do not weather well.

Soils derived from serpentine are toxic to many plants, because of high levels of nickel, chromium, and cobalt; growth of many plants is also inhibited by low levels of potassium and phosphorus and a low ratio of calcium/magnesium. The flora is generally very distinctive, with specialized, slow-growing species. Areas of serpentine-derived soil will show as strips of shrubland and open, scattered small trees (often conifers) within otherwise forested areas; these areas are called serpentine barrens.

Bowenite, a variety of antigorite, is an especially hard serpentine (5.5) of light to dark apple green color, often mottled with cloudy white patches and darker veining. It is the serpentine most frequently encountered in carving and jewelry. The name 'retinalite' is sometimes applied to yellow bowenite. The New Zealand material is called tangiwai.

Although not an official species, bowenite is the state mineral of Rhode Island, United States: this is also the variety's type locality. A bowenite cabochon featured as part of the "Our Mineral Heritage Brooch", was presented to U.S. First Lady Mrs. Lady Bird Johnson in 1967.

Williamsite is an American local varietal name for antigorite that is oil-green with black crystals of chromite or magnetite often included. Somewhat resembling fine jade, williamsite is cut into cabochons and beads. It is found mainly in Maryland and Pennsylvania.[20]

Serpentine from Pizzo Tremogge was traditionally identified as lizardite, although a full mineralogical investigation of this material has yet to be conducted. Recent preliminary data by Adamo et al. (2014) proved that antigorite and chrysotile also occur together with lizardite. Therefore, it seems useful to provide a further detailed characterization of the serpentine from Pizzo Tremogge, focusing on its use as a gem material. We investigated a suite of rough and cut samples provided by Mr. Pietro Nana (Sondrio, Italy), using gemological characterization, electron microprobe chemical analyses, and Raman spectroscopy. The latter is a reliable and nondestructive method for identifying the three serpentine minerals (Rinaudo et al., 2003).

Serpentine-group minerals include lizardite, chrysotile, and antigorite, which are polymorphs of the Mg-rich hydrous phyllosilicate with the approximate chemical formula Mg3Si2O5(OH)4. To some extent, Fe, Al, and Ni may be substituted for antigorite, and Al for Si (Deer et al., 2009). The basic structural unit consists of an Mg-rich sheet, linked on one side to a single tetrahedral silicate sheet, with hydrogen bonding between the layers (Evans et al., 2013). Lizardite, chrysotile, and antigorite are distinguished by their crystal microstructure, consisting of different arrangements of the layers. Lizardite and chrysotile are characterized by a flat and a curved/cylindrical crystal microstructure, respectively, while antigorite displays wavy layers resulting in a corrugated microstructure (Evans et al., 2013). Serpentine minerals form by the hydration of olivine-rich ultramafic rocks at relatively low temperature; this is the serpentinization process. The three serpentine-group minerals have different stability fields (Deer et al., 2009). In particular, lizardite is the first phase that commonly pseudomorphs after olivine; chrysotile occurs mainly as a filling in the fractures that crosscut serpentinite rocks, whereas antigorite is considered the high-temperature phase, growing from lizardite and chrysotile with increasing grade of metamorphism at temperatures above about 320C (Evans et al., 2013). Chrysotile is also the main constituent of commercial asbestos, which was used extensively for thermal and electric insulation until the discovery that its fine dust is harmful to human health (Fubini and Fenoglio, 2007).

Val Malenco is located at the border of southeastern Switzerland and northern Italy, between the Penninic and the Austroalpine domains of the Alps (Mntener et al., 2000). The regional geology appears complex due to extensive tectonic disruption associated with a stack of Alpine nappes (Penninic and Austroalpine nappe system). Three major structural complexes, shown in figure 4 from east to west, characterize this area:

In the Margna unit, the crystalline basement rocks show intercalations of carbonate rocks both of Paleozoic and Mesozoic age (Bedogn et al., 1993). The more ancient lithologies consist mainly of calcite-bearing marbles that preserve the amphibolite-facies paragenesis conditions of the enclosing rocks (gneisses and metagabbros). The composition of these marbles is not homogenous and is mainly related to the different proportions of carbonate and other associated minerals. Marbles in the area of Pizzo Tremogge (again, see figure 2), located 2800 m above sea level, are multicolored (yellow, yellow-green, and brown-orange) and show a rich mineral content, with magnesium silicates (clinohumite, olivine, serpentine, diopside, chlorite, and phlogopite), spinel, graphite, hematite, pyrite, and brucite. In most cases, olivine (forsterite) is completely replaced by yellow-green serpentine and chlorite (Bedogn et al., 1993).

Petrographic Examination and Raman Spectroscopy. When observed with a petrographic microscope, the samples showed a coarse-grained structure. Serpentine was the dominant mineral, alternating with carbonate (calcite and dolomite), quartz, brucite, and chlorite veins (figure 6).

The identification of serpentine minerals (lizardite, antigorite, and chrysotile) through petrographic examination is difficult, owing to their similar optical properties as well as their submicroscopic intergrowths (Rinaudo et al., 2003; Groppo et al., 2006). Four samples (S1, S2, S3, and S4) were therefore examined by petrographic Raman spectroscopy to determine their mineralogical composition. The Raman spectra of the four serpentine thin sections are shown in figure 7, whereas table 3 presents the bands observed and their assignments.

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