The alkaline earth metals are the elements in Group 2 of the Periodic Table: beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), barium (Ba), and radium (Ra).
Their minerals usually contain more than 25 atomic % of these elements.
Beryllium occurs in a hexgonal close-packed (hcp) crystal structure at room temperature (α-Be).
As indicated in the phase diagram on the left beryllium occurs as (β-Be) which is bcc at higher temperatures up to melting.
Native beryllium is not known to occur on the surface of the Earth, but may eventually be found among beryllium-bearing minerals in small amounts.
Taaffeeite was first identified from gem rough material, rather than from a crystal. Crystals are still extremely rare, probably the rarest and most desirable of the various gem species found in Sri Lanka. This crystal is a complete hexagonal crystal. It has good uniform purple color, complete all around. The crystal's rear face was polished slightly for a refractive index test, just to be sure of its verification (this is an important test for the species).
Name: "Named taaffeite in honor of Count Edward Charles Richard Taaffe, gemologist of Dublin (1898, Bohemia, Austria-Hungary - 1967, Dublin, Ireland), who discovered the mineral in 1945. It was the first case of a new mineral discovered as a facetted gem. The suffix was added by Armbruster et al. (2002) to indicate the polysome."
Chemical and X-ray analysis confirmed the principal constituents of taaffeite as beryllium, magnesium and aluminium, making taaffeite the first mineral to contain both beryllium and magnesium as essential components.
Member of the Taafeite Group > Högbomite Supergroup.
"The stacking of the spinel-type [S] (Mg
8) and nolanite-type [N] (BeMgAl
8) modules is N'SN'S. Malcherek & Schlüter (2016) describe a new polytype of the mineral, Magnesiotaaffeite-2N’2S2, with N'SSN" stacking sequence; this new polytype crystallizes in the trigonal system, space group P-3m1; it is Cr-bearing."
Taaffeite occurs in carbonate rocks alongside fluorite, mica, spinel and tourmaline, is increasingly found in alluvial deposits in Sri Lanka and southern Tanzania, as well as lower grade taaffeite in limestone sediments in China.
Unlike spinel, taaffeite displays the property of double refraction that allows distinction between these two minerals.
Bromellite is BeO, with 50 at % beryllium.
Magnesium has a hcp structure from room temperature up to melting. No other phases occur as is shown in the magnesium-end of the iron-magnesium phase diagram on the left.
Native magnesium is unlikely to occur on the surface of the Earth and is not known to occur.
Chloromagnesite, or chlormagnesite, has the formula MgCl2, with 33.3 at % magnesium.
Mg2SiO4 is the formula for forsterite, with 28.6 at % magnesium.
Forsterite is a member of the olivine group of minerals.
Calcium has a face-centered cubic (fcc) crystal structure at room temperature.
As shown in the phase diagram on the left, it does not change structure up to melting.
Native calcium is not known to occur on the surface of the Earth.
Bredigite is α-Ca2SiO4, with 28.5 at % calcium.
γ-dicalcium silicates Edit
γ-dicalcium silicate has the formula γ-Ca2SiO4, with 28.5 at % calcium.
Strontium at room temperature crystallizes in a fcc structure (α-Sr).
According to the phase diagram on the left, α-Sr transforms to γ-Sr (bcc) at 547°C.
Native strontium does not appear to occur on the surface of the Earth.
Barium is bcc (α-Ba) at room temperature as the phase diagram on the left indicates. It does change to an hcp structure at high pressures and temperatures.
Native barium is not known to occur on the surface of the Earth.
"Solid radium is bcc at room temperature. Radium melts at 973 K.63"
- Native alkaline earths may require very arid and reducing environments to occur.
See also Edit
- Magnesiotaaffeite, Mindat
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