Feldspar is a general term for feldspar minerals. Feldspar is the most widely distributed mineral in the crust, accounting for about 50% of the crust weight and 60% of the crust volume. Except for a few ultrabasic rocks such as kimberlite, all crystalline rocks contain feldspar, but there are few high-quality feldspar gems.

Feldspar minerals can often form huge crystals. For example, on Yimen Mountain in Urals, there is an ore body mined by a quarry, which is actually a huge Tianhe stone crystal. A kind of Labrador stone crystal with a length of 6 1cm was found in Labrador, Canada, which is rich in gem-grade Labrador stone (the origin of the name "Labrador stone").

Gems formed by feldspar minerals can be called feldspar gems. Some names of feldspar gems are consistent with mineral names, such as elongated stones and Tianhe stones; Other names are different from minerals, such as moonstone and actinolite.

Feldspar minerals include many mineral species, subspecies and varieties, forming many important gems. There are four kinds of feldspar gems confirmed in the national standard "Appraisal of Jewelry and Jade": moonstone, sunshine stone, Tianhe stone and Labrador stone.

The refractive index of moonstone given by China's national standard "Jewelry and Jade Identification" is1.518 ~1.526 (0.010). Based on this, it can be judged that the moonstone mineral is not Labrador, but only alkali feldspar. Another national standard in China, the name of jewelry and jade, clearly points out that the mineral name of moonstone is orthoclase. The moonstone minerals specified in these national standards are not completely consistent with the views of many gemologists.

Generally speaking, gems can only be gems formed by a specific mineral, while moonstone is a gem formed by several minerals. Therefore, there are different opinions about what kind of mineral moonstone is. It is generally believed that feldspar of various genera with moonlight effect can be called moonstone. However, some scholars believe that moonstone "should be limited to those Labrador gems that are blue-blue-white". Zhang Renshan put forward the opposite view in the book Emerald Diamond Jewelry. He thinks that "the moonstone has a refractive index of 1.520 ~ 1.525, which is glassy. In the transparent moonstone, there are often dazzling milky white or moonlight reflections, which are as bright as the bright moon, so it is called moonstone, and its color is white, gray or bluish white.

Some people think that "Heshibi" is a feldspar gem. During the Warring States Period, Bian He, a Chu, got a jade jade in Chushan and presented it to Li He, the king of Chu. Both of them were regarded as cheating the king with stones, and their left foot and right foot were cut off respectively. Later, Bian He held Yupu and wept bitterly at the foot of Jingshan Mountain. When King Wen heard of this, he sent someone to ask why, and ordered craftsmen to peel off the skin of Yupu to get beautiful jade, which was named "Heshibi". Later, Zhao got money, and King Qin Zhao wanted to exchange money with fifteen cities. Lin Xiangru, the envoy of Zhao, was sent to Qin with Bi. Lin Xiangru realized that he had no intention of exchanging the city for the jade, so he tactfully retrieved the jade from the palace and ordered his entourage to return home with it. So there are allusions and idioms of "priceless" and "returning the jade to Zhao". After Qin Shihuang unified China, he made the national seal with the seal, which was engraved with eight bird-and-insect-shaped seal characters written by Prime Minister Li Si.

What kind of jade it is has aroused the research interest and speculation of many scholars. Zhang and other scholars believe that He Shibi is a kind of Labrador stone with discoloration effect, which is quite similar to the discoloration phenomenon of Labrador stone, according to the fact that Du Guangting, a Taoist priest in the late Tang Dynasty and the Five Dynasties, witnessed He Shibi and described it as "blue from the side and white from the right" in his "Different Records". Some people think that the jade is moonstone or other jade. However, some scholars believe that Heshibi is neither Labrador stone nor moonstone. Choi-hi was spread to the later Tang Dynasty, and when the Khitans entered Luoyang, the imperial edict of Li Congke was burned in the Xuanwu Tower, and his whereabouts were unknown. We don't see the real thing, so the mystery of its jade quality can only be inferred, and it is difficult to draw a conclusion.

Feldspar minerals, all kinds of bright colors or feldspars with various optical effects, can be used as decorative gems, as well as ornamental stones and mineral crystals. There are many feldspar producing areas in the world. The sources of various feldspar will be introduced in relevant chapters.

