Minerals composed of SiO2 are called silica: quartz (the most common variety), trdymite, cristobalite, coesite, keatite, stishovite, diatomaceous earth, cryptocristallinic Varieties, agate, and silica glass.
SiO2 . Alkali ions (e.g. Na+) are included into The structure, if Si4+ is substituted by Al3+ . Thus, quartz contains normally contaminations of Li, Na, K, Al, Fe3+, Mn, and Ti .
Varieties:
• Cristobalite
Specification:
Cristobalite is stable from 1470° C to 1713° C (melting point) and metastable at lower temperatures.
Similar to tridymite, the cristobalit e structure is based on layers of six membered rings of tetrahedra. Opposite to tridymite, the triangles belonging to the apical bonding atoms are rotated by 60° between two neighboring layers. The cristobalit e structure is open and can accommodate large ions.
In thin sections, colorless and without Pleochroism.
Powder Diagram:
4.05 2.49 2.84 3.13 (11-695).
Natural Sources:
Described from Mexico (San Cristobal), and the USA (Yellowstone Park).
• Diatomaceous Earth
Specification:
Diatomaceous earth is an amorphous variety of silica, produced by a SiO2 concentrating animalcule. The naturally found diatomaceous earth is contaminated by additional minerals, and has to be heated to 700-800° C, to release its content of water (70%) and organic materials. The heating often creates cristobalite.
Chemistry/Composition:
SiO2+aq. It normally contains contaminations of Al, Fe, Ti, P, Ca, Mg, Na, and K.
Structure:
Amorphous.
Crystallographic Constants:
None.
Crystal Group:
None.
Color:
White to black (if contaminated by bituminous material).
Optical Properties:
n=1.40-1.46.
Pleochroism:
None.
Powder Diagram:
None.
Natural Sources:
Algeria, Denmark, France, Germany, Iceland, Japan, Kenya, Mexico, Romania, Russia, Spain, and the USA.
• Quartz
Specification:
Quartz is the most common variety of silica.
Chemistry/Composition:
SiO2 . Alkali ions (e.g. Na+) are included into the Structure, if Si4+ is substituted by Al3+ . Thus, quartz contains normally contaminations of Li, Na, K, Al, Fe3+, Mn, and Ti .
Structure:
a-Quartz is the stable configuration at moderate temperatures (<573° C) and atmospheric pressures. The basic structural elements are SiO4 tetrahedra, which are formed by one silicon atom and four surrounding oxygen atoms. The tetrahedra are bonded to each other by all four oxygen atoms, i.e., one tetrahedron is bonded with four different tetrahedra (chemical formula SiO2) . Additional tetrahedra are grouped and form two different helices: a) a trigonal one, which contains the silicon atoms at levels 0, 1/3 and 2/3 of the unit cell, and b) a hexagonal one, which contains the silicon atoms at levels 0, 2/3, 1/3, 0, 2/3 and 1/3. Only minor differences exist in a-quartz between the natural and theoretical positions of the atoms. a-quartz has a lower symmetry than ß-quartz, which has a stable structure between 573° C and 870° C. The theoretical positions of the atoms are identical to those found in nature. Both structures can be transferred into each other by small variations of the positions of the atoms.
Crystallographic Constants:
3.350 3.350 3.350 95.600 95.600 95.600.
Crystal Group:
Trigonal (a-quartz), hexagonal (ß-quartz).
Color:
Normally colorless, seldom white, gray, green, or black.
Optical Properties:
nO=1.5442 nE=1.5533.
Pleochroism:
In thin sections, colorless and without Pleochroism.
Powder Diagram:
3.34 4.25 1.38 1.82.
Natural Sources:
World-wide.
• Tridymite
Specification:
Tridymite is stable between 870° C and 1470° C and metastable at lower temperatures.
All tetrahedra in the tridymite structure are bonded to each other and create an open network of six-membered rings. The connected tetrahedra are placed in layers at the bases of the triangles, and the apices point into opposite directions alternatively. The apical oxygen atoms are the bonding atoms to the next layer. The basic oxygen atoms of one layer are placed directly above those of the underneath layer. The open crystal structure creates channels allowing the passage of large ions.
Crystallographic Constants:
9.88 17.1 16.3 90.000 90.000 90.000.
Crystal Group:
Hexagonal.
Color:
Normally colorless.
Optical Properties:
nx=1.469-1.477 ny=1.469-1.478 nz=1.473-1.481.
Pleochroism:
colorless in thin sections.
Powder Diagram:
4.10 4.32 3.82 2.96.
Natural Sources:
World-wide.
Medical Importance:
Key Hazards:
Fibrogenic, carcinogenic.
Involved Organs:
Lung.
