Ilaria Adamo

Green andradite stones: gemmological and mineralogical characterisation

A multi-methodological investigation of natural gem-quality andradite crystals occurring in serpentinites (Val Malenco, Italy; Nizhniy Tagil, Russia; Kerman, Iran, and Kaghan, Pakistan), or in skarn rocks (Erongo, Namibia, and Antetezambato, Madagascar) has been performed by means of gemmological standard testing, electron-microprobe chemical analyses in wavelength-dispersive mode (EMPA-WDS), ultraviolet, visible and near-infrared (UV-Vis-NIR) and mid-infrared (MID-IR) spectroscopy, and single-crystal X-ray diffraction, in order to describe the gemmological properties and the crystal chemistry of these marketable green stones. The EMPA-WDS data show that these green garnets are almost pure andradite [Adr (Ca3Fe2Si3O12)>96.1 mol%]. The Cr-content ranges, as Cr2O3, from <0.01 up to ~1.0 wt%. This chromophore element, if present, influences the garnet’s colour, which is mainly due to Fe3+. The Cr-content does not affect the unit-cell constant, at least within the range found here. The refined electron content at the Fe-site suggests that chromium occupies the octahedral site along with iron, as found in previous investigations. The MID-IR spectra show the presence of hydroxyl groups. The refined site occupancy factors of the Si-site, modelled with the scattering curve of silicon alone, range between 99.2(4) and 99.9(2), which barely suggests potential hydrogarnet substitution [ i.e. , (SiO4)4− ↔(O4H4)4−].

Gemmological investigation of a synthetic blue beryl: a multi-methodological study

Abstract A multi-methodological investigation of a synthetic Cu/Fe-bearing blue beryl [IV(Be2.86Cu0.14)∑=3.00VI(Al1.83Fe0.143+Mn0.032+Mg0.03)∑=2.03IV(Si5.97Al0.03)∑=6.00O18·(Li0.12Na0.04·0.40H2O)] has been performed by means of gemmological standard testing, electron microprobe chemical analyses, laser ablation inductively coupled plasma mass spectroscopy, thermo-gravimetric analyses, infrared spectroscopy and single-crystal X-ray diffraction in order to determine the gemmological properties, crystal structure and crystal-chemistry of this material. The increasing production of marketable hydrothermal synthetic beryls with ‘exotic’ colours and the small number of studies on the accurate location of chromophores in the crystal structure inspired this multi-methodological investigation. The X-ray structural refinements confirm that the space group of the Cu/Fe-bearing blue beryl is P6/mcc, with unit-cell parameters: 9.2483 ≤ a ≤ 9.2502 Å and 9.2184 ≤ c ≤ 9.2211 Å. The analysis of the difference Fourier maps of the electron density suggests that Cu is located at the tetrahedral site (Wyckoff 6f position) along with Be, whereas Fe shares the octahedral site with Al (4c position). No evidence of extra-framework Cu/Fe-sites (i.e. channel sites) has been found. The Li is probably located at the extra-framework 2b site. Infrared spectra show that the H2O molecules are present with two configurations: one with the H⋯H vector oriented ‖[0001] and the other with H⋯H vector oriented ⊥[0001].

Demantoid from Val Malenco, Italy: Review and Update

GEMS & GEMOLOGY WINTER 2009 emantoid is the Cr-bearing yellowish green to green variety of andradite [Ca3Fe2(SiO4)3] (O’Donoghue, 2006). Very popular in Russia (where it was first discovered) from about 1875 to the start of the Russian Revolution in 1917, this gem has enjoyed a resurgence in demand since the beginning of the 21st century (Furuya, 2007). One of the most notable localities for demantoid is Val Malenco, located in Sondrio Province in the Lombardy region of northern Italy. Several deposits in this area have produced well-formed rhombic dodecahedral crystals (e.g., figure 1, left) that are coveted by collectors (Bedogne and Pagano, 1972; Amthauer et al., 1974; Bedogne et al., 1993, 1999). A limited quantity (some thousands of carats) of Val Malenco demantoids have been cut, producing gemstones that are attractive but rarely exceed 2 or 3 ct (e.g., figure 1, right). Val Malenco demantoid was first documented by Cossa (1880), who studied a sample recovered by T. Taramelli in 1876. In the next century, Sigismund (1948) and Quareni and De Pieri (1966) described the morphology and some physical and chemical properties of this garnet. Subsequently, the demantoid was investigated by Bedogne and Pagano (1972), Amthauer et al. (1974), Stockton and Manson (1983), and Bedogne et al. (1993, 1999). Because some of these data are more than 20 years old, and some publications are in Italian, we prepared this review and update on the physical, chemical, and gemological properties of demantoid from Val Malenco. Note that demantoid—although commonly used as a trade or variety name—is not approved by the International Mineralogical Association as a mineral name (Nickel and Mandarino, 1987; O’Donoghue, 2006). However, for reasons of brevity and consistent with gemological convention, throughout this article we will use demantoid instead of andradite, variety demantoid.