The Smithsonian Museum in Washington, USA has: yellow orthoclase from Madagascar, 249.6 carats; Light green syenite cat's eye from Sri Lanka, 104.5 carats; Yellow Labrador stone produced in Utah, USA, 1 1. 1 carat; Light brown transparent orthoclase crystal is produced in Greenland and is 6cm long. The museum in Toronto, Ontario, Canada has 12.25 carats of cat's eye albite produced in Myanmar.

3.6.2 Physical and chemical properties and quality requirements of feldspar

3.6.2. 1 Physical and chemical properties of feldspar minerals

Feldspar family includes K-albisubfamily, plagioclase subfamily and barium feldspar subfamily.

The chemical general formula of feldspar can be written as MT4O8, in which m = Na, Ca, K, Ba and a small amount of monovalent or divalent alkali and alkaline earth metal ions, and li, Rb, Cs and Sr have larger plasma radius; T = Si, Al and a small amount of tetravalent or trivalent ions, and B, Ce, Fe3 ++ and Ti have smaller plasma radius.

In terms of chemical composition, most feldspar minerals are contained in the ternary system of Kalsi3O8-Naalsi3O8-CaAl2Si2O8, which is equivalent to the combination of three simple feldspar endmembers: potash feldspar (Or), albite (Ab) and anorthite (An). The miscibility of these three kinds of feldspar molecules is related to temperature.

Feldspar gem has glass luster, and the fracture surface is glass luster to pearl luster or grease luster. The mineralogical characteristics are shown in Table 3.6. 1 (cited from Wang Fuquan).

(1) albisubfamily

K-albite subfamily includes anorthite, orthoclase, cold feldspar, microcline, plagioclase and water feldspar.

The chemical composition of albite is KAlSi3O8-NaAlSi3O8 series, which contains a certain amount of CaAl2Si2O8. In the solid solution of potassium albite, it usually contains 5% ~ 10% (generally less than 5%).

The combination of potash feldspar molecules and albite molecules is called "potash feldspar series" or "alkali feldspar series", which forms a complete isomorphic series at high temperature; With the decrease of temperature, it melts into potassium phase and sodium phase, forming "striped feldspar"

Table 3.6. Gem Mineralogical Characteristics of1Feldspar Formation

(According to Arein, 1977)

Stripe feldspar is not a single mineral, but a two-phase mixture formed by albite (sometimes feldspar) embedded in potassium feldspar (usually microcline or syenite). When the main crystal is orthoclase, it is called "syenite feldspar", and when the main crystal is microcline, it is called "microcline feldspar". When the main crystal is albite, in which potassium feldspar is embedded, the striped feldspar and anti-striped feldspar called "anti-striped feldspar" are isomorphic mixed crystals formed by K [Alsi3O8] and Na [Alsi3O8] at high temperature, separated at low temperature, or formed by albitization of potassium feldspar.

Sannidine, orthoclase and microcline are three isomorphic variants of end member KAlSi3O8, which are collectively called "potash feldspar". Potassium feldspar generally contains more Ab components, sometimes as high as 50%.

A. Sanedin Sannidine, also known as "glass stone", is a high-temperature variant of potash feldspar, which crystallizes above 900℃ and contains a considerable amount of albite molecules (sometimes as high as 60%), often containing Ba, Ca, Rb, Fe and so on. Most of them are colorless and grayish white, and some of them are flesh red, light yellow, light brown and other colors, with high transparency and glass luster. Refractive index ng = 1.525 ~ 1.532, nm = 1.522 ~ 1.530, NP =1.518 ~/kloc-. Generally1.516 ~1.526. Generally, the birefringence is 0.0 10 (the refractive indexes of K-albite are all quoted from the "frame structure silicate gem" written by Zhang in Gemstone Collection).

B. Orthoclase Orthoclase is rarely pure K [Alsi3O8], and usually contains up to 20% albite molecules, sometimes up to about 50%, and a small amount of anorthite molecules; Potassium can be partially replaced by barium, and usually contains trivalent iron, calcium and trace amounts of gallium and rubidium. Orthoclase is usually flesh red, light yellow, light green, light red, gray, white, colorless and so on. Orthoclase with cat's eye, starlight, sunlight, moonlight and other effects is an important gem. Refractive index ng = 1.523 ~ 1.539, nm = 1.522 ~ 1.533, NP =1.516 ~/kloc. Generally1.518 ~1.526. The birefringence is usually 0.008.