Exposure/Epidemiology:
Ex p osure to silica sand and crystals may occur in mining or stone cutting, during the use in abrasive industries, and in foundry working. Exposed persons are packers of silica flour, employees in quarrying, sandblasting, tunnelling, pottery industries, glass manufacture, and electronic industries. Silica are usually present in the mining industries (e.g. mining of tin, hematite, graphite, copper, gold, and other metals, rocks or minerals). Diatomaceous earth is used in thermal and acoustic insulation, chemical and rubber industries, cosmetics, and as filtering and filling material. Exposure during farming has been described. About 3 million workers are exposed to silica in the USA (1983); in the UK, many women, usually pottery workers, suffer from silicosis.
Thresholds:
TWA
mg/m3
Australia
2
Belgium
0.1
Denmark
2
Germany (quartz)
0.15
(diatomaceous earth, burned)
0.3
(silica containing fine dust)
4
Great Britain
3
Switzerland
6
United States: ACGIH
0.1
Etiology/Pathophysiology:
Only those particles of SiO2, which are small enough (<7 µm of the aerodynamic diameter) can enter the brochioles and alveoli, pass to the interstitial tissue, and be transported to the regional lymph nodes (lymphotropism). Larger particles are exhaled. The smaller the particle size the larger is the portion of deposited particles (maximum deposition: particles measuring <5 µm in diameter). Macrophages ingest the SiO2 mineral fibers and are destroyed (cytotoxicity). The released fibers are again ingested. The repeated destruction of the macrophages activates fibroblasts with consequent collagen production, and silicotic nodules are formed (chronic silicosis). The acute inhalation of many silicon particles can destroy the macrophages still being located in the alveoli, and induce an acute intoxication (acute alveolitis or adult respiratory distress syndrome; acute silicosis). Particles measuring 0.5-2.0 µm in diameter have the highest fibrogenic potency. The fibrogenic activity of quartz is reduced if the particles are coated with a soluble amorphous surface layer (Beilby layer). Tridymite, the crystalline silica variety with the highest fibrogenic activity, has no Beilby layer. The SiOH molecules of the silica surface (silanol groups) are assumed to bond to macromolecules of cells (active quaternary phosphate ester groups of phospholipids, hydrogen in proteins), to denature proteins (trimethylammonium groups), and damage lipid membranes. The individual disposition is not exactly known, and depends upon the particular anatomy of the lower respiratory system, mucociliary clearing system, resorption, immunologic and enzymatic factors. The progression of the disease is enhanced by nonspecific factors such as smoking, chronic infections, air pollution, and dust. Stishovite (a seldom and high pressure modification of SiO2) has no or only weak fibrogenic activity, amorphous silica a weaker activity than crystalline silica, and cristobalite a ten-fold higher fibrogenic activity than that of quartz. Inhalation of mixed dusts containing aluminum and iron minerals can reduce the risk of silicosis.
This may be related to the lower toxicity of aluminated silica to cell membranes composed of a high sialic acid content (aluminated silica have a higher electrical charge than silica, and, therefore, a lower probability to have a close contact to highly charged membranes). The reduced rate of silicosis in persons living in the Sahara desert is thought to be related to a protective iron-oxide dust.
Lung Diseases:
Silica have a high fibrogenic potency. Exposed persons have an increased risk for tuberculosis, rheumatoid diseases and lung cancer. Five main types of the clinical course can be distinguished: a) acute, rapid progressive silicosis; and four chronic forms: b) simple silicosis; c) complicated silicosis; d) Caplan’s syndrome; e) silicotuberculosis. Silica are possibly carcinogenic; however, detailed data are not available.
Clinical Presentation:
Basically, two forms of silicosis are known: a) an acute and b) a chronic and progressive disease. Acute silicosis is only seen in patients with high exposure levels (tunnelling, sand blasting) and starts within days. Cough, dyspnea, and cyanosis are the symptoms of an adult respiratory distress syndrome with high lethality. Chronic silicosis normally starts within 10 years after exposure (radiographically within twenty years), and is progressive up to 30 years after the final exposure. Symptoms include cough, expectoration, sometimes perspiration without fever, dyspnea, and loss of weight. The development of chronic bronchitis and emphysema usually induces cor pulmonale. Silicosis can induce chronic respiratory obstructive diseases and bronchopneumonia. Extrathoracic lesions are found in the spleen, liver, bone marrow, and abdominal lymph nodes. The carcinogenic potency of SiO2 has not been definitively confirmed, and the association between silicosis and lung cancer is -at bestweak, and may be related to nonspecific inflammatory lesions (scar carcinoma). Scleroderma has a higher prevalence in patients with silicosis. The cause is not known.
Radiology:
Acute silicosis is characterized by shadows, which lie predominantly in the middle and lower lung zones, miliar infiltrations and sometimes edema. In chronic silicosis, circumscribed, round, small opacities (1-10 mm in diameter) predominantly in the upper lobes are seen. Calcification of the hilar lymph nodes, eggshell and reticular patterns or irregular densities in the upper lung zones may exist. In diatomaceous earth exposure nodularity is absent, and linear small, sometimes large opacities, predominantly in upper lung zones are found. Mixed dust pneumoconiosis is characterized by irregular opacities, Caplan’s syndrome by large necrobiotic nodules.