Aquamarine, Maxixe-type, and hydrothermal synthetic blue beryl: Analysis and Identification

MATERIALS AND METHODS We examined a total of 25 natural, treated, and synthetic blue beryl specimens (see, e.g., figure 1): four faceted Brazilian aquamarines (0.18 to 1.54 ct); one faceted and one rough aquamarine from Nigeria (1.81 and 54.02 ct, respectively); three faceted (0.08–0.13 ct) and two rough (7.30 and 8.20 ct) aquamarines (marketed as “True Blue” beryl) from the Yukon Territory, Canada; one faceted Maxixe-type (irradiated) blue beryl (1.77 ct); three faceted Tairus hydrothermal synthetic blue beryls (2.03–3.50 ct); AQUAMARINE, MAXIXE-TYPE BERYL, AND HYDROTHERMAL SYNTHETIC BLUE BERYL: ANALYSIS AND IDENTIFICATION

New Occurrence of Fire Opal from Bemia, Madagascar

GEMS & GEMOLOGY SUMMER 2010 O pals are water-bearing microand noncrystalline silica minerals, with the chemical formula SiO2•nH2O (see, e.g., Graetsch et al., 1994; Downing, 2003; O’Donoghue, 2006). One attractive variety is fire opal, which is characterized by a red-orange-yellow bodycolor, with or without play-of-color (O’Donoghue, 2006). This opal variety does not have the typical structure of play-of-color opal; rather, it is composed of random aggregates of hydrated silica nanograins ~20 nm in diameter (Fritsch et al., 2006; Gaillou et al., 2008b). The most famous locality for fire opal, one that has been producing fine material for more than 100 years, is the Querétaro area of Mexico (see, e.g., Koivula et al., 1983; Gübelin, 1986). Other sources include the United States, Turkey, Australia, Indonesia, Ethiopia, Somalia, Kazakhstan, Canada, and Brazil (Ball and Daniel, 1976; Smith, 1988; Bittencourt Rosa, 1990; Holzhey, 1991; Bank et al., 1997; Enseli et al., 2001; O’Donoghue, 2006). Opal, including the fire variety, is also known to come from various regions of Madagascar, particularly the Faratsiho deposit, located near the capital Antananarivo, in the center of the island (Lacroix, 1922). A new source of common opal, including fire opal (e.g., figure 1), was discovered a few years ago in the southeastern part of the island. According to A. and L. Pasqualini (pers. comm., 2010), who visited the deposit in May 2008, the opal is found near the city of Bemia, 70 km from the coast (figures 2 and 3). The opal occurs in Cretaceous rhyodacite volcanic rocks. Local people search for the opal by digging small pits, and ~200–400 kg of mixed-quality rough material has been produced. The opal generally occurs as nodules up to several centimeters in diameter or in veins up to 20–30 cm long, with large variations in quality and color. The rough opal is typically sent to the city of Antsirabe, 450 km north of Bemia, where it is fashioned into cabochons or faceted into fine gemstones that typically weigh up to 15 ct. Many of the various colors are typical of fire opal, but no play-of-color has been seen. Building on the work of Simoni and Caucia (2009, in Italian), the present article describes the standard gemological properties of Bemia opal, as well as the inclusions, powder X-ray diffraction patterns, chemical composition, and spectroscopic features.

NEPHRITE JADE FROM VAL MALENCO , ITALY : R EVIEW AND UPDATE

Alpe Mastabia, in the Val Malenco district of northern Italy, has been a source of nephrite jade since the early 2000s. Twenty-one samples from this locality were investigated by classical gemological methods; X-ray powder diffraction, combined with quantitative phase analysis; scanning electron microscopy in combination with energy-dispersive spectrometry; electron microprobe analysis; mass spectrometry; and mid-infrared spectroscopy. From a mineralogical standpoint, this jade consists mainly of tremolite amphibole, with variable amounts of other constituents, especially calcite (up to approximately 30 wt.%), but also pyroxene, apatite, and sulfide minerals. Its pale green color is related to the low iron content of the tremolite amphibole, whereas the other minerals are responsible for different colors (cal cite for white, molybdenite and galena for gray). On the basis of minor and trace-element composition, we can classify this jade as dolomite-related nephrite (para-nephrite). Although new material could be recovered from this area, future production will probably be limited by access difficulties.