C. cryolite It is generally believed that cryolite is a subspecies of orthoclase or a low-temperature variety of potash feldspar. Some scholars believe that cryolite is a metastable mixture, which has different structural States of tribasic feldspar, orthoclase and microcline. Cryolite contains more than 90% potassium feldspar molecules, colorless and transparent, sometimes milky white. According to systematic mineralogy, the terminal groups of cryolite are Or90 Ab9 An 1 (in turn, the ratio of molecular numbers of potash feldspar, albite and anorthite), often containing barium, colorless and transparent or milky white. Refractive index ng = 1.524 ~ 1.526, nm = 1.522 ~ 1.524, NP =1.518 ~/kloc-. The birefringence is 0.006.

D. microcline. Microveinlets are rarely pure K [ALSI3O8] and often contain 20% ~ 30% albite molecules. Micro-feldspar is a low-temperature variant of potassium feldspar, which crystallizes below 900℃. The common colors of microcline are white, gray, light yellow or light red, and some are rare black.

E. plagioclase K-ultralong stone, also known as K-ultralong stone, is a high-temperature sodium-rich alkali feldspar with a chemical composition of (Na, K) [ALSI3O8], in which the molecular weight of K-feldspar is less than 37%, the Na2O content is usually higher than K2O, and the chemical composition is closer to albite. The color is white, gray, light yellow, light red or green, transparent to translucent. Refractive index ng = 1.527 ~ 1.536, nm = 1.526 ~ 1.534, NP =1.522 ~1.529; The birefringence is 0.005 ~ 0.007.

F. hydrated ammonium feldspar. The chemical composition is NH4 [ALSI3O8] NH2O, which is the product of the interaction between hot springs containing ammonium, altered andesite and K-albite. -Generally, it is aphanitic, and its authigenic particle size is only 0.05 mm. Because of its small particle size, it cannot be used as a gem, but its mineral aggregate may form jade.

(2) plagioclase subfamily

The combination of feldspar molecules and anorthite molecules is called "anorthite series" or "plagioclase", which contains 5% ~ 10% of potash feldspar components (generally less than 5%) and a small amount of Ti, Fe3+, Fe2+, Mn, Mg, Ba, Sr ... plagioclase is generally colorless, but it is white or other colors due to dispersion of inclusions.

According to the molecular percentages of Ab and An contained in plagioclase, it can be divided into acid plagioclase (AN 0 ~ 30%), neutral plagioclase (AN 0 ~ 30% ~ 50%) and basic plagioclase (AN 50% ~ 100%), including the following six minerals: albite (AB1). Feldspar (also known as anorthite) (AB 90% ~ 70%, AN 10% ~ 30%), mesofeldspar (AB 70% ~ 50%, AN 30% ~ 50%), Labrador (AB 50% ~ 30%, AN 50% ~ 70%)

Varieties of Feldspar Gems in 3.6.2.2

Feldspar gems include moonstone, actinolite, chromotropic feldspar, Tianhe stone and feldspar with different colors and optical effects.

(1) moonstone

Some people call the moonstone "moonstone" (the name of Chinese characters in Japanese), while in ancient times it was called "Lunatone" and "sapphire" (the current national standards do not agree). Moonstone is mostly alkali feldspar and some plagioclase. Alkali feldspar is a general term for feldspar rich in potassium and sodium, including anorthite, orthoclase, microcline, plagioclase and albite (feldspar with the molecular weight of anorthite below 5%), as well as striped feldspar and anti-striped feldspar.

In alkali feldspar, two mineral phases, potash feldspar and albite, form hidden stripes and micro-stripes. When light hits this two-phase interface, it causes reflection, and the reflected light between different interfaces interferes with each other to produce iridescence, that is, light blue to milky white light (also called milky white light), which is the so-called "moonlight effect".

Moonstone was first discovered in the Swiss Adala Mountains. At present, the producing areas of high-quality moonstone are mainly Myanmar, Sri Lanka, the United States, Madagascar, Russia, etc. Followed by Tanzania, Brazil, Western Australia and so on. Moonstone is produced in Inner Mongolia, Hebei and Anhui, but its quality is different.

