The real, rare stu

For­get di­a­monds and ru­bies. Th­ese are the world’s rarest gems.

The Star Malaysia - Star2 - - ECOWATCH -

SCI­EN­TISTS have in­ven­to­ried and cat­e­gorised all of Earth’s rare min­eral species de­scribed to date, each sam­pled from five or fewer sites around the globe. In­di­vid­u­ally, sev­eral of the species have a known sup­ply world­wide smaller than a sugar cube.

Th­ese 2,550 min­er­als are far more rare than pricey di­a­monds and gems. But while their rar­ity would log­i­cally make them the most pre­cious of min­er­als, many would not work in a wed­ding ring set­ting. Sev­eral are prone to melt, evap­o­rate or de­hy­drate. And a few, vampire- like, grad­u­ally de­com­pose on ex­po­sure to sun­light.

Their great­est value to hu­man­ity lies in the tell- tale clues they of­fer about the sub- sur­face con­di­tions and el­e­ments that cre­ated them, as well as in­sights into the planet’s past bi­o­log­i­cal upheavals. In fact, rare min­er­als rep­re­sent Earth’s truest dis­tinc­tion from all other plan­ets, ac­cord­ing to au­thors of a pa­per in the jour­nal Amer­i­can Min­er­al­o­gist.

Sci­en­tists Robert Hazen of Carnegie In­sti­tu­tion and Jesse Ausubel of Rock­e­feller Univer­sity say that know­ing fully the min­eral sig­na­ture of our life- sup­port­ing planet – un­der­stand­ing the dis­tinct com­bi­na­tions of cir­cum­stances that cre­ate rare min­er­als – also hints at what an in­ter- plan­e­tary probe might find.

Their pa­per, On the Na­ture and Sig­nif­i­cance of Rar­ity in Min­er­al­ogy, es­tab­lishes the first sys­tem for cat­e­goris­ing rar­i­ties in the min­eral king­dom and pro­vides min­er­al­o­gists with a frame­work that par­al­lels one used for un­der­stand­ing rare plant and an­i­mal species.

Not so rare

The au­thors note the irony that pre­cious gems and other min­er­als highly val­ued by hu­mankind – in­clud­ing so- called “rare earth” min­er­als re­quired to make elec­tron­ics – do not meet the def­i­ni­tion of rare as far as planet Earth is con­cerned.

“Di­a­mond, ruby, emer­ald, and other pre­cious gems are found at nu­mer­ous lo­cal­i­ties and are sold in com­mer­cial quan­ti­ties, and thus are not rare in the sense used in this con­tri­bu­tion. Uses of the word ‘ rare’ in the con­text of ‘ rare earth el­e­ments’ or ‘ rare me­tals’ are sim­i­larly mis­lead­ing, as many thou­sands of tonnes of th­ese com­modi­ties are pro­duced an­nu­ally,” they wrote.

There are 5,090 recog­nised min­eral species. Of th­ese, fewer than 100 make up 99% of Earth’s crust. Around 2,550 are de­fined as rare – found at five or fewer lo­ca­tions world­wide. More than two- thirds of the known min­eral species, in­clud­ing the great ma­jor­ity of rare species, have been at­trib­uted to bi­o­log­i­cal changes in Earth’s near- sur­face en­vi­ron­ment. Each rare min­eral fits into th­ese cat­e­gories:

> Unique con­di­tions that cre­ated the min­eral

“Imag­ine mak­ing min­er­als at a kitchen stove us­ing a pres­sure cooker,” says Hazen. “What re­sults in the pot is a func­tion of vari­ables tem­per­a­ture, pres­sure and the in­gre­di­ents, and is one or more of just 72 chem­i­cal el­e­ments that make up Earth’s min­eral king­dom.”

Some min­er­als are rare be­cause, even though they form from the com­mon­est of in­gre­di­ents, they must be cooked at exquisitely con­trolled con­di­tions. For ex­am­ple, the min­eral ha­tru­rite is formed from three of Earth’s most abun­dant el­e­ments, cal­cium, sil­i­con, and oxy­gen, but only in tem­per­a­tures above 1,250° C and in the ab­sence of an­other ex­tremely com­mon el­e­ment, alu­minium.

By know­ing the idio­syn­cratic com­bi­na­tion of cir­cum­stances in­volved in a rare min­eral’s cre­ation, sci­en­tists can de­duce what el­e­ments are or are not present at a spe­cific depth and in some cases, such in­for­ma­tion as acid­ity at that level.

> Plan­e­tary con­straints

Other min­er­als are ex­tremely rare be­cause their in­gre­di­ents are al­most never found con­cen­trated in Earth’s crust. Thus, such scarce chem­i­cal el­e­ments as beryl­lium, hafnium and tel­lurium form rel­a­tively few min­er­als and most species are rare.

