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Hustota
Specific gravity

 
  Hustota je pro uren minerl velmi dleit. U chemicky mlo promnlivch minerl je toti pi dan teplot a tlaku vdy stejn a jej pesn stanoven nkdy sta k pm identifikaci. Je zvisl na chemickm sloen nerostu, na jeho atomov hmotnosti a na velikosti a stavb molekul, u krystalickch ltek pak na krystalov struktue. U krystalov formy te sloueniny je vt ne u formy amorfn.
Pi prci v ternu odhadne zkuen sbratel piblinou hustotu pouhm odvenm v ruce. Hovome pitom o minerlech lehkch s hustotou 1-2 (nkter bitumeny), stedn tkch s hustotou 2-4 (sem pat vtina nerost, nap. sdrovec a kemen), tkch s hustotou 4-6 (sfalerit, baryt) a velmi tkch s hustotou nad 6 (galenit, kasiterit). Nejvt hustotu maj ryz kovy, napklad zlato 15-16 a platina 14-20.
Pro rychl uren hustoty doma je vhodn a v podstat dostaujc metoda suspenzan. Urujeme zde relativn hustotu minerl, a to tak, e nezvtral lomek minerlu (asi 1 g) ponome do sklenn ndobky s tekutinou o znm hustot. Klesne-li minerl ke dnu, je jeho hustota vt ne hustota tekutiny, plave-li na hladin nebo po ponoen opt vyplave, je jeho hustota men. Vzn-li se, odpovd hustota nerostu hustot kapaliny.
Pro pesnj men pslunou tekutinu s nerostem edme nebo zahuujeme tak dlouho, a se minerl vzn (suspenzuje). Pomoc hustomru nebo pyknometru pak zjistme hustotu kapaliny a tm i hustotu minerlu. Jako tekutiny meme pout nap. bromoform (CHBr3) s nejvy hustotou 2,904 (zeuje se terem nebo benzolem), acetylentetrabromid (C2H2Br4) s hustotou 3,0 (zeuje se benzolem), metylenjodid (CH2J2), kter m hustotu 3,33 (zeuje se alkoholem, terem, benzolem). Pozor-tyto tekutiny jsou vtinou zdrav kodliv. Nevhodou tto metody je, e neznme vhodn tekutiny o vych hustotch.
Pro pesn a rychl stanoven hustoty je vhodn metoda dvojho ven, a to na vzduchu a ve vod. Je vhodn zejmna u neporznch vzork. K men hustoty touto metodou meme pout upraven laboratorn vhy tak, e na hek nad misku jednoho vahadla zavsme na hedvbnou nit dan minerl, kter zvme nejdv na vzduchu. Pro zven minerlu ve vod postavme na mstek pod upevnn minerl kdinku s destilovanou vodou, jej hladina m bt tak vysoko, aby byl minerl zcela ponoen. U minerl rozpustnch ve vod pouijeme jako kapalinu olej, alkohol aj.
Po zven provedeme vpoet hustoty podle vztahu:
kde m't je hmotnost minerlu na vzduchu, mt je hmotnost minerlu v kapalin a Sk je hustota kapaliny (hustota H2O pro 20 C = 0,9982 = 1 g/cm3).
Pro pesn stanoven hustoty velmi malch lomk minerl je mono pout metodu pyknometrickou. Vzorek vlome do sklenn ndoby (pyknometru) se znmm objemem. Po zaplnn mrnou kapalinou a zven se stanov objem vzorku. Metoda je nron na rychlost proveden a tm i na zrunost. Pro tuto metodu plat vztah:
piem m1 = hmotnost przdnho pyknometru, m2 = hmotnost pyknometru se vzorkem, M1 = hmotnost pyknometru s kapalinou, M2 = hmotnost pyknometru se vzorkem a mrnou kapalinou a Sk = hustota mrn kapaliny.
Hustota nerost me znan kolsat navtrnm a poruenm vzork. Rovn izomorfn pmsi jinch minerl a uzaveniny plyn a tekutin mohou znan ovlivnit vsledek men. Z tchto dvod je teba kad vzorek ped vlastnm menm podrobit zevrubn prohldce pod lupou nebo binokulrnm mikroskopem.
    Another physical property of minerals, used in their identification, is specific gravity. In minerals of constant chemical composition, it is always the same at fixed temperatures and pressures, its exact determination frequently helps in their identification. It depends upon the chemical composition of the mineral, on its atomic weight and the molecular size and arrangement; in crystalline substances it depends upon their crystal structure. Minerals, in single crystal form, have higher specific gravities than their polycrystalline equivalents.
An experienced collector can estimate the approximate specific gravity by simply holding the mineral and guessing its weight. There are light minerals of a specific gravity 1-2 (some bitumens), medium-heavy minerals of a specific gravity 2-4 (gypsum and quartz), heavy minerals of a specific gravity 4-6 (sphalerite and barytes), and very heavy minerals of a specific gravity of more than 6 (galena and cassiterite). Native metals, such as gold (15-16) and platinum (14-20) have the highest specific gravities.
A quick determination of specific gravity may be obtained by the suspension method. The relative specific gravity of minerals is determined when a piece of unweathered mineral (about 1 gm) is submerged in a liquid of known density. If the mineral sinks to the bottom, its specific gravity is higher than that of the liquid; if it floats, its specific gravity is lower.
For a more accurate measurement, the liquid is either diluted or concentrated until it achieves the density at which the mineral just floats. Then, it is easy to determine the specific gravity of the liquid by means of a densimeter (ie hydrometer) or pycnometer. A suitable liquid is bromoform CHBr3 with a density of 2.904 (diluted by ether or benzol), acetylene tetrabromide C2H2Br4 of a density of 3 (diluted by benzol), methylene iodide CH2I2 of a density of 3.33 (diluted in alcohol, ether or benzol). The only disadvantage of this method is that liquids of higher density are not readily available, or easy to handle and most of them are harmful. For a quick and exact determination of the specific gravity of a sample, it can be weighed twice, in air and then in water. This method can be applied best to non-porous minerals. A laboratory balance specially adapted for this purpose can be used. On a small hook above one of the scales is hung a small container on a silk thread, which is balanced on the other scale. The mineral sample is placed in the container and weighed in air. To estimate its weight in water, a beakercontaining distilled water is placed under the container so that it is complety immersed. With water-soluble minerals the immersion medium should be oil or alcohol.
When the weight of the mineral is known, its specific gravity can be calculated by means of the formula:
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