China's national standard "Jewelry and Jade Identification" stipulates that the refractive index of moonstone is1.518 ~1.526 (0.010) and the birefringence is 0.005 ~ 0.008. According to this standard, moonstone can only be a mineral of K-albisubfamily. However, many domestic scholars believe that moonstone includes cryolite moonstone, Sanidin moonstone, plagioclase moonstone, albite moonstone, Labrador moonstone and plagioclase moonstone.

A. cryolite moonstone. Most of the known moonstones are composed of cryolite. This kind of alabaster is foggy because of albite solid solution.

B. Saneidin Moonstone. Colorless transparent or translucent moonstone.

C. plagioclase striped feldspar moonstone. Also known as "Korean Moonstone", it contains 40% potash feldspar, 49% albite and 0/1%anorthite.

D. albite moonstone. This moonstone has the moonlight effect of white milky light; It is a kind of anti-striped feldspar with albite as the main crystal and potassium feldspar embedded, which has the effect of blue flash. More feldspar embedded in albite can also form albite moonstone.

E. Labrador moonstone. Labrador stones with yellowish brown tones and blue flashes do not change color, which is different from discolored Labrador stones.

F. plagioclase moonstone. In addition to Ab and An, minerals also contain a small amount of potassium feldspar and barium feldspar molecules.

(2) Actinolite

Sunshine stone, also known as "sun stone", "sandstone" or "Venus feldspar", is a kind of feldspar with inclusions such as hematite, goethite and iron mica arranged in layers, which scatters incident white light and condenses on the arc surface of arc-shaped gemstones, showing golden yellow or grayish red flash. This phenomenon is called "daylight effect". Actinolite can be divided into plagioclase actinolite and Shanidine actinolite due to different mineral compositions.

Sunlight stone is produced in Norway, the United States, Canada, Russia, India and Myanmar.

A. plagioclase actinolite The red or golden flame flash is caused by the inclusion of fine-grained specularite or goethite.

B. Sanidin Sunshine Stone. Colorless and transparent, it is golden yellow due to iron, and some are crimson and orange, with a flame-like flash (from Norway). Although the national standard "Name of Jewelry and Jade" considers that the mineral of actinolite is anorthite, some scholars believe that some actinolite is not anorthite, but anorthite.

(3) Colored feldspar

There are two main varieties:

A. color Labrador. Also known as "Rainbow Labrador Stone", unlike Moonstone, its background color is dark smoke gray, and continuous discoloration can be seen from different angles. Labrador is mainly produced in Canada, the United States, Germany, Ukraine and Finland. China has developed in Inner Mongolia and Hubei.

Regarding the cause of the discoloration effect of lapis lazuli, one view is that it is caused by flaky twins, and the other view is that it is caused by flaky inclusions or thin-layer strip separation.

B. color-changing striped feldspar. It is a kind of feldspar dominated by potassium feldspar, and potassium and albite are alternately arranged in strips.

(4) Tianhe stone

The green microcline with rubidium (Rb) and cesium (Cs) is called Tianhe Stone, which generally contains 0.4% ~ 3.3% of Rb 2O/Kloc-0 and 0.4% ~ 0.6% of Cs 2O 3. Tianhe stone is the main gem in microcline, which is in the form of sky blue, green or blue-green spots, or has cross-shaped net stripes and is often opaque. The coloring reason of Tianhe stone may be related to its lead content, or to the crystal structure defect caused by Pb replacing K.

Tianhe stone is mainly produced in the United States, Russia, South Africa, India (cat's eye and starlight feldspar gems are still produced) and Japan. In China, Xinjiang, Gansu, Inner Mongolia, Sichuan and Yunnan are the main producing areas.

(5) Orthoclase and microcline

Orthoclase and microcline can become gems because of starlight effect, cat's eye effect or transparency. Micro-plagioclase gemstones are colorless, white, reddish (also known as carnations), yellow, red, green to blue-green and black.

Quality Requirements of Feldspar Gems in 3.6.2.3

Colored moonstone is the best, such as blue gray, green, light yellow, orange to light brown, followed by colorless moonstone. High quality moonstone has floating blue light.

Tianhe stone is best in pure blue, followed by bluish blue, with high transparency and few impurities.