> Ephemeral min­er­als

Some min­er­als form un­der un­usual con­di­tions – ex­treme cold or dry en­vi­ron­ments, for ex­am­ple – but then sim­ply melt, evap­o­rate or de­hy­drate when ex­posed to dif­fer­ent sur­face con­di­tions. A crys­talline form of methane hy­drate, found in core sam­ples from Arc­tic drill sites, evap­o­rates at room pres­sure.

More than 100 min­eral species can per­sist in dry en­vi­ron­ments for many years, only to be washed away dur­ing rare rain events. Among the least sta­ble are rare min­eral species that ad­sorb mois­ture from the air then dis­solve in it. And a few, like edoy­lerite, metasiderona­trite and siderona­trite grad­u­ally de­com­pose on ex­po­sure to sun­light.

> Places ge­ol­o­gists rarely sam­ple

Some rare min­er­als sim­ply come from un­der- sam­pled re­gions, from ex­treme en­vi­ron­ments such as the flanks of erupt­ing vol­ca­noes, frigid and re­mote re­gions of Antarc­tica, and the deep­est reaches of the oceans. Other min­er­als that may be much more com­mon than are rep­re­sented in min­eral mu­se­ums in­clude a host of species that are dif­fi­cult to recog­nise based on their lack of bright colours or showy crys­tal faces. Most min­eral col­lec­tors favour eye- pop­ping spec­i­mens for their dis­play case.

Also, some min­er­als oc­cur only at the mi­cro or nano- scale. A num­ber of rare min­er­als known only from Otto Moun­tain in south­ern Cal­i­for­nia, for ex­am­ple, have been dis­cov­ered re­cently through the use of high­tech in­stru­ments.

Most min­eral ex­perts are fa­mil­iar with at best a hand­ful of the 2,550 ob­scure rar­i­ties, says Hazen, cit­ing the min­eral fin­gerite from El Salvador as “a per­fect storm of rar­ity.” Fin­gerite forms un­der ex­tremely re­stric­tive con­di­tions, from rare el­e­ments. It is wa­ter sol­u­ble and dis­ap­pears when rained upon, and comes from dan­ger­ous volcanic fumeroles near ac­tive vol­ca­noes, so is rarely col­lected. Con­se­quently, fin­gerite is only known from near the sum­mit of the Izalco Vol­cano in El Salvador.

In­sight­ful min­er­als

Rare min­er­als, the au­thors say, are key to un­der­stand­ing the di­ver­sity and dis­par­ity of Earth’s min­er­alog­i­cal en­vi­ron­ments. They are valu­able in un­der­stand­ing Earth as a com­plex evolv­ing sys­tem in which fluid- rock in­ter­ac­tions and bi­o­log­i­cal pro­cesses lead to new min­eral- form­ing niches.

“An­other pos­si­ble con­tri­bu­tion of rare min­er­als, though as yet spec­u­la­tive, re­lates to the ori­gins of life. While most ori­gins- oflife sce­nar­ios in­cor­po­rate com­mon min­er­als such as feldspars or clays, a num­ber of un­com­mon min­er­als, in­clud­ing species of sul­fides, bo­rates, and molyb­dates, have also been in­voked,” says Hazen.

“We live on a planet with re­mark­able min­er­alog­i­cal di­ver­sity, fea­tur­ing count­less vari­a­tions of colour and form, richly var­ied geo­chem­i­cal niches, and cap­ti­vat­ing com­po­si­tional and struc­tural com­plex­i­ties. Rare species, com­pris­ing as they do more than half of the di­ver­sity of Earth’s rich min­eral king­dom, thus pro­vide the clear­est and most com­pelling win­dow into the com­plex­i­ties of the evolv­ing min­er­alog­i­cal realm.” – Amer­i­can Min­er­al­o­gist

— Pho­tos: ROBERT DOWNS/ Univer­sity of Ne­vada

Ne­vadaite is formed un­der very re­stricted en­vi­ron­men­tal con­di­tions from scarce el­e­ments: vana­dium and cop­per. The crys­tals are colour­ful but mi­cro­scopic, and the min­eral is only known from just two lo­ca­tions: Eureka County, Ne­vada, and a cop­per mine in Kyr­gyzs­tan.

1 Ot­toite, known only from Otto Moun­tain, Cal­i­for­nia, in­cor­po­rates the ex­tremely rare el­e­ment tel­lurium and is mi­cro­scopic and very dif­fi­cult to spot in the field.

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2 Amicite is formed from very com­mon el­e­ments but re­quires ex­tremely pre­cise con­di­tions of tem­per­a­ture, pres­sure, and com­po­si­tion. It is known from only two sites.

3 Ich­nu­saite, cre­ated through a sub­ter­ranean mash- up of the ra­dioac­tive el­e­ment tho­rium and lead- like molyb­de­num. Only one spec­i­men has ever been found, in Sar­dinia.

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