Physical and chemical properties of some italian opals

The physical and compositional properties of some opals from different parts of Italy have been investigated through several methodologies like optical analysis, specific gravity, refractive indices, xrpd, ir spectroscopy, la-icp-ms. The opals show different colors: white, white brownish, white yellowish, white yellowish greenish and greyish. Black and metallic inclusions, consisting of todorokite, are sometimes present. Play of color have not been observed but some opals show small iridescence zones; opals are inactive to the long and short wavelength uv radiation (366 - 254 nm) with the exception of one sample and also phosphorescence is absent. Refractive index and specific gravity values are n = 1.43 - 1.44 and G = 2.07 - 2.33 g/cm 3 in agreement with literature. xrpd analyses highlighted Italian opals are A, CT and C types, but most of them can be classified as CT opals. IR spectroscopy data confirmed the opal classification. The most abundant elements are Mg (between 400 and 900 ppm), Fe (35-400 ppm), Ca (70-96 ppm) and Ni (20-70 ppm). Similarly to what observed in other opals worldwide, Fe appears to be the principal factor that determines the white brownish color and of the yellowish shade. Chromophore elements like V, Cr, Cu, Ti, Co and Ni are present in very low contents and do not influence the physical properties of the Italian opals. Mn is clearly detected (42 ppm) only in the sample n. 2 and is related to the presence of dendrites. Ca and Mg (non chromophore elements) are probably related to the matrix. On the whole the investigated Italian opals show a rather homogeneous trace element composition that appear well differentiated from that of other opals worldwide.

Investigation on the gemological, physical and compositional properties of some opals from Slovakia ("Hungarian" opals)

The physical and compositional properties of some “Hungarian” opals have been investigated through several methodologies such as optical analysis, specific gravity, refractive indices, AvaSpec spectroscopy, XRPD, IR and Raman spectroscopy, LA-ICP-MS. The “Hungarian” opal deposits, now in Slovakia, represented the largest and most significant gem opal deposits in Europe from the Roman Times to XIXth century. The analysed opals include brown semiopals, white opals and hyalite. In general, chromophore elements like V, Cr, Cu, Ti, Co are present in very low concentrations and do not influence the physical properties of the opals. Elements like Na, Mg, Al, K, Ca, Sc, Cr and Fe have been detected, in variable amounts, in all the investigated samples. The semiopals can be used as ornamental and semiprecious material, are mostly made up by CT opal close to pure tridymite and lower amounts of goethite, show high trace elements contents while the REE pattern show a clear Eu negative anomaly. These opals probably formed during an hydrothermal, or late magmatic stage. The white opals are made up by amorphous opal (A), show appreciable play of color and iridescence and are surely the most relevant for gemological purposes. The most pure white opals show low contents of trace elements and a rather homogeneous composition, that can be used as geographical marker. Conversely some opals affected by contamination from host rocks show much higher trace element contents. White opals probably formed during a low temperature tectonic event. The samples of hyalite are yellowish, opaque and greasy. They are made up by CT opal show relevant content of B and As while the other trace element are quite low, and are probably of volcanic origin.

Aquamarine from the Masino-Bregaglia Massif, Central Alps, Italy

GEMS & GEMOLOGY FALL 2009 he Masino-Bregaglia Massif (also known as the Bergell Massif) contains numerous granitic pegmatites hosting a remarkable variety of minerals—including aquamarine (figure 1)—that have attracted the interest of mineralogists and collectors since the late 18th century (e.g., Bedogné et al., 1995). Beryl from this area was initially mentioned by Repossi (1916) and Staub (1924). Subsequently, many other beryl occurrences were discovered in the massif. In their listing of the locations of historical beryl-bearing pegmatites, Hügi and Röwe (1970) indicated that the most important Italian deposits occurred in the areas of Val Bregaglia (Bregaglia Valley), Valle Mello, Cima di Zocca, Val Masino, Val Codera, and Alpe Vazzeda. In the 1970s, a limited amount of gem-quality aquamarine was recovered and cut from the Filone Silvana (Silvana dike), located in Val Codera. Masino-Bregaglia aquamarine crystals typically show a prismatic habit and measure several centimeters long, although some crystals attain ~15–20 cm in length. They range from light to dark greenish blue to blue or yellowgreen. Some gemand carving-quality aquamarine has been recovered, although the fact that most of the crystals contain numerous inclusions and fractures makes such material rare (Bedogné et al., 1995). To our knowledge, a gemological characterization of this aquamarine is lacking, except for the recent work of Caucia et al. (2008, in Italian). The present article builds on that work by supplying additional data obtained on a larger number of samples from four pegmatites in this area.

Tsavorite and other Grossulars from Itrafo, Madagascar

GEMS & GEMOLOGY FALL 2012 G with the chemical formula Ca3Al2(SiO4)3, is a species of the garnet group that exhibits colors ranging from colorless to pink, brown, yellow, orange, and green. The latter is known by the varietal name tsavorite when the color is a saturated green (O’Donoghue, 2006), whereas less-saturated material is often referred to as green grossular or mint green grossular in the trade. Although tsavorite is not approved as a mineral name by the International Mineralogical Association (Nickel and Mandarino, 1987; O’Donoghue, 2006), we will use the term in this article for the sake of brevity and consistency with gemological convention. The most important deposits of gem-quality tsavorite occur in Tanzania and Kenya (Bridges, 1974). Other notable sources include Pakistan’s Swat Valley (Jackson, 1992) and the Gogogogo area in southwestern Madagascar (Mercier et al., 1997; Johnson et al., 1999). A new source of fine gem-quality grossular (figure 1), including some tsavorite, was discovered in 2002 at the village of Itrafo in central Madagascar. This article presents a detailed characterization of this material.