Among Labrador stones, blue is the best, followed by yellow, pink, red and green. Colorless lithium soapstone, colored or colorless can be used as gems, but the transparency is high.

Sanidine should be colorless, transparent or translucent, without cracks and other defects, and the particle size should be large; It better have a cat's eye effect.

Plagioclase should be colorless or colored, transparent and large.

3.6.3 Basic Metallogenic Regularity of Feldspar Gems

Formation of feldspar gems

Magma, metamorphism and migmatization can all form feldspar. In pegmatite stage, feldspar megacrysts can be formed.

(1) K-albite deposit

K-albite mainly occurs in alkaline and acidic intrusive rocks, such as syenite, granite, granodiorite and its extrusive rocks, and also occurs in pegmatite and gneiss, but rarely in basic rocks. There are feldspar and plagioclase in the ejected rock. K-albite in pegmatite was formed from pegmatite action stage to hydrothermal metasomatism stage. K-albite is also common in high-temperature hydrothermal veins. The interaction between biotite and hydrothermal solution (water) can form potash feldspar and chlorite at the same time. Feldspar is a low-temperature hydrothermal product and also exists in low-grade metamorphic rocks and metasomatic rocks.

(2) plagioclase deposit

Plagioclase mainly occurs in calc-alkaline intrusive rocks such as plagioclase, gabbro, syenite, granite and granodiorite. The plagioclase in plagioclase is mostly feldspar and Labrador stone, and some of them are betonite. The plagioclase in pegmatite is albite and anorthite; Basalt and acid rhyolite rarely contain plagioclase phenocrysts, while andesite and quartz trachyte often contain more plagioclase phenocrysts. Albite phenocrysts are usually found in crystalline schists. Albite metasomatism can form a large number of albite, and regional sodium metasomatism can form albite gneiss with albite as high as 60%.

Basic metallogenic regularity of 3.6.3.2

(1) Moonstone (photo 3.6- 1)

Magmatic moonstone occurs in gabbro-plagioclase body; Pegmatite moonstone occurs in pegmatite, including diorite syenite pegmatite, muscovite pegmatite and alkali syenite pegmatite containing rare metals. Korean moonstone occurs in high-temperature metasomatic deposits and coexists with iron and copper minerals. In addition, it also exists in gneiss, granulite and other metamorphic rocks and placers.

Sanidine (photo 3.6-2) is a kind of high-temperature potash feldspar, and its occurrence is related to the temperature in the process of magma consolidation, so it occurs in extrusive rocks, lava rhyolite, coesite andesite, trachyte, sonic rocks, intermediate-acid tuff and ignimbrite, and it is porphyritic crystal. In the near-surface shallow diagenesis, due to the rapid solidification of magma, Sanidin can sometimes be seen. Potassium rhyolite porphyry often contains diorite crystals, which can show moonlight effect. Sanidine was also found in the contact metamorphic zone.

Feldspar is a low-temperature variety of potash feldspar, which was formed in the low-temperature hydrothermal stage after magmatic period, or produced in low-grade metamorphic rocks or metasomatic rocks. The alabaster in the alabaster granulite dike in Sri Lanka is white or blue moonstone.

Plagioclase mainly occurs in the deep xenoliths of Cenozoic alkaline basalt and sodium-rich basalt. Some occur in andesite, sonic rock and trachyte; Generally found in sodium-rich volcanic rocks, such as sodium trachyte and alkaline rhyolite; In alkaline syenite and nepheline syenite, it can be porphyritic or matrix; Some occur in granite.

Albite mainly occurs in alkaline igneous rocks, calc-alkaline igneous rocks, albite schists and granite pegmatites.

Plagioclase occurs in granite, quartz monzonite, granodiorite, syenite and corresponding extrusive rocks and pegmatites. Plagioclase in ejected rocks can occur in the form of phenocrysts.

Labrador rocks (Figures 3.6-3 and 3.6-4) are mainly produced in plagioclase, gabbro-plagioclase, gabbro-syenite, basalt and gabbro pegmatite.

(2) Sunlight Stone (photo 3.6-5)

Sunshine stone mainly occurs in timely veins in gneiss areas, such as Norway; Or in sodalite nepheline syenite, such as England; It is also found in pegmatites such as India and Madagascar. In addition, the United States, Canada, Russia and other countries also produce actinolite.

