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Sometime after midnight on February 8,1969, a large, bright meteor entered Earth's atmosphere and broke into thousands of pieces, plummeted to the ground, and scattered over an area 50 miles long and 10 miles wide in the state of Chihuahua in Mexico. The first meteorite from this fall was found in the village of Pueblito de Allende. Altogether, roughly two tons of meteorite fragments were recovered, all of which bear the name Allende for the location of the first discovery.
Individual specimens of Allende are covered with a black, glassy crust that formed when their exteriors melted as they were slowed by Earth's atmosphere. When broken open, Allende stones are revealed to contain an assortment of small, distinctive objects, spherical or irregular in shape and embedded in a dark gray matrix (binding material), which were once constituents of the solar nebula—the interstellar cloud of gas and dust out of which our solar system was formed.
The Allende meteorite is classified as a chondrite. Chondrites take their name from the Greek word chondros—meaning "seed"—an allusion to their appearance as rocks containing tiny seeds. These seeds are actually chondrules: millimeter-sized melted droplets of silicate material that were cooled into spheres of glass and crystal. A few chondrules contain grains that survived the melting event, so these enigmatic chondrules must have formed when compact masses of nebular dust were fused at high temperatures—approaching 1,700 degrees Celsius—and then cooled before these surviving grains could melt. Study of the textures of chondrules confirms that they cooled rather quickly, in times measured in minutes or hours, so the heating events that formed them must have been localized. It seems very unlikely that large portions of the nebula were heated to such extreme temperatures, and huge nebula areas could not possibly have lost heat so fast. Chondrules must have been melted in small pockets of the nebula that were able to lose heat rapidly. The origin of these peculiar glassy spheres remains an enigma.
Equally perplexing constituents of Allende are the refractory inclusions: irregular white masses that tend to be larger than chondrules. They are composed of minerals uncommon on Earth, all rich in calcium, aluminum, and titanium, the most refractory (resistant to melting) of the major elements in the nebula. The same minerals that occur in refractory inclusions are believed to be the earliest-formed substances to have condensed out of the solar nebula. However, studies of the textures of inclusions reveal that the order in which the minerals appeared in the inclusions varies from inclusion to inclusion, and often does not match the theoretical condensation sequence for those metals.
Chondrules and inclusions in Allende are held together by the chondrite matrix, a mixture of fine-grained, mostly silicateminerals that also includes grains of iron metal and iron sulfide. At one time it was thought that these matrix grains might be pristine nebular dust, the sort of stuff from which chondrules and inclusions were made. However, detailed studies of the chondrite matrix suggest that much of it, too, has been formed by condensation or melting in the nebula, although minute amounts of surviving interstellar dust are mixed with the processed materials.
All these diverse constituents are aggregated together to form chondritic meteorites, like Allende, that have chemical compositions much like that of the Sun. To compare the compositions of a meteorite and the Sun, it is necessary that we use ratios of elements rather than simply the abundances of atoms. After all, the Sun has many more atoms of any element, say iron, than does a meteorite specimen, but the ratios of iron to silicon in the two kinds of matter might be comparable. The compositional similarity is striking. The major difference is that Allende is depleted in the most volatile elements, like hydrogen, carbon, oxygen, nitrogen, and the noble gases, relative to the Sun. These are the elements that tend to form gases even at very low temperatures. We might think of chondrites as samples of distilled Sun, a sort of solar sludge from which only gases have been removed. Since practically all the solar system's mass resides in the Sun, this similarity in chemistry means that chondrites have average solar system composition, except for the most volatile elements; they are truly lumps of nebular matter, probably similar in composition to the matter from which planets were assembled.
1969年2月8日午夜过后的某个时候，一颗明亮的大流星进入地球大气层，坠落到地面，在墨西哥的奇瓦瓦州散布在50英里长、10英里宽的区域。。总共发现了大约两吨的陨石碎片，，以表示第一次发现的地点。
，由于地球大气层的减速，外壳融化后就形成了这种外壳。当破碎的打开,,独特的对象,球形或不规则形状和嵌入在一个黑暗的灰色矩阵(粘合剂),这曾经的太阳星云的气体和尘埃组成的星际云的太阳系成立。
。球粒陨石得名于希腊单词软骨——意思是“种子”——暗指它们的外表是含有微小种子的岩石。这些种子实际上是陨石球粒:几毫米大小的硅酸盐材料融化的液滴被冷却成玻璃和晶体。一些陨石球粒在融化过程中幸存下来，所以这些神秘的陨石球粒一定是在高温下(接近1700摄氏度)熔融时形成的，然后在这些幸存的陨石球粒融化之前冷却下来。对陨石球粒质地的研究证实，陨石球粒冷却速度相当快，时间以分钟或小时计，因此形成陨石球粒的加热事件必定是局部发生的。星云的大部分被加热到如此极端的温度似乎是非常不可能的，而且巨大的星云区域不可能以如此快的速度失去热量。陨石球粒一定是在星云的小区域融化的，这些小区域能够迅速地散热。这些特殊的玻璃球的起源仍然是个谜。
:不规则的白色团块往往比软骨块大。它们是由地球上不常见的矿物质组成，都富含钙、铝和钛，是星云中主要元素中最难融化的(抗融化)。难熔包裹体中所含的矿物质被认为是太阳系星云中最早期形成的物质。然而，对包裹体结构的研究表明，包裹体中矿物出现的顺序因包裹体的不同而不同，通常与这些金属的理论冷凝序列不匹配。
matrix)结合在一起。球粒基质是一种由细粒、主要是硅酸盐矿物组成的混合物，还包括铁金属和硫化铁的颗粒。曾经有一段时间，人们认为这些基质颗粒可能是原始的星云尘埃，这类物质构成了软骨和内含物。然而，对球粒陨石基质的详细研究表明，尽管少量幸存的星际尘埃与经过处理的物质混合在一起，但大部分球粒也是由星云内的冷凝或熔化形成的。
所有这些不同的成分聚集在一起，形成球粒陨石，，化学成分很像太阳。为了比较陨石和太阳的组成，我们有必要使用元素的比例，而不是简单地使用原子的丰度。毕竟，太阳中任何元素(比如铁)的原子都比陨石标本多得多，但这两种物质中铁与硅的比例或许可以相提并论。组成上的相似性是惊人的。，比如氢，碳，氧，氮和惰性气体，相对于太阳。这些元素即使在很低的温度下也会形成气体。我们可以把球粒看作是太阳蒸馏后的样品，一种只有气体被除去的太阳污泥。由于几乎所有太阳系的质量都驻留在太阳中，这种化学上的相似性意味着球粒陨石除了最易挥发的元素外，具有太阳系的平均组成;它们确实是星云物质的块状物，其组成可能与行星聚集的物质相似。
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