К вопросу образования двойников ставролита в метаморфических породах (на примере кристаллических сланцев Кейв, Кольский п-ов)

• Published in: Мінералогічний журнал
• Date: 2011
• Main Author: Нерадовский, Ю.Н.
• Format: Article
• Language: Russian
• Published: Інститут геохімії, мінералогії та рудоутворення ім. М.П. Семененка НАН України 2011
• Subjects: Мінералогiя
• Online Access: https://nasplib.isofts.kiev.ua/handle/123456789/64104
• Journal Title: Digital Library of Periodicals of National Academy of Sciences of Ukraine
• Cite this: К вопросу образования двойников ставролита в метаморфических породах (на примере кристаллических сланцев Кейв, Кольский п-ов) / Ю.Н. Нерадовский // Мінералогічний журнал. — 2011. — Т. 33, № 4. — С. 39-44. — Бібліогр.: 11 назв. — рос.

Исследован механизм зарождения и формирования двойников ставролита в кристаллических плагиоклазставролитовых сланцах. Показано, что образование двойников в рассматриваемых метаморфических породах
 происходило в результате механического воздействия на зародыши ставролита растягивающих сил пластического "течения" сланцев в процессе их формирования. Внутреннее строение двойников отражает метаморфизм
 сланцев и помогает восстановить динамику развития процесса. Досліджено механізм зародження і формування двійників ставроліту в кристалічних плагіоклаз-ставролітових сланцях. Показано, що утворення двійників у
 цих метаморфічних породах відбувалося внаслідок механічної дії розтягувальних сил пластичної "течії"
 сланців на зародки ставроліту в процесі їх формування. Внутрішня будова двійників відображає метаморфізм сланців і допомагає відновити динаміку розвитку процесу. The genesis of staurolite twins in crystalline plagioclasestaurolite
 schists of the Keivy schists (the Kola Peninsula)
 has been studied. It is shown that the twins in the metamorphic
 rocks under question have formed as a result of
 staurolite nuclei, being mechanically affected by stretching
 forces of an elastic "flow" of schists during their formation.
 The twins structure reflects metamorphism of the schists
 and helps restore the process dynamics. A peculiar feature
 of staurolite in the studied rocks is its zoning, remarkably
 distinct in every crystal. A special paper is dedicated to the
 study of the zoning. Major elements of zonal crystals are a
 nucleus and two I-genus zones. A thin II-genus zoning is
 developed in I-genus zones. The structure of staurolite
 crystals allows defining three stages of its formation. The
 first stage is characterized by the formation of nuclei, i. e.
 the embryos of future staurolite crystals on the initial stage
 of the crystalline schists formation as a result of metamorphic
 reaction. The second stage of the staurolite crystallization
 comprises the nucleus stretching and the second
 zone forming. The way the crystal grew in this period depended
 on the crystal space orientation. The motive forces
 for the crystals growth at the second stage were deformations
 of the comprising environment given to the "flow"
 and deformation of the mineral as a "participant perceiving
 external stresses". It is assumed that the nucleus
 stretching deformation in the direction of the schists "flow"
 occurred under the effect of forces of the elastic schist
 flow. During the stretching deformation the nucleus elongated
 with no continuity break or divided into two parts,
 both of these parts being remote from the long crystal axis.
 In the first case the twinning took place in a number of
 crystals. It is supposed that the stretching of crystals with
 no continuity break resulted from the nucleus material deformation
 according to the cell "migration of dislocations"
 and translation gliding. The latter is accompanied by healing
 of defects. The first signs of the twinning are observed
 at the boundaries of a staurolite crystal nucleus. A wedgelike
 incision with a twin embryo is observed on the nucleus
 of twin crystals. The embryo was initially oriented according
 to either {231} or {031}, which resulted in the formation
 of an oblique or cruciform twin. The crystal-2 happened
 to originate not on two sides of the crystal-1, but on
 one side. The opposite half of the crystal-2 was formed
 later. However, the crystal preserved its space orientation
 on a plane, which provided the suggestion that a "through"
 twinning plane existed in the crystal-1 during the crystal-2
 origination. Interestingly, the twinning joint is always close
 to the heart of the intergrown crystals. Thus, crystals in
 staurolite twins originated at different times and it is necessary
 to distinguish crystal-1 and crystal-2. Depending
 on the orientation according to {231} or {031}, crystal-2
 grew and crossed the nucleus of the host crystal obliquely
 of perpendicularly. The third stage of the staurolite crystallization
 is characterized by regular tectonic conditions and
 a growth of crystals in all directions. Once the nucleus
 stopped stretching and the second zone crystallized, the
 formation of twins finished. All crystals, including twin
 ones, got overgrown by planes of the third zone, i. e. obtained
 the current shape.