(3) Striped feldspar with moonlight effect and color change effect.

This feldspar occurs in syenite or monzonite.

(4) Tianhe Stone (photo 3.6-6)

Tianhe stone occurs in granite pegmatite. Pegmatite-type feldspar deposits are associated with granitic magma or alkaline magma, so they are often distributed near or at the top of granite bodies, while pegmatite-type feldspar deposits related to metamorphism and hypermetamorphism are often distributed in metamorphic rocks. In China, pegmatite feldspar deposits are mostly Caledonian and Hercynian in the north, and Yanshanian in the south.

(5) Orthoclase

Orthoclase exists in acidic, neutral and alkaline rocks and some metamorphic rocks, while its coarse crystals are common in pegmatites and dikes, such as Sri.

Cat's eye orthoclase is produced in Sri Lanka and Myanmar, and yellow transparent orthoclase crystal is produced in Madagascar. Microplagioclase occurs in acid rocks and alkaline rocks, but in pegmatite, it is mostly microcline striped feldspar.

3.6.4 Typical feldspar and gem deposits

3.6.4. 1 feldspar and gem deposits in magmatic rocks

(1) Labrador deposit in Warren, Ukraine.

Gabbro-plagioclase consists of gray to black layered slender rocks, about 200 meters long and 30 ~ 50 meters thick. Labrador crystal is several millimeters to several centimeters long, with golden yellow and purple blue iridescence.

(2) Moonstone in New Mexico, USA

Pegmatite intrudes into tertiary rhyolite tuff at the neck of rhyolite porphyry. Pegmatite is composed of quartz and wurtzite, and contains albite, biotite, magnetite, ilmenite, tantalite and so on. Some of them have moonlight effect in Senedin.

(3) Tianhe Stone in Inner Mongolia

Tianhe stone in Inner Mongolia occurs in albite granite vein or granite pegmatite vein.

Tianhe stone occurs in albite granite veins and was formed in granite activities in the late Yanshan period. Tianhe stone is light sky blue, light blue and blue-green, and its crystals are several millimeters to several centimeters long. Chemical composition of Tianhe stone (%): SiO 2 65.62, TiO2 0.002, Al2O3 18.04, Fe2O3 0. 16, FeO 0.02, MNo 0.003, MgO 0.45, Na2O 3. 19, K2O.

Tianhe stone in granite pegmatite vein occurs in feldspar-quartz block zone or long stone block zone, with light color and large crystal, which can reach more than 10cm.

(4) Anhui Lujiang Moonstone

Moonstone occurs in biotite monzonite, which is granular and porphyritic, with a content of about 40%. In the moonstone, 98% are potash feldspar molecules and 2% are sodalite molecules, which can be called "syenite cryptophyllite moonstone". The hidden texture in minerals can be diffracted, changing the color of white light, resulting in a light blue halo of moonstone in this area.

Feldspar and gem deposits in pegmatite in 3.6.4.2.

Xinjiang Hami (1) Tianhe stone deposit

The mining area is located in Tianshan uplift belt, and the exposed strata are mainly Precambrian, Carboniferous and Permian. Magmatic activities include Caledonian huagangyan and Variscan intrusive rocks; The intrusive rocks in the middle Variscan period are widely distributed and have multiple stages, including ultrabasic rocks, diorite, pyroxene diorite, quartz diorite, biotite granite and muscovite granite. According to Yang's research, pegmatite dikes containing Tianhe stone are distributed in medium-fine grained granite in the middle period of Variscan, and the dike differentiation is obvious. The biotite granite is more than 30 kilometers long, and there are granite pegmatite veins containing Tianhe stone and differentiated smoke crystals in the rock mass. Tianhe stone is mainly produced in feldspar-quartz block zone, and the granite pegmatite veins are well differentiated, with a content of more than 5%. Tianhe stone is plate-shaped, light blue and blue-green, with a general particle size of 10cm× 10cm×5.5cm and an individual particle size of 50cm×50cm× 10cm. Granite pegmatite dikes are simple in structure and contain chronological metasomatic bodies of Tianhe stone. Tianhe stone is short column, plate-shaped, blue-green and sky-blue, with particle size of1.5cm×1.2cm× 0.5cm, and part of it is 3.5cm×2cm×2cm. The mineral composition of Tianhe stone is shown in Table 3.6.2.

