Amethyst is a purple variety of quartz ( SiO < sub > 2 </ sub >) and owes its violet color to irradiation, iron impurities ( in some cases in conjunction with transition element impurities ), and the presence of trace elements, which result in complex crystal lattice substitutions.

The color of amethyst has been demonstrated to result from substitution by irradiation of trivalent iron ( Fe < sup > 3 +</ sup >) for silicon in the structure, in the presence of trace elements of large ionic radius, and, to a certain extent, the amethyst color can naturally result from displacement of transition elements even if the iron concentration is low.

Dendritic agates have fern like patterns in them formed due to the presence of manganese and iron oxides.

The intense heat of the flame made necessary the presence of heat shields made of iron ( βουκόλια, boukolia ), which are attested in the fleet inventories.

The iron chemistry in this previous classical model was elegant, but the required presence of the required diamagnetic high-energy singlet oxygen was never explained.

The locals also claimed that " the same spring is said to turn wood into iron ", but Defoe put this down to the presence of " copperas " in the water.

Iron's very common presence in rocky planets like Earth is due to its abundant production as a result of fusion in high-mass stars, where the production of nickel-56 ( which decays to the most common isotope of iron ) is the last nuclear fusion reaction that is exothermic.

Blacksmiths are credited with magical powers in many parts of the world, and it is significant that the Boudas are workers in iron and clay ; in the Life of N. Pearce ( i. 287 ) a European observer tells a story of a supposed transformation which took place in his presence and almost before his eyes ; but it does not appear how far hallucination rather than coincidence must be invoked to explain the experience.

The presence of iron as Fe < sup > 3 +</ sup > and Fe < sup > 2 +</ sup > helps understand oxygen fugacity, which in turn is a significant factor in petrogenesis.

In colloquial usage, the term is applied to red oxides, formed by the reaction of iron and oxygen in the presence of water or air moisture.

The reactivity of the iron center may be improved by the presence of the phenolate ligand of Tyr357 in the fifth iron ligand, which can assist in the oxidation of the Fe ( III ) to Fe ( IV ).

It weathers to iddingsite ( a combination of clay minerals, iron oxides and ferrihydrites ) readily in the presence of water.

The end of the 18th century saw the beginnings of the Industrial Revolution, and the presence of iron ore, limestone and large coal deposits in south-east Wales meant that this area soon saw the establishment of ironworks and coal mines, notably the Cyfarthfa Ironworks and the Dowlais Ironworks at Merthyr Tydfil.

The increasing presence of long-distance trade, an increase in local conflicts, and the introduction of bronze and iron metallurgy are trends denoting the end of the Mumun around 300 BC.

Liquid water was prevalent, and deep oceanic basins are known to have existed by the presence of banded iron formations, chert beds, chemical sediments and pillow basalts.

At that time when dietary iron may have been scarcer than today, the presence of the mutant allele may have provided an evolutionary or natural selection reproductive advantage by maintaining higher iron levels in the blood.

Each patient with the susceptible genotype accumulates iron at different rates depending on iron intake, the exact nature of the mutation and the presence of other insults to the liver such as alcohol and viral disease.

Carbon monoxide binds to iron in an end-on fashion, and is hindered by the presence of the distal histidine, which forces it into a bent conformation.

The volcanic rocks present from the prior eruptions feature olivine and have a small presence of hornblende.

The tails of comets ( which formed from the dust disk around the young Sun ) often have the spectral signature of olivine, and the presence of olivine has recently been verified in samples of a comet from the Stardust spacecraft.

Satellite gravity field measurements show a positive gravity anomaly centered on the caldera complex, suggesting the presence of a 600x300 km north-south elongated extinct magma chamber below, containing dense minerals ( probably mainly pyroxene, with olivine also possible ) that precipitated out of magma before eruptions.

Q-type asteroids are relatively uncommon inner-belt asteroids with a strong, broad 1 micrometre olivine and pyroxene feature, and a spectral slope that indiciates the presence of metal.

In addition, the MB spectra of rocks and outcrops displayed a strong decline in olivine presence,

It has a rotational period of 4. 7012 hours, and is classified as an R-Type asteroid due to the presence of strong absorption lines in olivine and pyroxene with little or no metals.

For the most part, eukaryotic cells sense the presence of chemotactic stimuli through the use of 7-transmembrane ( or serpentine ) heterotrimeric G-protein coupled receptors.

B. cereus competes with other microorganisms such as Salmonella and Campylobacter in the gut, so its presence reduces the numbers of those microorganisms.

The presence of a large scale high pressure subtropical ridge on each side of the equator reduces cloudiness at these low latitudes.

Trapping of the long-wave radiation due to the presence of clouds reduces the radiative forcing of the greenhouse gases compared to the clear-sky forcing.

Dolby HX Pro automatically reduces the bias signal in the presence of strong high frequency signals.

However, the presence of a large planet with a highly elliptical orbit in proximity to the habitable zone of the star reduces the likelihood of a terrestrial planet having a stable orbit within the habitable zone.

Thus the presence of an orthonormal basis reduces the study of a finite-dimensional inner product space to the study of R < sup > n </ sup > under dot product.

Furthermore, the presence of the spike reduces the commercial value of the wood, thereby reducing the economic viability of logging in the long term, while not threatening the life of the tree.

Conversely, a greater presence in the brain of opioid neurotransmitters such as endorphins reduces the frequency of yawning.

The presence of predators also reduces growth of a colony.

In 2007, he voted against the Mandatory Troop Rest Periods between Deployments to Iraq bill ( creates a mandatory rest period between deployments to Iraq for members of the Armed Forces, passed ) and against the Redeployment from Iraq Act ( reduces the presence of the U. S. armed forces in Iraq by April 1, 2008, passed ).

The form and consequences of conformity influence should undergo significant changes given the interposition of a medium that reduces the social presence of participants.

Recent literature has brought up the ideas that the Internet reduces social cues, facilitates a lowered sense of social presence and allows users to remain relatively anonymous.

Thus in the presence of h-principle, a differential topological problem reduces to an algebraic topological problem.

The fact that he bets at all indicates that he thinks he has an advantage in the bet, so the presence of vigorish reduces this edge by reducing the payout for a given amount wagered.

The presence of N < sub > 2 </ sub > also reduces oxidization known as solder dross.

Its presence, however, reduces both the amount of cholesterol the body absorbs from food and the reabsorption of the cholesterol component of bile.

The presence of multiple cylinders in reciprocating compressors reduces suction and discharge pulsations.

Nitrogen fixation is highly sensitive to the presence of oxygen, and therefore Azotobacter developed a special defensive mechanism against oxygen, namely a significant intensification of metabolism that reduces the concentration of oxygen in the cells.

The presence of water in magma reduces the melting temperature, and this may also play a role in enhancing Icelandic volcanism.

* A Cu compound ( in which copper is present in the + 2 oxidation state ) is added to the reaction in presence of a reducing agent ( e. g. sodium ascorbate ) which reduces the Cu from the (+ 2 ) to the (+ 1 ) oxidation state.

However at very low concentrations, the advantages of the coherent addition for CARS signal reduces and the presence of the incoherent background becomes an increasing problem.

This is necessary because, while the brake fluid is an incompressible liquid, air bubbles are compressible gas and their presence in the brake system greatly reduces the hydraulic pressure that can be developed within the system.

Also, the lighthouse is now automatic, which reduces the human presence only to the one of biologists assigned to the park's management.