Lung Function:
Lung Function displays no characteristic abnormal changes. In advanced stages of the disease, scarring of the lung induces obstruction.
Bronchoalveolar Lavage:
In bronchial lavage, one measures 20±15×106 cells of 84±8% macrophages, 14±7% lymphocytes and 2±2% granulocytes.
Pathology:
Gross:
Acute silicosis presents with heavy, firm, and indurated lungs. Some small black nodules may be found. The characteristic silicotic nodules are absent. Chronic silicosis presents with calcified (eggshell-like) hilar lymph nodes and multiple black, firm nodules which are concentrated towards the upper parts of both lungs. They measure a few millimeters in diameter and can become confluent into large areas with massive fibrosis in complicated silicosis. Perifocal emphysematous changes of the adjacent air spaces are usually severe.
Histology:
The findings include: a) acute silicosis: marked intra-alveolar eosinophilic exsudate which is PAS-positive, contains birefringent silica crystals and resembles alveolar proteinosis. Mild mononuclear interstitial inflammatory infiltrates are present; b) chronic silicosis: central hyalinized hyaline nodules with black dust particles. Many concentrically arranged histiocytes and fibroblasts are surrounded by acellular fibrous tissue with multiple diffusely distributed birefringent crystals. The nodules become confluent in lungs with complicated silicosis and present with caseous granulomas, if tuberculous infection is present. The adjacent air spaces are enlarged and show a marked rarification of the interalveolar septula. The intra-pulmonary proximal localized lymph nodes show marked scarring and an increased number of birefringent crystals.
Prognosis:
Silicosis is a progressive disease. 30% of all patients will develop one of the two main complications (massive fibrosis or tuberculosis). Recurrent pulmonary infections are frequent (followed by cardiorespiratory failure or cor pulmonale). Persons exposed to low levels may develop a slowly progressive silicosis (inactive silicosis).
Additional Diseases:
Anthracosilicosis:
The contemporaneous inhalation of coal dust and silicotic minerals results in pneumoconiosis. The severeness of the pulmonary fibrosis depends upon the amount of the inhaled SiO2 particles.
Siderosilicosis:
The contemporaneous inhalation of iron oxide and silicotic minerals results in siderosilicosis (Welder’s lung). Iron is stored in the lungs, and not associated with the fibrosis induced by SiO2 .
Systemic Sclerosis:
Patients with silicosis have a higher incidence of systemic sclerosis.
Caplan’s Syndrome:
Caplan’s syndrome is the combination of silicosis and rheumatoid arthritis, a rare disease.
Silicotuberculosis:
Silicotuberculosis, the combination of silicosis and tuberculosis, is the classical complication of silicosis. Patients with silicosis have an increased risk of infections by Mycobacterium tuberculosis. The infection may be associated with alterations of the immune system and disturbed mucociliary clearing system. Symptoms are loss of appetite and weight, night sweats, cough, expectoration, and dyspnea. Chest radiographs display typical signs of silicosis, in addition, cavitation and emphysema may be seen. The differential diagnosis between active and inactive silicotuberculosis may be difficult and is related to demonstration of living bacteria of Mycobacterium tuberculosis.
Pleural Plaques:
Pleural plaques have been described in patients with exposure to fibrous diatomaceous earth; for details, see under the headings asbestos .
Alveolar Proteinosis:
Alveolar proteinosis, normally seen in heavy smokers, is an unusual reaction to silica dust.
Kidneys:
Nephropathia (glomerulonephritis), induced by soluble silica have been described in patients with severe silicosis. Histomorphologically, silicotic granulomas can be demonstrated in the renal parenchyma.
Sarcoidosis:
Elevated serum angiotensine converting enzyme (ACE) levels have been described in both sarcoidosis and silicosis. Unlike silicosis, in sarcoidosis the alveolar macrophages stain positively with anti-ACE-antibodies.
Lung Cancer:
It is assumed that persons exposed to silica have a higher risk of lung cancer in both silicotic scars and nonaltered lung parenchyma.
Mesothelioma:
Some authors propose silica as an inducing agent for mesotheliomas; however, the data are still inconsistent.
Heart:
Epidemiologically, a higher mortality of chronic heart diseases was seen in ore miners. Confounding factors have not been estimated.
Remarks:
Biogenic silica have been found in some plants (phytoliths), and amorphous silica in algae, protozoa, and porifera (compare to diatomaceous earth). The oldest known men with silicosis were two persons living in the neolithic era. Women living in the Negev desert developed a specific variety of silicosis without characteristic histomorphologic silicotic reaction of the lung parenchyma. Exposure occurred during woolneedlework in tents. Women in the Transkei developed silicosis due to grinding their meals with sandstone.
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