Table 3.6.2 Chemical Composition Table of Tianhe Stone in Yunnan and Xinjiang (unit:%)

Note: * contains Fe2O3. (According to Li Zhifen)

(2) Tianhe Stone Deposit in Miyi County, Sichuan Province

The mining area is located on the axis of Kangdian, with pre-Sinian metamorphic rock series strata and magmatic rocks such as Jinning period, late Variscan period and Indosinian period. The pegmatite vein containing Tianhe stone occurs in the syenite fault zone in irregular vein or lenticular shape, with a length greater than 10m and a thickness less than1m.. Tianhe stone occurs in the long rock mass zone of pegmatite vein. Tianhe stone is bright green and blue, even in color, slightly glassy, transparent to translucent, with a crystal length of 8 ~ 12 cm and a maximum of 20cm. The mineral composition of Tianhe stone is shown in Table 3.6.2.

(3) Moonstone in Xuanhua, Hebei Province

In the granite pegmatite distributed in the Archean metamorphic rock series in Xuanhua area, albite moonstone coexists with synchrony, potash feldspar, biotite, magnetite and chalcopyrite. Albite moonstone with particle size larger than 10cm is common, colorless, light gray, light yellow and transparent, and light blue-purple opalescent light can be seen when rotating.

According to Lu Muxun's textual research, the chemical composition of Xuanhua Moonstone is between albite and plagioclase, and the grade of feldspar is calculated to be An 10, and the grade of Feier turntable is An9. The hardness is 5.4 and the density is 2.80g/cm3. Refractive index ng = 1.5395, nm = 1.5326, NP = 1.5303. The light blue opalescence is caused by sodium-rich low-temperature albite as the main phase, plagioclase with a certain amount of calcium as the secondary phase, and the two phases are arranged in ultra-thin parallel [0 10] rules. Chemical composition of moonstone (%): SiO 2 66.75, TiO2 0.0 1, Al2O3 20.83, Fe2O3 0.2 1, MgO 0. 16, CaO 0.94, Na2O 9. 12, K2O 0.

3.6.4.3 Sand Mine Stone Gem Deposit

Labrador moonstone placer in Inner Mongolia. According to Li Heineken's research, the distribution range of moonstone sand deposits in Labrador rocks is about 20 kilometers long and 4 kilometers wide. Modern placer originated from the ancient placer with Labrador gravel in the Lower Cretaceous.

Labrador stone is transparent to translucent, and its particle size is generally 1 ~ 2 cm long and the longest is 6cm. Light yellow to wine yellow, jewel-grade Labrador has high transparency, moonlight effect and a little cat's eye effect. Labrador rock contains 50% anorthite molecules and 50% albite molecules, belonging to Labrador rock close to feldspar.

3.6.5 Metallogenic Prospect of Feldspar Gems in Hebei Province

3.6.5. 1 Metallogenic prospect area with good metallogenic geological conditions

Feldspar gems in Hebei province are mainly distributed in volcanic rocks in Zhangjiakou area, followed by alkaline rocks, acidic rocks and granite pegmatites.

(1) triphenylamine and albite

Quartz porphyry, rhyolite and trachyte of the second member of the Upper Jurassic Zhangjiakou Formation distributed in Zhangjiakou, Chongli and Guyuan areas all contain diorite with a grain size of 2 ~ 8 mm and a content of < 5%. The mineral granularity of rhyolite is >: 0.05mm, which may form jade. The secondary andesite of Xiamaquan in Shangyi intruded into the clastic rocks of Middle Jurassic in the form of bedrock, and the secondary andesite contains tabular albite phenocrysts.

(2) plagioclase

The chromite spinel xenoliths distributed in tertiary Miocene Hannuoba basalt in Erdaobian, Zhou Ba and Zhangbei County contain plagioclase crystals, with the smallest particle size of 0.5 ~ 1 cm, generally 2 ~ 3 cm and the longest 13cm. Plagioclase crystals are tabular, colorless, transparent or milky white with yellowish and yellowish brown. The plagioclase found in the basalt of Hannuoba in Wanquan and Yangyuan is 2 ~ 2.5 cm long and 65,438 0 cm thick, which is tabular, colorless and transparent.

(3) Color-changing striped feldspar

Yanshan syenite in Jiashan, Chengde (Figure 3.6. 1) intruded into Jurassic and Triassic strata, with an area of 1 1km2. There is grayish green coarse-coarse pyroxene syenite in the middle of the rock mass, with an area of 0.15km2; ; Dark medium-coarse pyroxene syenite with an area of 0. 16km2 is distributed in the eastern edge of syenite body. Pyroxene syenite contains about 84% banded feldspar, 5% ~ 6% clinopyroxene, 2% ~ 3% amphibole and 2% ~ 4% chronological. Striped feldspar is thick, generally 3 ~ 4 cm long and 2 ~ 3 ~ 4 cm in diameter. The polished surface is dark green and blue, which is a kind of color-changing gem with great development value. At present, it is only used as a decorative board.

Fig. 3.6. 1 Geological Profile of Jiashan syenite in Chengde County (according to Wang et al. )

Pingquan Xiba and Pingquan Damiao pyroxene syenite covers an area of 2 1km2, and potash feldspar is cryptozonal feldspar with a content of 55% ~ 60% and a grain size of 4 ~ 5mm (see 4.7.5. 1 section for details).

(4) albite moonstone and Tianhe stone in Houtuotou, Pingshan County.

Moonstone and Tianhe stone occur in granite pegmatite in Archean gneiss distribution area. The plagioclase moonstone is surrounded by opaque plagioclase, which shows a bluish hazy flash after cutting and grinding. Tianhe stone is light blue-green, with a crystal length of 5 ~ 50 cm and a diameter of 1 ~ 7 cm, which is opaque and full of cracks. Rb2o0. 17% ~ 0.35%, mineral content 2.6%.

Granite pegmatites are often distributed in Archean metamorphic rock series in Hebei Province, and the genesis of these pegmatites may be related to migmatization, in which Tianhe stone or albite moonstone may be produced. Therefore, we should look for pegmatite feldspar gems in the migmatite development area.

Metallogenic Prospective Area with Metallogenic Clues in 3.6.5.2

(1) black microcline

It is an extremely rare feldspar gem variety, which occurs in Laiyuan granite body. Microveinlets exist in the form of porphyritic crystals, black and translucent, with a crystal length of 1 ~ 3 cm and a diameter greater than 1cm.

(2) plagioclase of Chengde Temple

Plagioclase pegmatite occurs in Damiao plagioclase with irregular pulse width of 1 ~ 2m and huge plagioclase crystal. This kind of pegmatite occurs in syenite and dark plagioclase.

(3) plagioclase from Wuliangsugou and Tuzigou in Longhua.

The plagioclase crystal in pegmatite is complete, grayish brown, brownish yellow, shiny, and the particle size is 3 ~ 5 mm

(4) Move to Tianhezhi of Hanerzhuang in the west.

Tianhe stone occurs in albite granite.

(5) Potassium feldspar slate of Chuanlinggou Formation.

Potassium feldspar slate is almost entirely composed of authigenic fine-grained potassium feldspar (see 6.4.6. 1 Jiechuanlinggou Formation for details). Chuanlinggou Formation is widely distributed in Hebei Province and may form jade or inkstone.

(6) albite

It should be noted that fine albite formed by albitization may form jade.

In addition, attention should be paid to the discovery of gem-grade phenocrysts in various magmatic rocks. For example, Hongshan complex is mainly composed of pyroxene syenite, biotite amphibole syenite and syenite. There are locally aegirine syenite and giant porphyry syenite. Tangquan porphyry granite in Chicheng County intrudes into Xiaojiakou ultrabasic rock, with an exposed area of 35km2, in which potash feldspar porphyry accounts for 40% ~ 60%, ordinary pyroxene porphyry accounts for 10% ~ 25%, the length of porphyry is 1 ~ 3cm, and the rock contains rare earth elements. The Hongshaliang porphyry granite in Jiangjiabao, Chicheng County contains 40% ~ 55% porphyry potash feldspar and 20% ~ 30% albite. The area of Chongli Guzuizi porphyry granite is 15km2, and the length of K-feldspar phenocrysts is 3-5cm, some of which exceed 10cm, accounting for 25%-70% of the rock area, and there are plagioclase phenocrysts.